En un mensaje anterior, Tom Lane escribió:
> I wrote:
> > Weird. I assume that your 'activa' field is 'bool'? I've been trying
> > to duplicate this misbehavior here, and as near as I can tell the system
> > handles selectivity estimates for boolean fields just fine. Whatever
> > percentage of 't' values was seen by the last VACUUM ANALYZE is exactly
> > what it uses.
>
> On second thought: 6.5.* can get confused if the column contains more
> NULLs than anything else. Dunno if you have a lot of nulls in activa,
> but if so you might try changing them all to explicit 'f' and then
> redoing the VACUUM ANALYZE. Next release will be smarter about keeping
> stats in the presence of many nulls.
>
> It'd be useful to double-check my theory that the system is
> misestimating the selectivity of the WHERE (u.activa) clause.
> You could try this:
> SELECT count(*) FROM usarios WHERE activa;
10571
> EXPLAIN SELECT count(*) FROM usarios WHERE activa;
> and see how far off the row count estimate in the EXPLAIN is
> from reality.
NOTICE: QUERY PLAN:
Aggregate (cost=498.84 rows=1 width=4) -> Seq Scan on usuarios (cost=498.84 rows=1 width=4)
EXPLAIN
Don't hesitate in asking any other info/test you may consider useful.
Regards!
Fernando P. Schapachnik
Administración de la red
VIA Net Works Argentina SA
Diagonal Roque Sáenz Peña 971, 4º y 5º piso.
1035 - Capital Federal, Argentina.
(54-11) 4323-3333
http://www.via-net-works.net.ar
