Table Diff Guide

SQLMesh's table diff tool allows you to compare the schema and data of two data objects. It supports comparison of a SQLMesh model across two environments or direct comparison of database tables or views.

It provides a method of validating models that can be used along with evaluating a model and testing a model with unit tests.

Note: Table diff requires the two objects to already exist in your project's underlying database or engine. If comparing models, this means you should have already planned and applied your changes to an environment.

Table diff comparisons

Table diff executes two types of comparison on the source and target objects: a schema diff and a row diff.

The schema diff identifies whether fields have been added, removed, or changed data types in the target object relative to the source object.

The row diff identifies changes in data values across columns with the same name and data type in both tables. It does this by performing an OUTER JOIN of the two tables then, for each column with the same name and data type, comparing data values from one table to those from the other.

The table diff tool can be called in two ways: comparison of a SQLMesh model across two project environments or direct comparison of tables/views. It executes the comparison using the database or engine specified in the SQLMesh project configuration.

Diffing models across environments

Compare a SQLMesh model across environments with the SQLMesh CLI interface by using the command sqlmesh table_diff [source environment]:[target environment] [model name].

For example, we could make two modifications to the SQLMesh quickstart model sqlmesh_example.incremental_model:

1. Change the row whose item_id is 3 to 4 with a CASE WHEN statement
2. Remove row whose item_id is 1 by adding a WHERE clause

MODEL (
    name sqlmesh_example.incremental_model,
    kind INCREMENTAL_BY_TIME_RANGE (
        time_column event_date
    ),
    start '2020-01-01',
    cron '@daily',
    grain (id, event_date)
);

SELECT
    id,
    CASE WHEN item_id = 3 THEN 4 ELSE item_id END as item_id, -- Change item_id 3 to 4
    event_date,
FROM
    sqlmesh_example.seed_model
WHERE
    event_date between @start_ds and @end_ds
    AND id != 1 -- Remove row whose item_id is 1

After running sqlmesh plan dev and applying the plan, the updated model will be present in the dev environment but not in prod.

Compare the two versions of the model with the table diff tool by running sqlmesh table_diff prod:dev sqlmesh_example.incremental_model.

The first argument prod:dev specifies that prod is the source environment to which we will compare the target environment dev. The second argument sqlmesh_example.incremental_model is the name of the model to compare across the prod and dev environments.

Because the grain is set to [id, ds] in the MODEL statement, SQLMesh knows how to perform the join between the two models. If grain were not set, the command would need to include the -o id -o ds option to specify that the tables should be joined on column id and ds. Specify -o once for each join column.

Table diff returns this output:

$ sqlmesh table_diff prod:dev sqlmesh_example.incremental_model

Schema Diff Between 'PROD' and 'DEV' environments for model 'sqlmesh_example.incremental_model':
└── Schemas match


Row Counts:
├──  COMMON: 6 rows
├──  PROD ONLY: 1 rows
└──  DEV ONLY: 0 rows

COMMON ROWS column comparison stats:
         pct_match
item_id       83.3

The "Schema Diff" section shows that the PROD and DEV schemas match because no columns have been added, removed, or change data type.

The "Row Counts" section shows that 6 rows were successfully joined and the 1 row we removed is only present in the PROD model.

The COMMON ROWS column comparison stats section shows that the item_id column values had an 83.3% match for the six joined rows (5 of the 6 row values were unchanged by our CASE WHEN statement). All non-join columns with the same data type in both tables are included in the comparison stats.

If we include the --show-sample option in the command, the output also includes rows from the different join components.

$ sqlmesh table_diff prod:dev sqlmesh_example.incremental_model --show-sample

Schema Diff Between 'PROD' and 'DEV' environments for model 'sqlmesh_example.incremental_model':
└── Schemas match


Row Counts:
├──  COMMON: 6 rows
├──  PROD ONLY: 1 rows
└──  DEV ONLY: 0 rows

COMMON ROWS column comparison stats:
         pct_match
item_id       83.3


COMMON ROWS sample data differences:
 id         ds  PROD__item_id  DEV__item_id
  3 2020-01-03              3           4.0


PROD ONLY sample rows:
 id         ds  item_id
  1 2020-01-01        2

The COMMON ROWS sample data differences section displays the row whose item_id value changed. The PROD__item_id column shows that item_id is 3 in the PROD table, and the DEV__item_id column shows that item_id is 4.0 in the DEV table.

The PROD ONLY sample rows section shows the one row that is present in PROD but not in DEV.

Diffing tables or views

Compare specific tables or views with the SQLMesh CLI interface by using the command sqlmesh table_diff [source table]:[target table].

The source and target tables should be fully qualified with catalog or schema names such that a SQL query of the form SELECT ... FROM [source table] would execute correctly.

Recall that SQLMesh models are accessible via views in the database. In the prod environment, the view has the same name as the model. For example, in the quickstart example project the prod incremental model is represented by the view sqlmesh_example.incremental_model. In the dev environment, __dev is appended to the schema name so the incremental model is represented by the view sqlmesh_example__dev.incremental_model.

We can replicate the comparison in the previous section by comparing the model views directly. Because we are passing the view names directly, the command needs to manually specify that the join should be on the id and ds columns with the -o id -o ds flags.

$ sqlmesh table_diff sqlmesh_example.incremental_model:sqlmesh_example__dev.incremental_model -o id -o ds --show-sample

Schema Diff Between 'SQLMESH_EXAMPLE.INCREMENTAL_MODEL' and 'SQLMESH_EXAMPLE__DEV.INCREMENTAL_MODEL':
└── Schemas match


Row Counts:
├──  COMMON: 6 rows
├──  SQLMESH_EXAMPLE.INCREMENTAL_MODEL ONLY: 1 rows
└──  SQLMESH_EXAMPLE__DEV.INCREMENTAL_MODEL ONLY: 0 rows

COMMON ROWS column comparison stats:
         pct_match
item_id       83.3


COMMON ROWS sample data differences:
 id         ds  s__item_id  t__item_id
  3 2020-01-03           3         4.0


SQLMESH_EXAMPLE.INCREMENTAL_MODEL ONLY sample rows:
 id         ds  item_id
  1 2020-01-01        2

The output matches, with the exception of the column labels in the COMMON ROWS sample data differences. The underlying table for each column is indicated by s__ for "source" table (first table in the command's colon operator :) and t__ for "target" table (second table in the command's colon operator :).