dbt Guide

gitlab-snowflake:
  target: dev
  outputs:
    dev:
      type: snowflake
      threads: 8
      account: gitlab
      user: {username}
      role: {rolename}
      database: {databasename}
      warehouse: DEV_XS
      schema: preparation
      authenticator: externalbrowser
    dev_l:
      type: snowflake
      threads: 16
      account: gitlab
      user: {username}
      role: {rolename}
      database: {databasename}
      warehouse: DEV_L
      schema: preparation
      authenticator: externalbrowser

 ~/repos/analytics/transform/snowflake-dbt
╰─$ make build-changes DOWNSTREAM="+1" FULL_REFRESH="True" TARGET="dev_xl" VARS="key":"value" EXCLUDE="test_model"

~/repos/analytics/transform/snowflake-dbt
╰─$ make lint-models
sqlfluff lint models/workspaces/workspace_data/mock/data_type_mock_table.sql

~/repos/analytics/transform/snowflake-dbt
╰─$ make lint-models FIX="True" MODEL="dim_date"
sqlfluff fix ./models/common/dimensions_shared/dim_date.sql

 ~/repos/analytics/transform/snowflake-dbt
╰─$ make safe-check

make DBT_MODELS="dim_subscription" clone-dbt-select-local-user-noscript

Configuration for contributing to dbt project

If you’re interested in contributing to dbt, here’s our recommended way of setting up your local environment to make it easy.

• Fork the dbt project via the GitHub UI to your personal namespace

• Clone the project locally

• Create a virtual environment (venv) for dbt following these commands

  cd ~
  mkdir .venv # This should be in your root "~" directory
  python -m venv .venv/dbt
  source ~/.venv/dbt/bin/activate
  pip install dbt
  

• Consider adding alias dbt!="source ~/.venv/dbt/bin/activate" to your .bashrc or .zshrc to make it easy to start the virtual environment

• Navigate to the dbt project in the same terminal window - you should see (dbt) at the start of the command prompt

• Run pip install -r editable_requirements.txt. This will ensure when you run dbt locally in your venv you’re using the code on your machine.

• Run which dbt to ensure it’s pointing to the venv

• Develop code locally, commit your changes as you would, and push up to your namespace on GitHub

When you’re ready to submit your code for an MR, ensure you’ve signed their CLA.

Style and Usage Guide

Model Structure

As we transition to a more Kimball-style warehouse, we are improving how we organize models in the warehouse and in our project structure. The following sections will all be top-level directories under the models directory, which is a dbt default. This structure is inspired by how dbt Labs structures their projects.

Sources

All raw data will still be in the RAW database in Snowflake. These raw tables are referred to as source tables or raw tables. They are typically stored in a schema that indicates its original data source, e.g. netsuite

Sources are defined in dbt using a sources.yml file.

• We use a variable to reference the database in dbt sources, so that if we’re testing changes in a Snowflake clone, the reference can be programmatically set

• When working with source tables with names that don’t meet our usual convention or have unclear meanings, use identifiers to override source table names when the original is messy or confusing. (Docs on using identifiers)

  # Good
  tables:
    - name: bizible_attribution_touchpoint
      identifier: bizible2__bizible_attribution_touchpoint__c
  
  # Bad
  tables:
    - name: bizible2__bizible_attribution_touchpoint__c
  

Source Models

We are enforcing a very thin source layer on top of all raw data. This directory is where the majority of source-specific transformations will be stored. These are “base” models that pull directly from the raw data and do the prep work required to make facts and dimensions and should do only the following:

• Rename fields to user-friendly names
• Cast columns to appropriate types
• Minimal transformations that are 100% guaranteed to be useful for the foreseeable future. An example of this is parsing out the Salesforce ID from a field known to have messy data.
• Placement in a logically named schema

Even in cases where the underlying raw data is perfectly cast and named, there should still exist a source model which enforces the formatting. This is for the convenience of end users so they only have one place to look and it makes permissioning cleaner in situations where this perfect data is sensitive.

The following should not be done in a source model:

• Removing data
• Joining to other tables
• Transformations that fundamentally alter the meaning of a column

For all intents and purposes, the source models should be considered the “raw” data for the vast majority of users.

Key points to remember:

• These models will be written to a logically named schema in the prep database based on the data source type. For example:
  • Zuora data stored in raw.zuora would have source models in prep.zuora
  • GitLab.com data with tables stored in raw.tap_postgres.gitlab_db_* would have source models in prep.gitlab_dotcom
  • Customers.gitlab.com data with tables stored in raw.tap_postgres.customers_db_* would have source models in prep.customers_db
• These models should be organized by source - this will usually map to a schema in the raw database
• The name of source models should end with _source
• Only source models should select from source/raw tables
• Source models should not select from the raw database directly. Instead, they should reference sources with jinja, e.g. FROM {{ source('bamboohr', 'job_info') }}
• Only a single source model should be able to select from a given source table
• Source models should be placed in the /models/sources/<data_source/ directory
• Source models should perform all necessary data type casting, using the :: syntax when casting (You accomplish the same thing with fewer characters, and it presents as cleaner).
  • Ideally, source models should cast every column. Explicit is better than implicit. Test your assumptions
• Source models should perform all field naming to force field names to conform to standard field naming conventions
• Source fields that use reserved words must be renamed in Source models
• Source models for particularly large data should always end with an ORDER BY statement on a logical field (usually a relevant timestamp). This essentially defines the cluster key for the warehouse and will help to take advantage of Snowflake’s micro-partitioning.

For a visual of how the source models relate to the raw tables and how they can act as a clean layer for all downstream modeling, see the following chart:

graph LR

  subgraph "RAW Database"

    subgraph "tap_postgres Schema"
      GitLabRaw[GitLab Users Raw Table ]
      VersionRaw[Version DB Pings Raw Table ]
    end

    subgraph "zuora Schema"
      ZuoraRaw[Zuora Account Raw Table ]
    end

  end

  subgraph "PREP Database"

    subgraph "gitlab_dotcom Schema"
      GitLabRaw --> gitlabusers[GitLab Users Source Model ]
    end

    subgraph "version_db Schema"
      VersionRaw --> versionpings[Version DB Pings Source Model ]
    end

    subgraph "zuora Schema"
      ZuoraRaw --> ZuoraAccount[Zuora Account Source Model ]
    end

    subgraph "preparation Schema"
      dbtModels[More dbt models ]
      gitlabusers --> dbtModels
      versionpings --> dbtModels
      ZuoraAccount --> dbtModels
    end

  end

  subgraph "PROD Database"

    subgraph "common schema"
      dbtModels --> dbtModels2[More dbt models ]
      gitlabusers --> dbtModels2
      versionpings --> dbtModels2
      ZuoraAccount --> dbtModels2
    end

    subgraph "common_mapping schema"
      dbtModels --> dbtModels3[More dbt models ]
      gitlabusers --> dbtModels3
      versionpings --> dbtModels3
      ZuoraAccount --> dbtModels3
    end

  end

Sensitive Data

In some cases, there are columns whose raw values should not be exposed. This includes things like customer emails and names. There are legitimate reasons to need this data however, and the following is how we secure this data while still making it available to those with a legitimate need to know.

