Re: Subqueries and the optimizer

I had the same problem (VERY SLOW when using a WHERE xxx IN (SELECT ...)), but fortunately, a join solved my problem.  However, the WHERE xxx IN (SELECT ...) syntax is often more natural.  I'm also waiting for 7.4.

-- Dean

Dmitri Bichko wrote on 2003-05-20 13:45:

I wish it were as easy as a join - the query is much simplified for the purpose of the example, in reality the subselect is more complicated and includes a GROUP BY (which, at least as far as I know, makes subqueries the only way of doing this).

Thanks anway, guess I'll wait for 7.4 with this (and just split them up into two queries for the time being),
Dmitri

-----Original Message-----
From: Dean Gibson (DB Administrator) [mailto:dba-sql@ultimeth.net]
Sent: Tuesday, May 20, 2003 2:28 PM
To: pgsql-general@postgresql.org
Subject: Re: [GENERAL] Subqueries and the optimizer


Try:

SELECT blast_id FROM genes, ll_out_mm WHERE blast_batch_id = 2 AND
genes.locus_id = ll_out_mm.locus_id;

Using more recent versions of PostgreSQL, you can also write:

SELECT blast_id FROM genes JOIN ll_out_mm USING ( locus_id) WHERE
blast_batch_id = 2;

-- Dean

Dmitri Bichko wrote on 2003-05-20 10:50:
>So, I have a table with an index:
>
>dev=# explain SELECT blast_id FROM genes WHERE blast_batch_id = 2 AND
>locus_id IN (1,2);
>                                              QUERY PLAN
>-----------------------------------------------------------------------
-
>------------------------------
> Index Scan using idx_genes_locus_id, idx_genes_locus_id on genes
>(cost=0.00..88.21 rows=14 width=4)
>   Index Cond: ((locus_id = 1) OR (locus_id = 2))
>   Filter: (blast_batch_id = 2)
>
>So far so good, but when I try it with a subquery:
>
>dev=# explain SELECT blast_id FROM genes WHERE blast_batch_id = 2 AND
>locus_id IN (SELECT locus_id FROM ll_out_mm);
>QUERY PLAN
>-----------------------------------------------------------------------
-
> Seq Scan on genes  (cost=0.00..21414353.48 rows=11003 width=4)
>   Filter: ((blast_batch_id = 2) AND (subplan))
>   SubPlan
>     ->  Seq Scan on ll_out_mm  (cost=0.00..1267.64 rows=59264 width=4)
>
>
>How can I nudge the optimizer in the direction of using the index in
the
>second case as well?  Or is it supposed to be doing this in this case.
>
>Thanks,
>Dmitri
>
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