Re: Sequence last value is different among primary and secondary

On 15/11/2022 08:24, Kenny Bachman wrote:
> 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

--

Paul Smith Computer Services
Tel: 01484 855800
Vat No: GB 685 6987 53

Sign up for news & updates at http://www.pscs.co.uk/go/subscribe