Static Masking

For a given model using static masking, the source format is followed as above. There is no hashing of columns in the source model. This should be treated the same as the raw data in terms of security and access.

Sensitive columns are documented in the schema.yml file using the meta key and setting sensitive equal to true. An example is as follows.

  - name: sfdc_contact_source
    description: Source model for SFDC Contacts
    columns:
         - name: contact_id
           tests:
              - not_null
              - unique
         - name: contact_email
           meta:
              sensitive: true
         - name: contact_name
           meta:
              sensitive: true

Two separate models are then created from the source model: a sensitive and non-sensitive model.

The non-sensitive model uses a dbt macro called hash_sensitive_columns which takes the source table and hashes all of the columns with sensitive: true in the meta field. There is no specific join key specified since all columns are hashed in the same way. Another column can be added in this model as a join key outside of the macro, if needed. The sfdc_contact model is a good example of this. 2 columns are hashed but an additional primary key of contact_id is specified.

In the sensitive model, the dbt macro nohash_sensitive_columns is used to create a join key. The macro takes the source table and a column as the join key and it returns the hashed column as the join key and the remaining columns unhashed. The sfdc_contact_pii model is a good example of the macro in use.

All hashing includes a salt as well. These are specified via environment variables. There are different salts depending on the type of data. These are defined in the get_salt macro and are also set when using the dbt container for local development.

In general, team members should not be permitted to see the salt used in the query string within the Snowflake UI. In table models this goal is met by using the Snowflake built-in ENCRYPT function. For models materialized into views, the ENCRYPT function seems to not work. Instead, a workaround using secure views is utilized. A secure view limits DDL viewing to the owner only, thus limiting visibility of the hash. To create a secure view, set secure equal to true in the model configuration. A view that utilizes the hashing functionality as described, but is not configured as a secure view, will likely not be queryable.

Dynamic Masking

When the sensitive data needs to be known by some users but not all then dynamic masking can be applied.

Sensitive columns to be masked dynamically are documented in the schema.yml file using the meta key and setting masking_policy equal to one of the Data Masking Roles found in the roles.yml file. An example is as follows.

  - name: sfdc_contact_source
    description: Source model for SFDC Contacts
    columns:
         - name: contact_id
           tests:
              - not_null
              - unique
         - name: contact_email
           meta:
              masking_policy: general_data_masking
         - name: contact_name
           meta:
              masking_policy: general_data_masking

A post-hook running the macro mask_model will need to be configured for any model that will need dynamic masking applied.

The mask_model macro will first retrieve all of the columns of the given model that have a masking_policy identified. That information is passed to an other macro, apply_masking_policy, witch orchestrates the creation and application of Snowflake masking policies for the given columns.

The first step of the apply_masking_policy is to get the data type of the columns to be masked as the polices are data type dependant. This is done with a query to the data base information_schema table with the following query:

SELECT
  t.table_catalog,
  t.table_schema,
  t.table_name,
  t.table_type,
  c.column_name,
  c.data_type
FROM "{{ database }}".information_schema.tables t
INNER JOIN "{{ database }}".information_schema.columns c
  ON c.table_schema = t.table_schema
  AND c.table_name = t.table_name
WHERE t.table_catalog =  '{{ database.upper() }}'
  AND t.table_type IN ('BASE TABLE', 'VIEW')
  AND t.table_schema = '{{ schema.upper() }}'
  AND t.table_name = '{{ alias.upper() }}'
ORDER BY t.table_schema,
  t.table_name;

Where the database, schema, and alias are passed to the macro at the time it is called.

With the qualified column name and the data type, masking policies are created for a given database, schema, and data type for the specified masking policy. And once the policies have been created they are applied to the identified columns.

It should be noted that permissions are based on an allow list of roles. Meaning permission has to be granted to see the unmasked data at query time:

CREATE OR REPLACE MASKING POLICY "{{ database }}".{{ schema }}.{{ policy }}_{{ data_type }} AS (val {{ data_type }})
  RETURNS {{ data_type }} ->
      CASE
        WHEN CURRENT_ROLE() IN ('transformer','loader') THEN val  -- Set for specific roles that should always have access
        WHEN IS_ROLE_IN_SESSION('{{ policy }}') THEN val -- Set for the user to inherit access bases on there roles
        ELSE {{ mask }}
      END;

Only one policy can be applied to a column so users that need access will have to have the permissions granted using the applied masking role.

Staging

Prior to our implementation of Kimball modeling, most all of our models would have fallen into Staging category.

This directory is where the majority of business-specific transformations will be stored. This layer of modeling is considerably more complex than creating source models, and the models are highly tailored to the analytical needs of business. This includes:

• Filtering irrelevant records
• Choosing columns required for analytics
• Renaming columns to represent abstract business concepts
• Joining to other tables
• Executing business logic
• Modelling of fct_and dim_ tables following Kimball methodology

Workspaces

We provide a space in the dbt project for code that is team specific and not meant to adhere to all of the coding and style guides. This is in an effort to help teams iterate faster using solutions that don’t need to be more robust.

Within the project there is a /models/workspaces/ folder where teams can create a folder of the style workspace_<team> to store their code. This code will not be reviewed by the Data Team for style. The only concern given prior to merge is whether it runs and if there are egregious inefficiencies in the code that could impact production runs.

To add a new space:

• Create an issue in the analytics project and open a new merge request

• Create a new folder in /models/workspaces/ e.g. workspace_security

• Add an entry to the dbt_project.yml file for the new workspace. Include the schema it should write to:

  # ------------------
  # Workspaces
  # ------------------
  workspaces:
    +tags: ["workspace"]
  
    workspace_security: # This maps to the folder in `/models/workspaces/`
      +schema: workspace_security # This is the schema in the `prod` database
  

• Add your .sql files to the folder

• Add any entries to the CODEOWNERS file

• Use the dbt Workspace Changes MR template and follow the instructions there to submit the MR for review and final merge

Newly added code will take up to 24 hours to appear in the data warehouse.

The Data Team reservers the right to reject code that will dramatically slow the production dbt run. If this happens, we will consider building a separate dbt execution job just for the workspaces.

General

• Model names should be as obvious as possible and should use full words where possible, e.g. accounts instead of accts.

• Documenting and testing new data models is a part of the process of creating them. A new dbt model is not complete without tests and documentation.

• Follow the naming convention of analysis type, data source (in alpha order, if multiple), thing, aggregation

  -- Good
  retention_sfdc_zuora_customer_count.sql
  
  -- Bad
  retention.sql
  

• All {{ ref('...') }} statements should be placed in CTEs at the top of the file. (Think of these as import statements.)

  • This does not imply all CTE’s that have a {{ ref('...') }} should be SELECT * only. It is ok to do additional manipulations in a CTE with a ref if it makes sense for the model.
  • If only a small number of fields are required from a model containing many columns then it can be performant to list them in the CTE, otherwise it is better to use SELECT *. To do this, the simple_cte macro can be used.

