Once the identity property is set on a column, it can't be removed. Setting the seed or increment to a value other than 1 results in the following error: The use of seed and increment values other than 1 is not supported with memory optimized tables. In memory-optimized tables, the seed and increment must be set to 1, 1. Only one identity column can be created per table. For more information, see Replicate Identity Columns. If a table with an identity column is published for replication, the identity column must be managed in a way that is appropriate for the type of replication used. If you can't use identity values because of these restrictions, create a separate table holding a current value and manage access to the table and number assignment with your application. These restrictions are part of the design in order to improve performance, and because they're acceptable in many common situations. This can result in gaps when the subsequent identity values are generated. If a particular insert statement fails, or if the insert statement is rolled back then the consumed identity values are lost and aren't generated again. Reuse of values - For a given identity property with specific seed/increment, the identity values aren't reused by the engine. Using a sequence generator with the NOCACHE option can limit the gaps to transactions that are never committed. If gaps aren't acceptable, then the application should use its own mechanism to generate key values. This can result in gaps in the identity value upon insert. If values must be consecutive, then the transaction should use an exclusive lock on the table or use the SERIALIZABLE isolation level.Ĭonsecutive values after server restart or other failures - SQL Server might cache identity values for performance reasons and some of the assigned values can be lost during a database failure or server restart. For more information, see Using IDENTITY to create surrogate keys in a Synapse SQL pool.Ĭonsecutive values within a transaction - A transaction inserting multiple rows isn't guaranteed to get consecutive values for the rows because other concurrent inserts might occur on the table. Azure Synapse Analytics doesn't support PRIMARY KEY or UNIQUE constraint or UNIQUE index.
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