Re: How to influence the planner

On Fri, 31 Aug 2007, Tom Lane wrote:

> 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.

Changing to enable_seqscan = on does solve this problem, thanks
Is there some method of crafting a query that will assert my wishes to the planner

mda=# EXPLAIN ANALYZE select * from (select * from t1 where length(bar) = 0) a order by foo;
QUERY PLAN
------------------------------------------------------------------------
Index Scan using t1_pkey on t1 (cost=0.00..46698482.18
rows=60038 width=334) (actual time=4784.869..2317363.298 rows=32705
loops=1)
Filter: (length(bar) = 0)
Total runtime: 2317395.137 ms
(3 rows)

When is enable_seqscan = off appropriate

>
> regards, tom lane
>
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