• If you want to separate out some complex SQL into a separate model, you absolutely should to keep things DRY and easier to understand. The config setting materialized='ephemeral' is one option which essentially treats the model like a CTE.

Model Configuration

There are multiple ways to provide configuration definitions for models. The dbt docs for configuring models provide a concise explanation of the ways to configure models.

Our guidelines for configuring models:

• The default materialization is view
• The default schema is prep.preparation.
• Disabling any model should always be done in the dbt_project.yml via the +enabled: false declaration
• Configs should be applied in the smallest number of locations:
  • If <50% of models in a directory require the same configuration, then configure the individual models
  • If >=50% of models in a directory require the same configuration, strongly consider setting a default in the dbt_project.yml, but think about whether that setting is a sensible default for any new models in the directory

Versions

Model versions within dbt allow for a controlled transition between model changes that may break or significantly impact downstream uses. When versions are defined for a model all enabled versions will be produced in the target database and schema. This allows users of the model to access the versions side by side so that any breaking changes can be addressed while still delivering reports and analysis using an earlier version. This is most effective when columns are removed or significant refactor of several models is taking place over multiple development cycles.

Defining Model Versions

Model versions are implemented by creating a new model file with the same name as the target model but with an added version suffix, and by defining version properties in the relevant schema.yml file. More details can be found in the dbt documentation for model versions.

# dim_date.sql
# dim_date_v2.sql

models:
  name: dim_date
  ...
  versions:
    - v: 1
      defined_in: dim_date.sql # Only needed if there is no suffix on the model file.
    - v: 2
  latest_version: 1

Referencing Model Versions

With the provided model definitions, two models will be created when the model is included in a selection, such as dbt run --models dim_date; database.schema.dim_date_v1 and database.schema.dim_date_v2. Additionally, with the create_latest_version_view post-hook, a view will be created of the latest version; database.schema.dim_date. This view allows users to always be on the latest version of a model without needing to know what that version is.

Within dbt, if a specific version of a model needs to be used to build an other model, it can be defined in the ref function:

{{ ref('dim_date', v=2) }}

If a version is not defined in the ref function then the latest version of the model will be used.

Depends On

In normal usage, dbt knows the proper order to run all models based on the usage of the {{ ref('...') }} syntax. There are cases though where dbt doesn’t know when a model should be run. A specific example is when we use the schema_union_all or schema_union_limit macros. In this case, dbt thinks the model can run first because no explicit references are made at compilation time. To address this, a comment can be added in the file, after the configuration setting, to indicate which model it depends on:

{{config({
    "materialized":"view"
  })
}}

-- depends_on: {{ ref('snowplow_sessions') }}

{{ schema_union_all('snowplow_', 'snowplow_sessions') }}

dbt will see the ref and build this model after the specified model.

Database and Schema Name Generation

In dbt, it is possible to generate custom database and schema names. This is used extensively in our project to control where a model is materialized and it changes depending on production or development use cases.

Databases

The default behavior is documented in the “Using databases” section of the dbt documentation. A macro called generate_database_name determines the schema to write to.

We override the behavior of this macro with our own generate_database_name definition. This macro takes the configuration (target name and schema) supplied in the profiles.yml as well as the schema configuration provided in the model config to determine what the final schema should be.

Schemas

The default behavior is documented in the “Using custom schemas” section of the dbt documentation. A macro called generate_schema_name determines the schema to write to.

We override the behavior of this macro with our own generate_schema_name definition. This macro takes the configuration (target name and schema) supplied in the profiles.yml as well as the schema configuration provided in the model config to determine what the final schema should be.

Development behavior

In FY21-Q4, we switched to having development databases instead of schemas. This means that the schemas that are used in production match what is used in development, but the database location is different. dbt users should have their own scratch databases defined, such as TMURPHY_PROD and TMURPHY_PREP, where models are written to.

This switch is controlled by the target name defined in the profiles.yml file. Local development should never have prod or ci as the target.

Macros

Naming conventions

• File name must match the macro name

Structure

• Macros should be documented in either the macros.yml file or in a macros.md file in descriptions are long
• Use the arguments property in macros.yml to describe the input variables

dbt-utils

In our dbt project we make use of the dbt-utils package. This adds several macros that are commonly useful. Important ones to take note of:

• group_by - This macro build a group by statement for fields 1…N
• star - This macro pulls all the columns from a table excluding the columns listed in the except argument
• surrogate_key - This macro takes a list of field names and returns a hash of the values to generate a unique key

Seeds

Seeds are a way to load data from csv files into our data warehouse (dbt documentation). Because these csv files are located in our dbt repository, they are version controlled and code reviewable. This method is appropriate for loading static data which changes infrequently. A csv file that’s up to ~1k lines long and less than a few kilobytes is probably a good candidate for use with the dbt seed command. A seed file should be placed in the project folder that corresponds to the functional team that has ownership of the information found therein. This folder structure also corresponds to a schema in the PREP database so that the data can be easily used in further development.

Organizing columns

When writing a base model, columns should have some logical ordering to them. We encourage these 4 basic groupings:

• Primary data
• Foreign keys
• Logical data - This group can be subdivided further if needed
• Metadata

Primary data is the key information describing the table. The primary key should be in this group along with other relevant unique attributes such as name.

Foreign keys should be all the columns which point to another table.

Logical data is for additional data dimensions that describe the object in reference. For a Salesforce opportunity this would be the opportunity owner or contract value. Further logical groupings are encouraged if they make sense. For example, having a group of all the variations of contract value would make sense.

Within any group, the columns should be alphabetized on the alias name.

An exception to the grouping recommendation is when we control the extraction via a defined manifest file. A perfect example of this is our gitlab.com manifest which defines which columns we extract from our application database. The base models for these tables can be ordered identically to the manifest as it’s easier to compare diffs and ensure accuracy between files.

• Ordered alphabetically by alias within groups

  -- Good
  
  SELECT
    id                    AS account_id,
    name                  AS account_name,
  
    -- Foreign Keys
    ownerid               AS owner_id,
    pid                   AS parent_account_id,
    zid                   AS zuora_id,
  
    -- Logical Info
    opportunity_owner__c  AS opportunity_owner,
    account_owner__c      AS opportunity_owner_manager,
    owner_team_o__c       AS opportunity_owner_team,
  
    -- metadata
    isdeleted             AS is_deleted,
    lastactivitydate      AS last_activity_date
  FROM table
  

• Ordered alphabetically by alias without groups

  -- Less Good
  
  SELECT
    id                    AS account_id,
    name                  AS account_name,
    isdeleted             AS is_deleted,
    lastactivitydate      AS last_activity_date,
    opportunity_owner__c  AS opportunity_owner,
    account_owner__c      AS opportunity_owner_manager,
    owner_team_o__c       AS opportunity_owner_team,
    ownerid               AS owner_id,
    pid                   AS parent_account_id,
    zid                   AS zuora_id
  FROM table
  

• Ordered alphabetically by original name

  -- Bad
  
  SELECT
    account_owner__c      AS opportunity_owner_manager,
    id                    AS account_id,
    isdeleted             AS is_deleted,
    lastactivitydate      AS last_activity_date
    name                  AS account_name,
    opportunity_owner__c  AS opportunity_owner,
    owner_team_o__c       AS opportunity_owner_team,
    ownerid               AS owner_id,
    pid                   AS parent_account_id,
    zid                   AS zuora_id
  FROM table
  

Tags

Tags in dbt are a way to label different parts of a project. These tags can then be utilized when selecting sets of models, snapshots, or seeds to run.

