Re: How to influence the planner

Richard Ray <rray(at)mstc(dot)state(dot)ms(dot)us> writes:
> On Fri, 31 Aug 2007, Michael Glaesemann wrote:
>> EXPLAIN ANALYZE will help you see what the planner is doing to produce the
>> results.

> mda=# EXPLAIN ANALYZE select * from t1 where length(bar) = 0 order by foo;
> QUERY PLAN
> ---------------------------------------------------------------
> Index Scan using t1_pkey on t1 (cost=0.00..46698478.18
> rows=60038 width=334) (actual time=4612.740..2349582.520 rows=32705
> loops=1)
> Filter: (length(bar) = 0)
> Total runtime: 2349614.258 ms
> (3 rows)

> mda=# EXPLAIN ANALYZE select * from t1 where length(bar) = 0;
> QUERY PLAN
> ---------------------------------------------------------------
> Seq Scan on t1 (cost=100000000.00..102020349.17 rows=60038
> width=334) (actual time=39.065..108645.233 rows=32705 loops=1)
> Filter: (length(bar) = 0)
> Total runtime: 108677.759 ms
> (3 rows)

The problem here is you've got enable_seqscan = off. Don't do that.
That will make it use an index if it possibly can, whether using one
is a good idea or not. In this case, since the useful condition on
length(bar) is not indexable, the best available index-using scan
uses the index to implement order by foo ... which is pointless here
in terms of saving runtime.

> I'm running PostgreSQL 8.1.0 on Fedora Core 6

Please update. There are a *lot* of bugs fixed in the 8.1.x series
since then.

regards, tom lane