On Thu, 28 Aug 2003, Michael Guerin wrote:
> Stephan Szabo wrote:
>
> >On Thu, 28 Aug 2003, Michael Guerin wrote:
> >
> >
> >
> >>Stephan Szabo wrote:
> >>
> >>
> >>
> >>>On Wed, 27 Aug 2003, Michael Guerin wrote:
> >>>
> >>>
> >>>
> >>>
> >>>
> >>>>I'm running into some performance problems trying to execute simple
> >>>>queries.
> >>>>
> >>>>postgresql version 7.3.3
> >>>>.conf params changed from defaults.
> >>>>shared_buffers = 64000
> >>>>sort_mem = 64000
> >>>>fsync = false
> >>>>effective_cache_size = 400000
> >>>>
> >>>>ex. query: select * from x where id in (select id from y);
> >>>>
> >>>>There's an index on each table for id. SQL Server takes <1s to return,
> >>>>postgresql doesn't return at all, neither does explain analyze.
> >>>>
> >>>>
> >>>>
> >>>>
> >>>IN(subquery) is known to run poorly in 7.3.x and earlier. 7.4 is
> >>>generally much better (for reasonably sized subqueries) but in earlier
> >>>versions you'll probably want to convert into an EXISTS or join form.
> >>>
> >>>
> >>>
> >>>
> >>>
> >>>
> >>Something else seems to be going on, even switching to an exists clause
> >>gives much better but poor performance.
> >>count(*) where exists clause: Postgresql 19s, SQL Server <1s
> >>count(*) where not exists: 23.3s SQL Server 1.5s
> >>
> >>
> >
> >What does explain analyze show for the two queries?
> >
> >
> >
> >
> explain analyze select count(*) from tbltimeseries where exists(select
> uniqid from tblobjectname where timeseriesid = uniqid);
> Aggregate (cost=5681552.18..5681552.18 rows=1 width=0) (actual
> time=22756.64..22756.64 rows=1 loops=1)
> -> Seq Scan on tbltimeseries (cost=0.00..5680051.34 rows=600336
> width=0) (actual time=22.06..21686.78 rows=1200113 loops=1)
> Filter: (NOT (subplan))
> SubPlan
> -> Index Scan using idx_objectname on tblobjectname
> (cost=0.00..4.70 rows=1 width=4) (actual time=0.01..0.01 rows=0
> loops=1200673)
> Index Cond: ($0 = uniqid)
> Total runtime: 22756.83 msec
> (7 rows)
Hmm... I'd thought that it had options for a better plan than that.
What do things like:
explain analyze select count(distinct timeseriesid) from tbltimeseries,
tblobjectname where timeseriesid=uniquid;
and
explain analyze select count(distinct timeseriesid) from
tbltimeseries left outer join tblobjectname on (timeseriesid=uniqid)
where uniqid is null;
give you?