Tags can be added in YAML files or in the config settings of any model. Review the dbt_project.yml file for several examples of how tags are used. Specific examples of adding tags for the Trusted Data Framework are shown below.

Within the analytics and data-tests projects we enforce a Single Source of Truth for all tags. We use the Valid Tags CSV to document which tags are used. Within merge requets, there is a validation step within every dbt CI job that will check this csv against all tags used in the project and fail the job if there is a mismatch. In the future, we aim to include more metadata about tags within this csv file.

Be aware that tags applied at any level do not apply to any tests. Tags for tests have to be explicitly applied for every test within the schema.yml file.

Warehouse Size

Configuring the warehouse size within the model can be a way to ensure needed performance on a model. This is intended to be performed on a model by model bases and for models with known performance needs; for example, the model contains a large recursive CTE that will fail if not run on a LARGE or bigger warehouse.

This configuration can be done using the generate_warehouse_name macro within the model configuration and is designed to work with both production, TRANSFORMER, and development DEV warehouses. To configure the warehouse size, you need to pass the desired size to the macro where the correct warehouse name will be generated. This will only work for currently existing warehouses.

{{
  config(
    snowflake_warehouse = generate_warehouse_name('XL')
  )
}}

Sample Data in Development

To streamline local development on local models, a way to sample, or use a subset of the data, is made available to the developers. This tool will allow developers the option of using sample data, or full data depending on what the situation calls for, allowing them to iterate quickly on the structure of models using sample data and then switch to full data when validation is needed. Using this in conjunction with local cloning of tables should improve the developer cycle time.

When selecting the tool to use, the developer should consider speed gained from the tool and the consequence of leaving the sampling in the code. A Random Sample is easy to add to the code, but if left in the code base, it puts the quality of the production data at risk, whereas a Sample Table will take longer to set up, but has no risk to the production data.

Random Sample

With the macro sample_table the developer will be able to select a random percent of the target table. This macro takes a percentage value which represent the amount of the target table to be returned. While the sample is random it is also deterministic, meaning that if the target table has not changed each query will return the same rows. If this macro is left in the code it will be executed in production and ci environments so it should be removed before merging in to the main code branch.

Example Use:

SELECT *
FROM {{ ref('dim_date') }} {{ sample_table(3) }}

-- Resolve to:
SELECT *
FROM dev_prod.common.dim_date SAMPLE SYSTEM (3) SEED (16)

Sample Table

With a sample table the developer creates a table that represents a sub set of the data in the target table that existing models will use instead of the original table. This requires configuring the desired samples and performing operations to create the sample tables. When configuring the sample the sample clause can use the sample_table macro or any filtering statement that can be used after the FROM statement such as LIMIT, WHERE, or QUALIFY. These sample tables will only be used in non production and ci environments and will not appear in the model lineage.

Workflow steps:

• Clone the target sample tables

  • Using a command such as clone-dbt-select-local-user ensure that there is a full data table to sample from.

• Configure the sample for each table to be created

  • Samples are configured in the samples macro in the samples_yml variable

    
    samples:
      - name: dim_date
        clause: "{{ sample_table(3) }}"
    
    # Or
    
    samples:
      - name: dim_date
        clause: "WHERE date_actual >= DATEADD('day', -30, CURRENT_DATE())"
    

• Construct the sample tables

  • Using the run-operation command execute the create_sample_tables macro

    dbt run-operation create_sample_tables
    

• Develop and iterate as needed

• Final test run using the full data

  • Override the local_data variable to be full-data as part of the dbt execution

    dbt run -s dim_date --vars 'local_data: full_data'
    

• Remove sample configuration

  • Remove the list of samples from the samples_yml before merging the code changes

Example Use:

-- Sample configuration
/*
samples:
  - name: dim_date
    clause: "WHERE date_actual >= DATEADD('day', -30, CURRENT_DATE())"
*/

-- dbt run-operation create_sample_tables executes the following command
CREATE OR REPLACE TRANSIENT TABLE dev_prod.common.dim_date__sample AS SELECT * FROM dev_prod.common.dim_date WHERE date_actual >= DATEADD('day', -30, CURRENT_DATE());

-- In model ref function will retrieve the sample table
SELECT *
FROM {{ ref('dim_date') }}

-- Resolves to:
SELECT *
FROM dev_prod.common.dim_date__sample

How Sampling Macros Work

For more details on how the macros used in sampling function see the following documentaion:

• create_sample_tables
• generate_sample_table_sql
• get_sample_relation
• is_table_sampled
• sample_table
• samples
• ref

Trusted Data Framework

See the Trusted Data Framework section of our Platform page for a deeper dive into the philosophy behind the Trusted Data Framework.

Schema To Golden Data Coverage

We implement 12 categories of Trusted Data Framework (TDF) monitors and tests (monitors are created in and executed by Monte-Carlo, tests are created with and executed by dbt):

1. Freshness monitors Monitor for unusual delays in table and field updates
2. Schema monitors Monitor fields that are added, removed or changed
3. Volume monitors Monitor for unusual changes in table size based on the numbers of rows
4. Field health Monitor Monitor fields for dips or spikes in stats like % null, % unique, and more. Our ML sets the thresholds.
5. SQL rule monitor Write a SQL statement to check for any expressable condition across 1 or more tables in your data.
6. JSON schema monitor Monitor for schema changes in JSON data added to a table field.
7. Dimension tracking Monitor for changes in the distribution of values within a low-cardinality table field.
8. Schema tests to validate the integrity of a schema
9. Column Value tests to determine if the data value in a column matches pre-defined thresholds or literals
10. Rowcount tests to determine if the number of rows in a table over a pre-defined period of time match pre-defined thresholds or literals
11. Custom SQL tests any valid SQL that doesn’t conform to the above categories

Our tests are stored in 2 primary places - either in a YAML file within our main project or within our Data Tests project.

Schema and Column Value tests will usually be in the main project. These will be in schema.yml and sources.yml files in the same directory as the models they represent.

Rowcount, and any other custom SQL tests will always be in the Data Tests project. This is a private project for internal GitLab use only.

Tagging

Tagging the tests is an important step in adding new tests. Labeling the test with a dbt tag is how we parse and identify tests when building trusted data dashboards. There are 2 ways to tag tests depending on their type.

The first is by adding tags in the YAML definition. This can be done at the highest level of the YAML definition for source tests, or on the column level for model tests.

