> On 06 Sep 2017, at 09:45, Haribabu Kommi <kommi.harib...@gmail.com> wrote: > > On Tue, Jul 25, 2017 at 9:33 PM, Tomas Vondra <tomas.von...@2ndquadrant.com > <mailto:tomas.von...@2ndquadrant.com>> wrote: > On 7/25/17 12:55 AM, Tom Lane wrote: > Tomas Vondra <tomas.von...@2ndquadrant.com > <mailto:tomas.von...@2ndquadrant.com>> writes: > It seems to me that VACUUM and ANALYZE somewhat disagree on what > exactly reltuples means. VACUUM seems to be thinking that reltuples > = live + dead while ANALYZE apparently believes that reltuples = > live > > The question is - which of the reltuples definitions is the right > one? I've always assumed that "reltuples = live + dead" but perhaps > not? > > I think the planner basically assumes that reltuples is the live > tuple count, so maybe we'd better change VACUUM to get in step. > > Attached is a patch that (I think) does just that. The disagreement was > caused by VACUUM treating recently dead tuples as live, while ANALYZE treats > both of those as dead. > > At first I was worried that this will negatively affect plans in the > long-running transaction, as it will get underestimates (due to reltuples not > including rows it can see). But that's a problem we already have anyway, you > just need to run ANALYZE in the other session. > > Thanks for the patch. > From the mail, I understand that this patch tries to improve the > reltuples value update in the catalog table by the vacuum command > to consider the proper visible tuples similar like analyze command. > > - > num_tuples); > + > num_tuples - nkeep); > > With the above correction, there is a problem in reporting the number > of live tuples to the stats. > > postgres=# select reltuples, n_live_tup, n_dead_tup > from pg_stat_user_tables join pg_class using (relname) > where relname = 't'; > reltuples | n_live_tup | n_dead_tup > -----------+------------+------------ > 899818 | 799636 | 100182 > (1 row) > > > The live tuples data value is again decremented with dead tuples > value before sending them to stats in function lazy_vacuum_rel(), > > /* report results to the stats collector, too */ > new_live_tuples = new_rel_tuples - vacrelstats->new_dead_tuples; > > The fix needs a correction here also. Or change the correction in > lazy_vacuum_rel() function itself before updating catalog table similar > like stats.
This patch is marked Waiting for Author, have you had a chance to look at this to address the comments in the above review? cheers ./daniel -- Sent via pgsql-hackers mailing list (pgsql-hackers@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-hackers
