Re: why select count(*) consumes wal logs - Mailing list pgsql-general

Ravi Krishna <srkrishna1@aol.com> writes:
> I loaded 133 million rows to a wide table (more than 100 cols) via COPY. The table has
> no index at this time. Since I am the only user I don't see any other activity.
> Now when I run select count(*) on the table where I just loaded data, it runs for ever,
> more than 10min and still running. Intrigued, I checked locks and saw nothing.  Then I noticed something
> strange.  When select count(*) runs, PG is writing to wal_logs, and that too a large amount. Why?

That represents setting the yes-this-row-is-committed hint bits on the
newly loaded rows.  The first access to any such row will set that bit,
whether it's a select or a VACUUM or whatever.

            regards, tom lane