## Source Labeling in sources.yml
version: 2

sources:
  - name: zuora
    tags: ["tdf","zuora"]

## Model Labeing in schema.yml
version: 2

models:
  - name: zuora_accounting_period_source
    description: Source layer for Zuora Accounting Periods for cleaning and renaming
    columns:
      - name: accounting_period_id
        tags: ["tdf","zuora"]
        tests:
          - not_null
          - unique

Each of these examples will apply the tags to all tests nested in the underlying hierarchy.

The second way of adding tags is via the config declaration at the top of a test file:

{{ config({
    "tags": ["tdf","zuora"]
    })
}}

WITH test AS (...)

General Guidance

• Every model should be tested in a schema.yml file
• At minimum, unique fields, not nullable fields, and foreign key constraints should be tested (if applicable)
• The output of dbt test should be pasted into MRs
• Any failing tests should be fixed or explained prior to requesting a review

Schema Tests

Schema tests are designed to validate the existence of known tables, columns, and other schema structures. Schema tests help identify planned and accidental schema changes.

All Schema Tests result in a PASS or FAIL status.

Schema Test Example

Purpose: This test validates critical tables exist in the Zuora Data Pipeline.

We’ve implemented schema tests as a dbt macro. This means that instead of writing SQL, a user can add the test by simply calling the macro. This is controlled by the raw_table_existence macro.

-- File: https://gitlab.com/gitlab-data/analytics/-/blob/master/transform/snowflake-dbt/tests/sources/zuora/existence/zuora_raw_source_table_existence.sql
{{ config({
    "tags": ["tdf","zuora"]
    })
}}

{{ raw_table_existence(
    'zuora_stitch',
    ['account', 'subscription', 'rateplancharge']
) }}

Column Value Tests

Column Value Tests determine if the data value in a column is within a pre-defined threshold or matches a known literal. Column Value Tests are the most common type of TDF test because they have a wide range of applications. Column Value tests are useful in the following scenarios:

• change management: pre-release and post-release testing
• ensuring sums/totals for important historical data meets previously reported results
• ensuring known “approved” data always exists

Column value tests can be added as both YAML and SQL. dbt natively has tests to assert that a column is not null, has unique values, only contains certain values, or that all values in a column are represented in another model (referential integrity).

We also use the dbt-utils package to add even more testing capabilities.

All Column Value Tests result in a PASS or FAIL status.

Column Value Test Example 1

Purpose: This test validates the account ID field in Zuora. This field is always 32 characters long and only has numbers and lowercase letters.

Because we use dbt, we have documentation for all of our source tables and most of our downstream modeled data. With in the yaml documentation files, we’re able to add tests to individual columns.

# File: https://gitlab.com/gitlab-data/analytics/-/blob/master/transform/snowflake-dbt/models/sources/zuora/sources.yml
    - name: account
    description: '{{ doc("zuora_account_source") }}'
    columns:
        - name: id
        description: Primary Key for Accounts
        tags: ["tdf","zuora"]
        tests:
            - dbt_utils.expression_is_true:
                expression: "id REGEXP '[0-9a-z]{32}'"

Rowcount Tests

The Rowcount test is a specialized type of Column Value test and is broken out because of its importance and utility. Rowcount tests determine if the number of rows in a table over a period of time meet expected pre-defined results. If data volumes change rapidly for legitimate reasons, rowcount tests will need to be updated appropriately.

Rowcount Test Example 1

Purpose: This test validates we always had 18,849 Zuora subscription records created in 2019.

This test is implemented as a dbt macro. This means that instead of writing SQL, a user can add the test by simply calling the macro. This is controlled by the source_rowcount macro.

-- https://gitlab.com/gitlab-data/data-tests/-/blob/main/tests/sources/zuora/rowcount/zuora_subscription_source_rowcount_2019.sql
{{ config({
    "tags": ["tdf","zuora"]
    })
}}

{{ source_rowcount(
    'zuora',
    'subscription',
    18489,
    "autorenew = 'TRUE' and createddate > '2019-01-01' and createddate < '2020-01-01'"
) }}

Rowcount Test Example 2

Purpose: We have a fast-growing business and should always have at least 50 and at most 200 new Subscriptions loaded from the previous day. This is controlled by the model_new_records_per_day macro.

-- https://gitlab.com/gitlab-data/data-tests/-/blob/main/tests/sources/zuora/rowcount/zuora_subscription_source_model_new_records_per_day.sql
{{ config({
    "tags": ["tdf","zuora"],
    "severity": "warn",
    })
}}

{{ model_new_rows_per_day(
    'zuora_subscription_source',
    'created_date',
    50,
    200,
    "date_trunc('day',created_date) >= '2020-01-01'"
) }}

Custom SQL

You may have a test in mind that doesn’t fit into any of the above categories. You can also write arbitrary SQL as a test. The key point to keep in mind is that the test is passing if no rows are returned. If any rows are returned from the query, then the test would fail.

An example of this from the dbt docs:

{{ config({
    "tags": ["tdf","fct_payments"]
    })
}}

-- Refunds have a negative amount, so the total amount should always be >= 0.
-- Therefore return records where this isn't true to make the test fail
SELECT
    order_id,
    sum(amount) AS total_amount
FROM {{ ref('fct_payments' )}}
GROUP BY 1
HAVING total_amount < 0

Any valid SQL can be written here and any dbt models or source tables can be referenced.

Merge Request Workflow

There are a few scenarios to think about when adding or updating tests.

The first scenario is modifying or adding a test in a YAML file within our main project. This follows our standard MR workflow and nothing is different. Run the CI jobs as you normally would.

The second scenario is adding any test or golden data records within the data-tests project against tables that are being updated or added via an MR in the analytics project. This is the most common scenario. In this case, no pipelines need to be executed in the data-tests project MR. The regular dbt pipelines in the analytics MR can be run and the only change is the branch name of the data-tests project needs to be passed to the job via the DATA_TEST_BRANCH environment variable.

The third scenario is when tests are being added to the data-tests project, but no golden data CSV files are being updated or added, AND there is no corresponding MR in the analytics project. In this scenario, you will see some CI jobs that will run your tests against production data. This is useful for ensuring minor changes (such as syntax, tags, etc.) work.

The fourth scenario is when golden data CSV files are being added or updated and there is no corresponding analytics MR. In this case, we do not want to test against production as the golden data files are inserted into the database as tables. In this scenario, you will see additional CI jobs. There are some to create a clone of the warehouse, some to run dbt models stored in the analytics project against the clone, and some to run the tests against the clone.

This flowchart should give a rough guide of what to do. Follow the instructions in the relevant MR template in the project for more detailed instructions.

graph TD
  A[Do you want to add YAML or SQL tests?] -->|YAML| B[Open an Analytics MR <br/>and proceed as usual]
  A -->|SQL| C[Open a Data Tests MR]
  A -->|Both| D[Open an Analytics and Data Tests MR]


  D --> E[Run dbt jobs as usual in analytics <br/>but provide the branch name of the <br/> Data Tests project as `DATA_TEST_BRANCH`]

  C --> F[Are you updating or adding<br/> a Golden Data CSV?]

