Returns the start_lsn column value for the specified capture instance from the cdc.change_tables system table. This value represents the low endpoint of the validity interval for the capture instance.
Returns 0x00000000000000000000 when the capture instance does not exist or when the caller is not authorized to access the change data associated with the capture instance.
This function is typically used to identify the low endpoint of the change data capture timeline associated with a capture instance. You can also use this function to validate that the endpoints of a query range fall within the capture instance timeline before requesting change data. It is important to perform such checks because the low endpoint of a capture instance changes when cleanup is performed on the change tables. If the time between requests for change data is significant, even a low endpoint that is set to the high endpoint of the previous change data request might lie outside the current timeline. For more information about the change data capture timeline, see Configuring Change Data Capture.
A. Returning the minimum LSN value for a specified capture instance
The following example returns the minimum LSN value for the capture instance HumanResources_Employee in the AdventureWorks2008R2 database.
USE AdventureWorks2008R2; GO SELECT sys.fn_cdc_get_min_lsn ('HumanResources_Employee')AS min_lsn;
B. Verifying the low endpoint of a query range
The following example uses the minimum LSN value returned by sys.fn_cdc_get_min_lsn to verify that the proposed low endpoint for a change data query is valid for the current timeline for the capture instance HumanResources_Employee. This example assumes that the previous high endpoint LSN for the capture instance was saved and is available to set the @save_to_lsn variable. For the purposes of this example, @save_to_lsn is set to 0x000000000000000000 to force the error-handling section to run.
USE AdventureWorks2008R2; GO DECLARE @min_lsn binary(10), @from_lsn binary(10),@save_to_lsn binary(10), @to_lsn binary(10); -- Sets @save_to_lsn to the previous high endpoint saved from the last change data request. SET @save_to_lsn = 0x000000000000000000; -- Sets the upper endpoint for the query range to the current maximum LSN. SET @to_lsn = sys.fn_cdc_get_max_lsn(); -- Sets the @min_lsn parameter to the current minimum LSN for the capture instance. SET @min_lsn = sys.fn_cdc_get_min_lsn ('HumanResources_Employee'); -- Sets the low endpoint for the query range to the LSN that follows the previous high endpoint. SET @from_lsn = sys.fn_cdc_increment_lsn(@save_to_lsn); -- Tests to verify the low endpoint is valid for the current capture instance. IF (@from_lsn < @min_lsn) BEGIN RAISERROR('Low endpoint of the request interval is invalid.', 16, -1); END ELSE -- Return the changes occurring within the query range. SELECT * FROM cdc.fn_cdc_get_all_changes_HumanResources_Employee(@from_lsn, @to_lsn, 'all'); GO