Hi, > Andreas Brandl <ml@xxxxxxxxxxxxxx> writes: > > we're currently investigating a statistics issue on postgres. We > > have some tables which frequently show up with strange values for > > n_live_tup. If you compare those values with a count on that > > particular table, there is a mismatch of factor 10-30. This causes > > the planner to come up with very bad plans (we also have this issue > > on bigger table like the one below). > > The planner doesn't use n_live_tup; the only thing that that's used > for > is decisions about when to autovacuum/autoanalyze. So you have two > problems here not one. So, you're saying that having a mismatch between n_live_tup and the actual row count is not that much of a problem (besides it influences when to auto-vacuum/analyze), right? I'm just curious: where does the planner take the (approximate) row-count from? > Can you provide a test case for the n_live_tup drift? That is, > something that when done over and over causes n_live_tup to get > further > and further from reality? I'll try to implement a minimal piece of code showing this, although I'm not sure if this will work. Might there be a link between n_live_tup drifting and doing unnecessary (blind) updates, which do not change any information of a row? Thank you! Best regards Andreas -- Sent via pgsql-general mailing list (pgsql-general@xxxxxxxxxxxxxx) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-general
