Re: pg_class.reltuples VS pg_stat_all_tables.n_live_tup for estimation of table

William Dunn <dunnwjr(at)gmail(dot)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