From: Tom Lane <tgl@sss.pgh.pa.us>
To: Jorge Arevalo <jorgearevalo@libregis.org>
cc: David G Johnston <david.g.johnston@gmail.com>,
 pgsql-general@postgresql.org
Subject: Re: Query optimization
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Comments: In-reply-to Jorge Arevalo <jorgearevalo@libregis.org>
 message dated "Wed, 29 Oct 2014 19:06:03 +0100"
Date: Wed, 29 Oct 2014 14:53:18 -0400
Message-ID: <29612.1414608798@sss.pgh.pa.us>
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Sender: pgsql-general-owner@postgresql.org

Jorge Arevalo <jorgearevalo@libregis.org> writes:
> SELECT value1,value2,value3,value4,
> value5, hstore(ARRAY['field9', 'field10', 'field11', 'field12', 'field13',
> 'field14'], ARRAY[field9, field10, field11, field12, field13, field14) as
> metadata, value7, (select array((select row(f1, f2) from table2 p where
> p.f3 = field7))) as values_array FROM table1

The parentheses/brackets don't seem to match up real well here ...

> This is the result of EXPLAIN ANALYZE

>                                                                    QUERY
> PLAN
> -------------------------------------------------------------------------------------------------------------------------------------------------
>  Index Scan using table1_pkey on table1  (cost=67846.38..395773.45
> rows=8419127 width=88) (actual time=7122.704..22670.680 rows=8419127
> loops=1)
>    InitPlan 2 (returns $1)
>      ->  Result  (cost=67846.29..67846.29 rows=1 width=0) (actual
> time=7009.063..7009.065 rows=1 loops=1)
>            InitPlan 1 (returns $0)
>              ->  Seq Scan on table2 p  (cost=0.00..67846.29 rows=12689
> width=20) (actual time=14.971..5069.840 rows=2537787 loops=1)
>                    Filter: (f3 = field7)

Hm.  If I'm reading that right, you're building an array containing
2537787 entries, each of which is a composite datum containing two
columns of unmentioned datatypes.  I suspect a big chunk of your
runtime is going into manipulating that array -- PG is not terribly
efficient with big arrays containing variable-width values.

I'm also a bit confused as to why the planner is saying that the (SELECT
ARRAY(...)) bit is an InitPlan and not a SubPlan.  That implies that
"field7" in the innermost WHERE clause is not a reference to table1 but a
reference to table2.  Is that really what you meant?  IOW, are you sure
this query is performing the right calculation in the first place?

			regards, tom lane


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