Re: First query very slow. Solutions: memory, or settings, or SQL?

On Tuesday 21 July 2009 04:36:41 Phoenix Kiula wrote:
> On Mon, Jul 20, 2009 at 2:04 PM, Peter Eisentraut<peter_e(at)gmx(dot)net> wrote:
> > On Monday 20 July 2009 06:45:40 Phoenix Kiula wrote:
> >> explain analyze select * from sites where user_id = 'phoenix' order by
> >> id desc limit 10;
> >>
> >> QUERY PLAN
> >> ------------------------------------------------------------------------
> >>--- ----------------------------------------------------------- Limit
> >> (cost=344.85..344.87 rows=10 width=262) (actual
> >> time=5879.069..5879.167 rows=10 loops=1)
> >> -> Sort (cost=344.85..345.66 rows=323 width=262) (actual
> >> time=5879.060..5879.093 rows=10 loops=1)
> >> Sort Key: id
> >> -> Index Scan using new_idx_sites_userid on sites
> >> (cost=0.00..331.39 rows=323 width=262) (actual time=44.408..5867.557
> >> rows=2178 loops=1)
> >> Index Cond: ((user_id)::text = 'phoenix'::text)
> >> Total runtime: 5879.414 ms
> >> (6 rows)
> >
> > The row estimate for the index scan is off. Try reanalyzing, or increase
> > the statistics target.
>
> How did you tell that the row estimate is off? I mean which numbers?

Compare the rows=N figures for the estimate and the actual time.

> Also, my statistics are already set pretty high. On "USER_ID" they are
> at 100. Which columns should I increase the stats on, those in the
> WHERE clause or those in the SELECT bit? The USER_ID and the
> USER_KNOWN, which are in the WHERE clause, both have statistics of
> 100! And USER_KNOWN is just a binary value (0 or 1) so I wonder what
> purpose stats would serve?

Well then that should be OK.