  F -->|Yes| G[Run a clone - full or shallow - then run dbt jobs]
  F -->|No| H[Run dbt jobs against production]

In the case where you have a merge request in data-tests and one in analytics, the analytics MR should be set as a dependency of the data-tests MR. This means that the analytics MR must be merged prior the data-tests MR being merged.

Running the newly introduced dbt tests in the data-tests project

Steps to follow in order to run the tests you implemented in the data-tests project from your machine, while developing them:

1. Push your changes to the remote branch you are working on the data-tests project
2. Go to your analytics project locally, create a new branch (git checkout -b <branch_name>) with the same name as the one at data-tests & modify the Makefile to edit the DATA_TEST_BRANCH to match your branch name on the data-test project
3. From the analytics project run make run-dbt
4. You should see some logs, which also show the revision data-tests was installed from, where you should see your branch
5. From where you currently are (which should be the snowflake-dbt directory) run the corresponding command for testing your own model

Example

To run the zuora_revenue_revenue_contract_line_source rowcount tests, we can use the following command, which should work without any issues:

dbt --partial-parse test --models zuora_revenue_revenue_contract_line_source

⚠️ Please note, whenever you make changes to the underlying tests in the data-tests project, you need to push those changes to the remote and re-run steps 3-5, to start a dbt container with the latest changes from your branch.

Data extraction (RAW data layer)

Data extraction is loading data from the source system to Snowflake data warehouse in the RAW data layer.

Data transformation (Prod data layer)

Data transformation is downstream transformation via dbt for Dimensions, Facts, Marts and reports models.

Snapshots

dbt snapshots are

• SCD 2 tables, built on top of mutable (SCD1) source tables
• that record changes to the source table over time

One might assume that for each source table, there are multiple snapshot tables- one snapshot for each point in time- but that would be incorrect.

Instead, each source table will have only one corresponding snapshot table, and the snapshot table- due to it’s SCD2 nature- contains the entire history of the source table.

For more on snapshots, including examples, go to dbt docs.

Take note of how we talk about and define snapshots.

Create snapshot tables with dbt snapshot

Snapshot definitions are stored in the snapshots folder of our dbt project. We have organized the different snapshots by data source for easy discovery.

The following is an example of how we implement a snapshot:

{% snapshot sfdc_opportunity_snapshots %}

    {{
        config(
          unique_key='id',
          strategy='timestamp',
          updated_at='systemmodstamp',
        )
    }}

    SELECT *
    FROM {{ source('salesforce', 'opportunity') }}

{% endsnapshot %}

Key items to note:

• The database and schema are configured in dbt_project.yml. The database is an environmental variable while the schema is set to snapshots.
• Always select from a source table. Even if some deduplication is required, a source table must be selected from, as selecting from a downstream dbt model is prone to failure
• As snapshots are stored in RAW, your role will need to be explicitly granted access to the schema or tables
• Follow the naming convention {source_name}_{source_table_name}_snapshots for your file name
• Avoid any transformations in snapshots aside from deduplication efforts. Always clean data downstream
• Unless you don’t have a reliable updated_at field, always prefer using timestamp as a strategy (over check). Please find documentation about strategy here

Snapshot Model Types

While the DBT Snapshot tables are built directly over sources to capture changed records, tables are built over the snapshots to be used for analysis.

1. A DBT Snapshot model captures changed records for a single table.
   • The single table being snapshotted may be a Source table or a table already being used for analysis.
   • A Snapshot table is defined with {% snapshot table_name %} in the configuration section of the model.
   • The snapshot model has to be added to the sources.yml file to be recognized by other models. This is further described in the Make snapshots table available in prod database section below.
   • These tables have to be referenced as Source tables by other models because they are built in RAW. ie. {{ source(location,name)}} syntax.
2. DBT models built over snapshot tables include snapshot data and are used for historical analysis. Multiple snapshots may be joined and logic added to provide a historical view of fully built out derived dimensions.

DBT Snapshot Model:

The strategy to determine when a new snaphot record is written can be configured 2 different ways:

• timestamp uses a Date Column in the table to determine if that date has changed since the last snapshot was taken
• check uses a list of columns to determine if any of the columns have changed since the last time the snapshot was taken.

The record version is determined by the dbt_valid_from and dbt_valid_to columns. These TIMESTAMP columns are created automatically by DBT and utilized by the snapshot model to determine the timeframe of each snapshotted row.

• When a new snapshot record is written, dbt_valid_from has the current date time and. dbt_valid_to is NULL to show this is the most recent snapshot row.
• dbt_valid_to in the previous version of the record is updated with the same current date time as the new record.

DBT Snapshots, by default, are loaded incrementally. Records that have changed are picked up each time the snapshot operation runs.

DBT Model built over Snapshots:

• Building Models on top of snapshots is further defined below.
• DBT Snapshots are Source level data and are not queried directly for user analysis. These tables may include the dbt_valid_from and dbt_valid_to columns to identify the record version which comes from the Dbt Snapshot table.
• date spining is used to show the record value for any day in history. Rows are included for all dates between dbt_valid_from and dbt_valid_to. This allows for easy analysis and joins for particular dates.
• These models may be configured as incremental because the underlying dbt snapshots are appended to and not modified.
• A published snapshot model may need to be refreshed if the structure or logic is changed.

Snapshot Model Type Features

The different types of snapshots are determined by basic features used when building models with snapshot data.

Some basic features of Snapshot models are:

• DBT Snapshot - The DBT Snapshot model is built in RAW over a single table and is not available for analysis
• Over Snapshot - Model built directly over the DBT Snapshot model
• Spined Dates - Model built over one or more Snapshot models that includes Date Spining
• History Rebuild - Model built over one or more Snapshot models using the same logic used to build out the SCD Dimension. The history in these models can be rebuilt if the columns or logic changes.

Snapshot Model Type Examples:

• Snapshot Methods used for ARR Data can be found HERE
• Here are examples of snapshot models with the variation of features that help determine the type:

Altering Snapshot Tables within dbt snapshot

dbt does a great job of handling schema changes in snapshots, but given the breadth of our main project repository there is the possibility of collisions when we’re also changing source schemas in an extraction. Adding columns to extractions source schemas of snapshotted tables should be done in a prior and seperate merge request from the changes to the snapshot. dbt references the existing schema in the source tables (in RAW) when running snapshots and so the snapshots need to run at least once with the new schema present in the source data before changes can be made to the snapshots themselves with reference to these new columns.

Testing Snapshots

Testing of a snapshot can be done in a merge request using the specify_snapshot CI job. Engineers should test locally using Airflow, as the proper environment variables are handled based on the git branch. Testing should NOT be done while on the master branch. It is not recommended to test locally by setting the SNOWFLAKE_SNAPSHOT_DATABASE environment variable. This should never be set to RAW as it will overwrite production data.

Snapshots and GDPR

Sometimes the data team receives requests to delete personal data from the Snowflake Data Warehouse, because of GDPR. To address these deletions, we use dbt macros. A macro scans all applicable data that needs to be removed, this also applies to snapshot tables. These maros could be used manually following this issue template and are also scheduled to be run in Airflow following this runbook and handbook page.

