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In-Reply-To: <201003311746.38192.luvar@plaintext.sk>
References: <201003311746.38192.luvar@plaintext.sk>
Date: Tue, 6 Apr 2010 12:45:59 -0600
Message-ID: <p2kdcc563d11004061145g928107c0gc73c2b860eee677@mail.gmail.com>
Subject: Re: Some question
From: Scott Marlowe <scott.marlowe@gmail.com>
To: luvar@plaintext.sk
Cc: pgsql-performance@postgresql.org
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2010/3/31 =C4=BDubom=C3=ADr Varga <luvar@plaintext.sk>:
> Hi, stright to my "problem":
> If I try to select constant 1 from table with two rows, it will be someth=
ing
> like this:
>
> explain
> SELECT * FROM t_route
> =C2=A0 =C2=A0 =C2=A0 =C2=A0WHERE t_route.route_type_fk =3D (SELECT id FRO=
M t_route_type WHERE type =3D 2)
> =C2=A0 =C2=A0 =C2=A0 =C2=A0limit 4;
>
> "Limit =C2=A0(cost=3D1.02..1.91 rows=3D4 width=3D2640)"
> " =C2=A0InitPlan"
> " =C2=A0 =C2=A0-> =C2=A0Seq Scan on t_route_type =C2=A0(cost=3D0.00..1.02=
 rows=3D1 width=3D8)"
> " =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0Filter: ("type" =3D 2)"
> " =C2=A0-> =C2=A0Seq Scan on t_route =C2=A0(cost=3D0.00..118115.25 rows=
=3D535090 width=3D2640)"
> " =C2=A0 =C2=A0 =C2=A0 =C2=A0Filter: (route_type_fk =3D $0)"
>

Looking at this it looks like you're using prepared queries, which
can't make as good of a decision as regular queries because the values
are opaque to the planner.

Can you provide us with the output of explain analyze of that query?