Обсуждение: pg_class.reltuples VS pg_stat_all_tables.n_live_tup for estimation of table
Hello,
Does anyone which is a more accurate estimate of a table's live rows: pg_class.reltuples (http://www.postgresql.org/docs/current/static/catalog-pg-class.html) OR pg_stat_all_tables.n_live_tup (http://www.postgresql.org/docs/current/static/monitoring-stats.html#PG-STAT-ALL-TABLES-VIEW)? In my test database their values are not the same.
- Is pg_class.reltuples an estimation of live tuples only, or is it of all tuples (both live and dead)? I would guess it's live only but that is a guess
- In the database I am testing:
- pg_class.reltuples <> pg_stat_all_tables.n_live_tup
- pg_class.reltuples <> (pg_stat_all_tables.n_live_tup + pg_stat_all_tables.n_dead_tup)
- pg_class.reltuples <> pg_stat_all_tables.n_live_tup
I understand that pg_stat_all_tables is probably the more expensive view to query but I am already querying both so I only care which is more accurate.
Thanks!
Will
Will J. Dunn
William Dunn <dunnwjr@gmail.com> writes: > Does anyone which is a more accurate estimate of a table's live > rows: pg_class.reltuples ( > http://www.postgresql.org/docs/current/static/catalog-pg-class.html) > OR pg_stat_all_tables.n_live_tup ( > http://www.postgresql.org/docs/current/static/monitoring-stats.html#PG-STAT-ALL-TABLES-VIEW)? They're both inaccurate with different sources of inaccuracy. I dunno that you should assume that one is necessarily better than the other. > - Is pg_class.reltuples an estimation of live tuples only, or is it of > all tuples (both live and dead)? I would guess it's live only but that is a > guess Hm. I'm pretty sure the planner takes it as counting live tuples only, but it looks like VACUUM thinks it includes recently-dead-but-not-yet- removable tuples. We might need to do some adjustment there. regards, tom lane
Re: pg_class.reltuples VS pg_stat_all_tables.n_live_tup for estimation of table
От
William Dunn
Дата:
Thanks so much Tom!
Will J. Dunn
On Wed, Jun 17, 2015 at 3:48 PM, Tom Lane <tgl@sss.pgh.pa.us> wrote:
William Dunn <dunnwjr@gmail.com> writes:
> Does anyone which is a more accurate estimate of a table's live
> rows: pg_class.reltuples (
> http://www.postgresql.org/docs/current/static/catalog-pg-class.html)
> OR pg_stat_all_tables.n_live_tup (
> http://www.postgresql.org/docs/current/static/monitoring-stats.html#PG-STAT-ALL-TABLES-VIEW)?
They're both inaccurate with different sources of inaccuracy. I dunno
that you should assume that one is necessarily better than the other.
> - Is pg_class.reltuples an estimation of live tuples only, or is it of
> all tuples (both live and dead)? I would guess it's live only but that is a
> guess
Hm. I'm pretty sure the planner takes it as counting live tuples only,
but it looks like VACUUM thinks it includes recently-dead-but-not-yet-
removable tuples. We might need to do some adjustment there.
regards, tom lane