There are 2 flavours:

1. The GDPR deletion request applies to all GitLab sources, and therefore all tables in the data warehouse need to be checked and updated. Macro. Use this macro if the issue that requesting a GDPR deletion states Deletion Request (Full) (in either the title or issue description).
2. The GDPR deletion request applies to only GitLab.com related sources and therefore only GitLab.com related tables in the data warehouse need to be checked and updated. Macro. Use this macro only if the issue that requesting a GDPR deletion states Deletion Request (GitLab.com Only (in either the title or issue description).

Specific to the second flavour, check when creating a new snapshot model or rename an existing snapshot model, if the dbt macro covers the models involved. Check if the filtering in the macro applies to the applicable snapshot tables in case of a GDPR deletions request for GitLab.com only related sources.

Make snapshots table available in prod database

Once a snapshot is taken, it becomes, and should be treated as, a data source.

We currently follow the legacy method for generating models based on snapshots. This means we don’t have source models. Base models for snapshots are available in the folder /models/snapshots of our dbt project. Key items to note:

• Before writing a snapshot base model, don’t forget to add it in the sources.yml file This entry is required for the snapshot to be recognized and used by other models. (keep this file sorted)
• The name of the table in the data warehouse should be consistent with our data warehouse design guideline. Ideally we would like to stick to {source_name}_{source_table_name}_snapshots as our naming convention. But dbt doesn’t allow duplicated file names in projects. In order to avoid this the snapshot and the snapshot base model having the same name, we follow this pattern:
  • The name of the base model file will be the name of the source snapshot table to which we suffix _base. Ex: we have a gitlab_dotcom_members_snapshots snapshot file here and a base model of this snapshot here named gitlab_dotcom_members_snapshots_base.
  • We use the dbt config alias argument to rename the table by removing the _base suffix and keep the table name clean
• If a base model built upon the snapshotted source table exists, please re-use the query that has been already written and apply the following modifications:
  • Remove the deduplication process, it is not necessary.
  • Always add dbt_scd_id as a primary key to your snapshot base model and rename it to something more explicit (documentation about snapshot meta-fields can be found here)
  • Add columns dbt_valid_from and dbt_valid_to to your query
  • Good example here with the snapshot base model and the source model

Building models on top of snapshots

In some cases there is a need to have a record per day, rather than a record per changed record with timeframe constraints dbt_valid_from and dbt_valid_to. In this case a technique called date spining can be used to create a model with daily snapshots.

In date spining, a snapshot model is joined to a date table based on dbt_valid_from and dbt_valid_to. A good example of daily snapshot model is dim_subscriptions_snapshots table where source model of zuora_subscription_snapshots is joined to dim_dates based on dbt_valid_from and dbt_valid_to. This join results with one record per subscription per day with subscription version that was active on given day (called snapshot_date).

Another possibility to generate daily records is using dbt utility function date_spine. We use this function currently to generate date details source model.

We also have a convenience macro create_snapshot_base that utilizes date_spine to generate model with daily records out of any snapshot table. For example implementation look at sfdc_opportunity_snapshots_base model.

Incremental models on top of snapshots

If you are using date spining to generate record for each day, consider materializing the model as incremental. This way only new records will be added based on the snapshot_date condition. For an example implementation look at the mart_arr_snapshots model

Testing Downstream Impact

To manually review the downstream impacts a change to a model may have use the asset and filed level lineage in Monte Carlo.

Dropping DBT Models

Models are dropped by removing files in your local IDE session and committing these changes to be run in the CI Pipes on the MR. Snowflake tables in Production have to be removed separately by a DE. This should be specified on the MR and communicated directly to the DE. Some tables may need to be retained as history, even though the Dbt Models are removed. Here is an Example MR that shows Models being deprecated with some of the tables being retained in the database.

Model Efficiency

A model’s efficiency is a measure of how well the model uses the Snowflake resources to produce the model. At this time, it is not a measure of the queries used as we have not found a way to procedurally and reliably quantify the actions taken within a query. The efficiency score of a model can be determined for each model invocation and is a based on three numbers determined from the queries executed by the model. The component numbers are intended to provide an insight into where to investigate when the overall number does not meet the intend targets. These scores can be aggregated to show the overall efficiency of a grouping of models, such as a run or day.

Method

For each model the queries executed are first filtered and aggregated. Only the specific query types of CREATE_VIEW, INSERT, DELETE, CREATE_TABLE_AS_SELECT, MERGE, CREATE_VIEW, SELECT, EXTERNAL_TABLE_REFRESH are considered and the query properties of bytes_scanned, bytes_spilled_to_remote_storage, bytes_spilled_to_local_storage, partitions_total, partitions_scanned are aggregated for calculation. Once aggregated the following metrics are calculated:

Local Storage Efficiency

\[E_l = min{\frac{s-S_l}s,0}\]

• Where \(S_l\) is the model Bytes Spilled to Local Storage
• Where \(E_l\) is the model Local Storage Efficiency
• Where \(s\) is the model Bytes Scanned

The metric is calculated as the model bytes scanned less the model bytes spilled to local storage divided by the model bytes scanned and limited to values between 0 and 1. This calculation allows for a number that is independent of other models but still comparable to other models.

Remote Storage Efficiency

\[E_r = min{\frac{s-S_r}s,0}\]

• Where \(S_r\) is the model Bytes Spilled to Remote Storage
• Where \(E_r\) is the model Remote Storage Efficiency
• Where \(s\) is the model Bytes Scanned

The metric is calculated as the model bytes scanned less the model bytes spilled to remote storage divided by the model bytes scanned and limited to values between 0 and 1. This calculation allows for a number that is independent of other models but still comparable to other models.

Partition Scan Efficiency

\[E_p = if\ p\ >\ 1\ then\ min{\frac{p-S_p}p,0}\ else\ 1\]

• Where \(S_p\) is the model Partitions Scanned
• Where \(E_p\) is the model Partition Scan Efficiency
• Where \(p\) is the model Total Partitions

If there is more than one model partition then the metric is calculated as the model total partitions less the model partitions scanned divided by the model total partitions and limited to values between zero and one, otherwise the metric value is set to one. This calculation allows for a number that is independent of other models but still comparable to other models. It is expected that most models will not be able to achieve a partitions scan efficiency value of one as some number of partitions will always need to be scanned, but efforts should be made to improve the metric as much as possible.

Efficiency Score

\[E = [(E_l * w_l) + (E_r * w_r) + (E_p * w_p)]*100\]

• Where \(E\) is the model Efficiency Score
• Where \(E_p\) is the model Partition Scan Efficiency
• Where \(E_r\) is the model Remote Storage Efficiency
• Where \(E_l\) is the model Local Storage Efficiency
• Where \(w_p\) is the model Partition Scan Efficiency weight
• Where \(w_r\) is the model Remote Storage Efficiency weight
• Where \(w_l\) is the model Local Storage Efficiency weight

The compound score is calculated as the weighted average of the Local Storage Efficiency, Remote Storage Efficiency, and Partition Scan Efficiency metrics. The metric weights are determined arbitrarily by the needs and focus of the business.

