Re: Sequence last value is different among primary and secondary

Hello Paul,

Yes, you are right, it makes sense.
Thank you so much for the explanation.

Best,

Paul Smith <paul(at)pscs(dot)co(dot)uk>, 15 Kas 2022 Sal, 11:54 tarihinde şunu yazdı:

> 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
>
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>
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