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Limitations and known issues with Azure Synapse Link for SQL

  • Artikkeli

This article lists the limitations and known issues with Azure Synapse Link for SQL.

Limitations

The following sections list limitations for Azure Synapse Link for SQL.

Azure SQL Database and SQL Server 2022

  • Source tables must have primary keys.
  • Only a writeable, primary replica is supported as the data source for Azure Synapse Link for SQL.
  • The following data types aren't supported for primary keys in the source tables.
    • real
    • float
    • hierarchyid
    • sql_variant
    • timestamp
  • Source table row size can't exceed 7,500 bytes. For tables where variable-length columns are stored off-row, a 24-byte pointer is stored in the main record.
  • When source tables are being initially snapshotted, any source table data containing large object (LOB) data greater than 1 MB in size is not supported. These LOB data types include: varchar(max), nvarchar(max), varbinary(max). An error is thrown and data is not exported to Azure Synapse Analytics. Use the stored procedure sp_configure to increase the configured maximum value for max text repl size option, which defaults to 64 K. A configured value of -1 indicates no limit, other than the limit imposed by the data type.
  • Tables enabled for Azure Synapse Link for SQL can have a maximum of 1,020 columns (not 1,024).
  • While a database can have multiple links enabled, a given table can't belong to multiple links.
  • When a database owner doesn't have a mapped login, Azure Synapse Link for SQL runs into an error when enabling a link connection. User can set database owner to a valid user with the ALTER AUTHORIZATION command to fix this issue.
  • If the source table contains computed columns or columns with data types that dedicated SQL pools don't support, the columns aren't replicated. Unsupported columns include the following.
    • image
    • text
    • xml
    • timestamp
    • sql_variant
    • UDT
    • geometry
    • geography
  • A maximum of 5,000 tables can be added to a single link connection.
  • The following table data definition language (DDL) operations aren't allowed on source tables when they're enabled for Azure Synapse Link for SQL. All other DDL operations are allowed, but they aren't replicated to Azure Synapse Analytics.
    • Switch Partition
    • Add/Drop/Alter Column
    • Alter Primary Key
    • Drop/Truncate Table
    • Rename Table
  • If data definition language (DDL) + data manipulation language (DML) is executed in an explicit transaction (between BEGIN TRANSACTION and END TRANSACTION statements), replication for corresponding tables fails within the link connection.

    Note

    If a table is critical for transactional consistency at the link connection level, please review the state of the Azure Synapse Link table in the Monitoring tab.

  • Azure Synapse Link for SQL can't be enabled if any of the following features are in use for the source table.
    • Change Data Capture
    • Temporal history table
    • Always encrypted
    • In-memory tables
    • Columnstore index
    • Graph
  • System tables can't be replicated.
  • The security configuration from the source database will NOT be reflected in the target dedicated SQL pool.
  • Enabling Azure Synapse Link for SQL creates a new schema named changefeed. Don't use this schema, as it is reserved for system use.
  • Source tables with collations that are unsupported by dedicated SQL pools, such as UTF-8 and certain Japanese collations, can't be replicated. See supported collations in Synapse SQL pools.
    • Additionally, Azure Synapse Link for SQL does not support some Thai language collations:
      • Thai100CaseInsensitiveAccentInsensitiveKanaSensitive
      • Thai100CaseInsensitiveAccentSensitiveSupplementaryCharacters
      • Thai100CaseSensitiveAccentInsensitiveKanaSensitive
      • Thai100CaseSensitiveAccentInsensitiveKanaSensitiveWidthSensitiveSupplementaryCharacters
      • Thai100CaseSensitiveAccentSensitiveKanaSensitive
      • Thai100CaseSensitiveAccentSensitiveSupplementaryCharacters
      • ThaiCaseSensitiveAccentInsensitiveWidthSensitive
  • Single row updates (including off-page storage) of > 370 MB are not supported.
  • When Azure Synapse Link for SQL on Azure SQL Database or SQL Server 2022 is enabled, the aggressive log truncation feature of Accelerated Database Recovery (ADR) is automatically disabled. This is necessary because Azure Synapse Link for SQL accesses the database transaction log. This behavior is similar to changed data capture (CDC). Active transactions continue to hold the transaction log truncation until the transaction commits and Azure Synapse Link for SQL catches up, or transaction aborts. This might result in the transaction log filling up more than usual and should be monitored so that the transaction log does not fill.

