Re: Replica and transaction

On Tue, 2025-02-18 at 18:59 +0200, kaido vaikla wrote:
> I found a statement:
>
> Long-running transactions can cause replication lag because the changes made by
> these transactions are not replicated until the transaction is committed.
> (https://www.percona.com/blog/replication-lag-in-postgresql/)
>
>
> It means:
> change made by transaction is not replicated until the transaction is committed?
>
> And for example
> time0: wal on primary
> time1: tx1 begin
> time2: tx2 begin
> time3: tx2 rollback
> time4: tx3 begin
> time5: tx3 commit
> time6: tx1 commit
> time7: walsender sends tx1, tx3 to replica, tx2 skipped because rollback
> ?

I'd say that the article is talking about streaming replication, and the
statement is manifestly wrong in the context of streaming replication.
All changes, no matter if they are committed or not, will be replicated
right away. They just don't become *visible* before the COMMIT is replayed.

With logical replication, uncommitted changes are not replicated unless
the subscription was created with "streaming" set to something else than
"off".

Yours,
Laurenz Albe

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