Re: Sequence last value is different among primary and secondary

Hello Team,

I got a weird situation about the sequence value in PG 14.5 (rhel 8.6). The sequence's last value is different between primary and secondary nodes.

This sequence output belongs to the PRIMARY node:

SELECT * FROM "ACCOUNT_ITEM_SEQ" ;
 last_value | log_cnt | is_called
------------+---------+-----------
     105485 |      31 | t

This output belongs to the SECONDARY node:

SELECT * FROM "ACCOUNT_ITEM_SEQ" ;
 last_value | log_cnt | is_called
------------+---------+-----------
     105516 |       0 | t

There is no replication gap or lag between the nodes.

What is the reason behind this difference?

I believe this is to do with the way PostgreSQL does sequences. It doesn't update the WAL every time a sequence increments, just every 32 times to avoid writing lots of unnecessary entries given that the sequence generally updates in a predictable manner. The 'log_cnt' keeps track of how many increments are to go before the next WAL update.

I guess the secondary is just updated with the next 'step', so that if there's a failover, the sequence number is at a 'safe' position without causing duplicates. (note that the primary 'last_value + log_cnt' = secondary 'last_value')

Eg, see https://stackoverflow.com/questions/66456952/what-does-log-cnt-mean-in-the-postgres-sequence

Paul