sys.fn_cdc_map_time_to_lsn (Transact-SQL)

Raksts
12/29/2022

Applies to: SQL Server

Returns the log sequence number (LSN) value from the start_lsn column in the cdc.lsn_time_mapping system table for the specified time. You can use this function to systematically map datetime ranges into the LSN-based range needed by the change data capture enumeration functions cdc.fn_cdc_get_all_changes_<capture_instance> and cdc.fn_cdc_get_net_changes_<capture_instance> to return data changes within that range.

Transact-SQL syntax conventions

Syntax

sys.fn_cdc_map_time_to_lsn ( '<relational_operator>', tracking_time )

<relational_operator> ::=
    { largest less than
    | largest less than or equal
    | smallest greater than
    | smallest greater than or equal
    }

Arguments

'<relational_operator>' { largest less than | largest less than or equal | smallest greater than | smallest greater than or equal }

Used to identify a distinct LSN value in within the cdc.lsn_time_mapping table with an associated tran_end_time that satisfies the relation when compared to the tracking_time value.

relational_operator is nvarchar(30).

tracking_time

Is the datetime value to match against. tracking_time is datetime.

Return type

binary(10)

Remarks

To understand how the sys.fn_cdc_map_time_to_lsn function can be used to map datetime ranges to LSN ranges, consider the following scenario.

Assume that a consumer wants to extract change data on a daily basis. That is, the consumer wants only changes for a given day up to and including midnight. The lower bound of the time range would be up to but not including midnight of the previous day. The upper bound would be up to and including midnight of the given day. The following example shows how the function sys.fn_cdc_map_time_to_lsn can be used to systematically map this time-based range into the LSN-based range needed by the change data capture enumeration functions to return all changes within that range.

DECLARE @begin_time DATETIME,
    @end_time DATETIME,
    @begin_lsn BINARY (10),
    @end_lsn BINARY (10);

SET @begin_time = '2007-01-01 12:00:00.000';
SET @end_time = '2007-01-02 12:00:00.000';

SELECT @begin_lsn = sys.fn_cdc_map_time_to_lsn('smallest greater than', @begin_time);

SELECT @end_lsn = sys.fn_cdc_map_time_to_lsn('largest less than or equal', @end_time);

SELECT *
FROM cdc.fn_cdc_get_net_changes_HR_Department(@begin_lsn, @end_lsn, 'all` `');

The relational operator smallest greater than is used to restrict changes to those that occurred after midnight on the previous day. If multiple entries with different LSN values share the tran_end_time value identified as the lower bound in the cdc.lsn_time_mapping table, the function will return the smallest LSN ensuring that all entries are included. For the upper bound, the relational operator largest less than or equal to is used to ensure that the range includes all entries for the day including those that have midnight as their tran_end_time value. If multiple entries with different LSN values share the tran_end_time value identified as the upper bound, the function will return the largest LSN ensuring that all entries are included.

Permissions

Requires membership in the public role.

Examples

The following example uses the sys.fn_cdc_map_time_to_lsn function to determine whether there are any rows in the cdc.lsn_time_mapping table with a tran_end_time value that is greater than or equal to midnight. This query can be used to determine, for example, whether the capture process has already processed the changes committed through midnight of the previous day, so that the extraction of change data for that day can proceed.

DECLARE @extraction_time DATETIME,
    @lsn BINARY (10);

SET @extraction_time = '2007-01-01 12:00:00.000';

SELECT @lsn = sys.fn_cdc_map_time_to_lsn('smallest greater than or equal', @extraction_time);

IF @lsn IS NOT NULL
BEGIN
    <some action>
END

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