Azure SQL Database only

  • Azure Synapse Link for SQL isn't supported on Free, Basic, or Standard tier with fewer than 100 DTUs.
  • Azure Synapse Link for SQL isn't supported on SQL Managed Instances.
  • Service principal isn't supported for authenticating to source Azure SQL DB, so when creating Azure SQL DB linked Service, choose SQL authentication, user-assigned managed identity (UAMI) or service assigned managed Identity (SAMI).
  • If the Azure SQL Database logical server has both a SAMI and UAMI configured, Azure Synapse Link uses SAMI.
  • Azure Synapse Link can't be enabled on the secondary database after a GeoDR failover, if the secondary database has a different name from the primary database.
  • If you enable Azure Synapse Link for SQL on your database as a Microsoft Entra user, Point-in-time restore (PITR) fails. PITR only works when you enable Azure Synapse Link for SQL on your database as a SQL user.
  • If you create a database as a Microsoft Entra user and enable Azure Synapse Link for SQL, a SQL authentication user (for example, even sysadmin role) won't be able to disable/make changes to Azure Synapse Link for SQL artifacts. However, another Microsoft Entra user is able to enable/disable Azure Synapse Link for SQL on the same database. Similarly, if you create a database as an SQL authentication user, enabling/disabling Azure Synapse Link for SQL as a Microsoft Entra user doesn't work.
  • Cross-tenant data replication is not supported where an Azure SQL Database and the Azure Synapse workspace are in separate tenants.

SQL Server 2022 only

  • Azure Synapse Link for SQL can't be enabled on databases that are transactional replication publishers or distributors.
  • With asynchronous replicas in an availability group, transactions must be written to all replicas before publishing to Azure Synapse Link for SQL.
  • Azure Synapse Link for SQL isn't supported on databases with database mirroring enabled.
  • Restoring an Azure Synapse Link for SQL-enabled database from on-premises to Azure SQL Managed Instance isn't supported.

Caution

Azure Synapse Link for SQL is not supported on databases that are also using Azure SQL Managed Instance Link. Caution that in these scenarios, when the managed instance transitions to read-write mode, you might encounter transaction log full issues.

Known issues

  • Applies To - Azure Synapse Link for Azure SQL Database and SQL Server 2022

  • Issue - When you delete an Azure Synapse Analytics workspace it is possible that running links might not be stopped, which causes the source database to think that the link is still operational and could lead to the transaction log to not be truncated, and fill.

  • Resolution - There are two possible resolutions to this situation:

    1. Stop any running links before deleting the Azure Synapse Analytics workspace.
    2. Manually clean up the link definition in the source database.
      1. Find the table_group_id that needs to be stopped using the following query. 
        SELECT table_group_id, workspace_id, synapse_workgroup_name
FROM [changefeed].[change_feed_table_groups]
WHERE synapse_workgroup_name = <synapse workspace name>;
      2. Drop each link identified using the following procedure. 
        EXEC sys.sp_change_feed_drop_table_group @table_group_id = <table_group_id>;
      3. Optionally, if you're disabling all of the table groups for a given database, you can also disable change feed on the database with the following command. 
        EXEC sys.sp_change_feed_disable_db;

Re-enable change feed on a table for which it was recently disabled will show an error

  • Applies To - Azure Synapse Link for Azure SQL Database and SQL Server 2022
  • This is an uncommon behavior.
  • Issue - When you try to enable a table that has been recently disabled with its metadata not yet been cleaned up and state marked as DISABLED, an error is thrown stating A table can only be enabled once among all table groups.
  • Resolution - Wait for sometime for the disabled table system procedure to complete and then try to re-enable the table again.
  • Applies To - Azure Synapse Link for Azure SQL Database and SQL Server 2022
  • Issue - For SQL databases enabled with Azure Synapse Link, when you use SSDT Import/Export and Extract/Deploy operations to import/setup a new database, the changefeed schema and user aren't excluded in the new database. However, the tables for the change feed are ignored by DacFX because they are marked as is_ms_shipped=1 in sys.objects, and those objects never included in SSDT Import/Export and Extract/Deploy operations. When enabling Azure Synapse Link on the imported/deployed database, the system stored procedure sys.sp_change_feed_enable_db fails if the changefeed user and schema already exist. This issue is encountered if you have created a user or schema named changefeed that is not related to Azure Synapse Link change feed capability.
  • Resolution -
    • Manually drop the empty changefeed schema and changefeed user. Then, Azure Synapse Link can be enabled successfully on the imported/deployed database.
    • If you have defined a custom schema or user named changefeed in your database that is not related to Azure Synapse Link, and you do not intend to use Azure Synapse Link for SQL, it is not necessary to drop your changefeed schema or user.
    • If you have defined a customer schema or user named changefeed in your database, currently, this database cannot participate in the Azure Synapse Link for SQL.

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