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)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*
*willjdunn.com <http://willjdunn.com>*
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
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Thanks so much Tom!
*Will J. Dunn*
*willjdunn.com <http://willjdunn.com>*
On Wed, Jun 17, 2015 at 3:48 PM, Tom Lane <tgl@sss.pgh.pa.us> wrote:
Show quoted text
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 butthat 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