Dbt is a powerful library used for data transformation and analysis. It supports various data warehouse solutions through different dbt adapters. Each adapter is designed to handle the unique syntax, functions, and configurations of specific data warehouses.
The error
In this article, we will discuss a common error that occurs when using dbt with Azure Synapse Dedicated Pool. The error message looks like this:
18:11:08 ('42000', '[42000] [Microsoft][ODBC Driver 17 for SQL Server][SQL Server]Merge statements with a WHEN NOT MATCHED [BY TARGET] clause must target a hash distributed table.
The error is encountered while taking a snapshot of a table using the dbt-synapse adapter.
Reproducing the Error
Let’s understand how this error occurs. Imagine we have a model called model_gamma, which is a table containing columns like id and modified_date. To keep track of updates to this table, we create a snapshot named snapshot_gamma.
snapshot_gamma Initially, when we run the dbt snapshot command, it creates the first snapshot successfully. However, the error arises when trying to take another snapshot of model_gamma. This is because dbt attempts to modify the snapshot_gamma table, leading to the error.
The Cause
The error is related to the use of the SQL MERGE command under the hood when taking snapshots with Azure Synapse. Specifically, the error occurs when using the MERGE command with a WHEN NOT MATCHED BY TARGET clause.
snapshot_gamma, located in the project's target folder The MERGE command performs a full table scan and combines INSERT, UPDATE, and DELETE statements. The error suggests that if the WHEN NOT MATCHED BY TARGET clause is used, the target table (snapshot_gamma) must be hash-distributed.
Round-Robin vs Hash
Azure Synapse supports different distribution styles like Round-Robin and Hash. These are different approaches of storing data in the data warehouse.
The Round-Robin approach ensures data is evenly distributed across nodes. Each node gets an equal share of data, but data points with similar values might not end up in the same node.
The Hash approach uses a hash function (sort of a rule of thumb) to determine which node to save a data point in. Data points with equal values go into the same node, improving data locality and potentially benefiting certain queries like the MERGE statement.
The Solution
To fix the error, we need to change the distribution style of the snapshot_gamma table to hash-distributed. We can pass specific configurations to our models and snapshots on Azure Sypanse thanks to dbt-synapse.
First, delete the original snapshot_gamma table:
drop table snapshot.snapshot_gamma;
Then, modify the snapshot file to indicate the desired distribution approach. That is, a HASH distribution along the id column:
snapshot_gamma, after specifying a Hash distribution After making these changes, run the dbt snapshot command again to create the first snapshot table.
snapshot_gamma from scratch
Finally, confirm that the snapshot works without errors when taken afterward.
snapshot_gamma a second time
By understanding the error and the differences between Round-Robin and Hash distribution in Azure Synapse, we can effectively fix the issue and ensure smooth data processing with dbt. Happy querying.