Usage

Model efficiency can be looked at for an individual model or for a collection of models. When looking at a collection of models, it is recommended that efficiency metrics and scores be weighted, for example by the total bytes scanned, across the models in the collection. When the metrics and score are below target values, each metric can indicate an area of exploration to improve the efficiency: Local Storage Efficiency and Remote Storage Efficiency indicate insufficient memory for the bytes processed by the model and low Partition Scan Efficiency indicates that the model may not be set up to prune partitions as part of the needed table scans.

Reporting

To report on the overall efficiency of dbt models for the organization the latest invocation of each model is used and weighted by the total bytes scanned. This method provides a view over time of the changes to the aggregate efficiency of all of the models and represents a lagging indicator of the effect of changes to models and new models added to the collection. Each metric and score are available in the reporting so that the cause of changes can be drilled into and areas of improvement can be identified.

Model Performance

Performance should be balanced between model execution times, which directly affect Snowflake Credit spend, and developer time, which impacts how long a developer is waiting during development, troubleshooting, and bug fixes. Generally, model execution should strike a balance between running as fast as possible and with as little cost as possible. This can be a difficult balance as some of the methods to improve run time also increase the cost, however there are methods and techniques that can be used to achieve both improved run time and decreased cost. As the number and complexity of models grow, general model run time performance criteria is needed. The overall performance goals for the data warehouse are that:

• All models should individually execute in less than 60 minutes.
• All models should be run on the smallest warehouse allowable

Categorization

To provide a basis for communication and criteria for applying methods of improvements, categorizations for model and table size have been developed as follows:

Model Run Time

Defined as the total run time of all queries associated with the execution of a model. Categorization Details

Model Size

Defined as the rows and bytes of the table produced by the model. Categorization Details

Model Efficiency

Defined as the total bytes spilled compared to the total bytes written of all the queries of a model. Categorization Details

Methods

When looking for opportunities to improve model performance, the overall impact of the effort should be kept in mind. For example, spending a lot of effort on a model with a run time classification of S and model efficiency classification of Poor is not likely to yield a significant improvement on overall dbt invocations. The methods provided in this section are meant to be a guide for performance improvements, they are not a strict set of rules that can be enforced without thought to their implication.

Model Materialization

The way a model is materialized will directly impact how long the model runs during execution. Some materializations will improve a model’s performance, but decrease the performance of downstream models and queries while others will improve performance at the cost of increased complexity. When possible, quantifiable criteria are used to guide the materialization recommendations, but it is more likely that the developer will have to use their best judgement to select the materialization.

• View models should be used when the transformations are light or the impact of the transformations are minor on the performance of downstream models.
• Ephemeral models should be used when a transformation is simple and needs to be used across multiple other models.
• Table models should be used when there is complex query logic that negatively impacts downstream queries if it were to be performed with each query.
• Incremental models should be used when the table is a size M or larger, the model run time is a size L or larger, and existing source data changes very little day to day.
  • Incremental models should be set to never full refresh if the table a XL size or larger, the source data does not update old records, and there is low risk of the schema of the source data changing.
  • Incremental models can use a temporary table or a view for the incremental load and may have different performance. Each option should be evaluated when creating a new incremental model. Further details can be found here.

Query Optimization

Query optimization involves looking at the way the query executes and making changes that will allow the model to run more efficiently. General guidance for optimizing query performance is as follows:

• Reduce spilling to local and remote storage by reducing the columns and rows that are used in the query
• Ensure that the order JOIN clauses are operating on as few rows as feasible
• Minimize fan out of rows and cartesian results in JOIN clauses

Clustering

The application of clustering, and automatic reclustering, will be very dependent on the situation and would typically be placed on the source tables in the lineage of the model where a performance increase is desired. Clustering should be considered in the following circumstances:

• Partition pruning is poor in the model build query.
• There are specific and consistent columns used for filtering and joining in queries against the source table.

Adjusting the Warehouse size

Adjusting the warehouse size for improved performance is not straight forward as there are many factors that go into how a query will perform. Warehouse size adjustment may be considered under the following circumstances:

• Increasing Warehouse Size
  • The model efficiency is regularly categorized as poor or very poor.
• Decreasing Warehouse Size
  • The model efficiency is good and the model run time is XS.

Other potential methods

• Search Optimization
  • https://community.snowflake.com/s/article/Search-Optimization-When-How-To-Use

System Level Optimization

There are a number of other potential system level optimizations that can be considered to improve overall performance.

• Invocation Threads
• Max Concurrent Query level
• Auto Scaling Policy
• Warehouse Max Clusters

Guidance for Checking Model Performance

Retrieve Model Performance Statistics

Using the model_build_performance runbook retrieve the model performance statistics.

Check model Run Time

If the model run time is less than L then the effort needed to improve the model is not likely to yield significant returns at this time and the model can be left as it is.

Check the Model Efficiency

A Model Efficiency worse than Acceptable, represents significant spillage thus reducing the performance because of the Remote and Local disk reads.

• Reduce the amount of data needed to be stored in memory during the query execution.
  • Reduce number of Columns used
  • Reduce number of Rows used
  • Check Filter order
  • Check Join order
  • Check for Redundant Joins

Check for incremental viability

Not all models need to be incremental due to the increased design complexity.

• If the model loads new data regularly, but most of the data does not change it may be a good candidate for an incremental model.
• If the source data can be filtered in a way to return new or changed records, such as an updated_date field, it may be a good candidate for an incremental model.
• If the model represents the current state of data that changes it is not a good candidate for an incremental model.
• If the model is based on a snapshot model, it is not a good candidate for an incremental model.

Check never full refresh viability

• Event stream data that does not change is a prime candidate for never full refreshing.
• Check data source guidelines from the handbook.

Check Warehouse Size viability

Increasing the warehouse size will not always mean an increase in performance and an improvement in cost.

• If refactoring has not improved the Model Efficiency, an increase to the Warehouse Size may be warranted.
• If Model Efficiency is Good but the model is reaching the timeout limit for the warehouse then an increase to the Warehouse Size may be warranted.

Upgrading dbt

See the runbook for instructions on how to independently and asyncronously upgrade dbt.

Staying up to date

Our policy is that we should always be on a version of dbt-core that does have critical support. Check the linked schedule to see planned releases and support windows. For minor releases that are released while we are still on a version with critical support, we will evaluate on a quarterly basis to determine whether the minor release warrants an update.

When a major release happens, we should upgrade to the new major version before the second minor release on the new major version. So for example, we should be on v2.0.0 before v.2.2.0 is released. The extra time allowance is in place to account for breaking changes between major versions.

Scheduling a dbt upgrade

dbt upgrades should take place on a Tuesday of a week in which there are no major worldwide holidays or Family and Friends days. This is to enable enough time for team members to correct any breaking changes that weren’t caught in testing without having to work through the weekend. In a worst case scenario, the upgrade can be rolled back on Wednesday so that normal operations can resume for the remainder of the week.

Snowflake Warehouse Sizing

The Snowflake warehouse sizing handbook page has guidelines on properly sizing dbt models.

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