I think the most important thing to understand SQL Server CDC is to realized the LSN value in the CDC tables are the commit LSN. Than, it is more easy to understand the the LSN is mapping to the tran_end_time in the cdc.lsn_time_mapping table.
select date('1970-01-01') - date('1969-01-01')
select ABS((EXTRACT(EPOCH FROM '1969-01-01'::TIMESTAMP) - EXTRACT(EPOCH FROM '1970-01-01'::TIMESTAMP)) / 86400)
Microsoft SQL Server automatically generate table valued functions for the change instance when CDC tracking is enabled on tables.
Unfortunately, these functions may report the misleading error message:
An insufficient number of arguments were supplied for the procedure or function cdc.fn_cdc_get_net_changes_ ...
The real meaning of the error message is:
- one or more arguments are null;
- the LSN is outside the range of min(start_lsn) and the max(start_lsn) in the cdc.lsn_time_mapping table; or
- the from_lsn is less than the start_lsn of the capture_instance listed in the system table cdc.change_tables;
- the from_lsn is greather than the to_lsn.
In short the lsn values specifying the range must be existing in the cdc.lsn_time_mapping table.
But, I am using the system function sys.fn_cdc_map_time_to_lsn to map the time to lsn. How come I ended up with a lsn does not exist in the cdc.lsn_time_mapping table? Yes, it will happen. The relational operator “smallest greater than” and “smallest greater than or equal” map the time to a lsn greater the the given time. For example, the max time in the cdc.lsn_time_mapping table is 2017-01-01, the return value will be null if the givn time is later than “2017-01-01”.
It is confusing.
select used_bytes/pow(1024,3) as used_gb, *
where tablename = ‘my_table_name’
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