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Date: Tue, 13 May 2014 21:57:10 +0100
Subject: Re: Constraint exclusion won't exclude parent table
From: Tim Kane <tim.kane@gmail.com>
To: Tom Lane <tgl@sss.pgh.pa.us>
CC: "pgsql-performance@postgresql.org" <pgsql-performance@postgresql.org>
Message-ID: <CF9840D4.7F03D%tim.kane@gmail.com>
Thread-Topic: [PERFORM] Constraint exclusion won't exclude parent table
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>=20
>=20
> Yeah, it's not expected that that's going to cost much.  I am suspicious
> that what you are looking at is mostly measurement overhead: during
> EXPLAIN ANALYZE, each plan node has to do two gettimeofday() calls per
> call, and there are lots of platforms where that is significant relative
> to the actual work done per node.
>=20
> You might try comparing the overall times for select count(*) from ...
> rather than EXPLAIN ANALYZE for these two cases.  If those times are
> much closer together than what you're getting from EXPLAIN ANALYZE,
> then you've got a machine with expensive gettimeofday() and you have
> to take your measurements with an appropriate quantum of salt.
>=20
> regards, tom lane

Interesting..=20

Direct query:
Time: 374336.514 ms

Indirect query:
Time: 387114.059 ms

Mystery solved. Thanks again Tom.

For what it=E2=80=99s worth: Linux 3.2.0-4-amd64  Debian 3.2.46-1+deb7u1 x86_64
>=20



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<html><head></head><body style=3D"word-wrap: break-word; -webkit-nbsp-mode: s=
pace; -webkit-line-break: after-white-space; color: rgb(0, 0, 0); font-size:=
 14px; font-family: Calibri, sans-serif;"><blockquote style=3D"margin:0 0 0 40=
px; border:none; padding:0px;"><div><br></div></blockquote><span id=3D"OLK_SRC=
_BODY_SECTION"><div><div><blockquote style=3D"margin:0 0 0 40px; border:none; =
padding:0px;"><div><br></div><div>Yeah, it's not expected that that's going =
to cost much.&nbsp;&nbsp;I am suspicious</div><div>that what you are looking=
 at is mostly measurement overhead: during</div><div>EXPLAIN ANALYZE, each p=
lan node has to do two gettimeofday() calls per</div><div>call, and there ar=
e lots of platforms where that is significant relative</div><div>to the actu=
al work done per node.</div><div><br></div><div>You might try comparing the =
overall times for select count(*) from ...</div><div>rather than EXPLAIN ANA=
LYZE for these two cases.&nbsp;&nbsp;If those times are</div><div>much close=
r together than what you're getting from EXPLAIN ANALYZE,</div><div>then you=
've got a machine with expensive gettimeofday() and you have</div><div>to ta=
ke your measurements with an appropriate quantum of salt.</div><div><br></di=
v><div><span class=3D"Apple-tab-span" style=3D"white-space:pre">	</span><span cl=
ass=3D"Apple-tab-span" style=3D"white-space:pre">	</span><span class=3D"Apple-tab-=
span" style=3D"white-space:pre">	</span>regards, tom lane</div></blockquote></=
div></div></span><div><br></div><div>Interesting..&nbsp;</div><div><br></div=
><div>Direct query:</div><div>Time: 374336.514 ms</div><div><br></div><div>I=
ndirect query:</div><div>Time: 387114.059 ms</div><div><br></div><div>Myster=
y solved. Thanks again Tom.</div><div><br></div><div>For what it&#8217;s wor=
th:&nbsp;Linux 3.2.0-4-amd64 &nbsp;Debian 3.2.46-1+deb7u1 x86_64</div><span =
id=3D"OLK_SRC_BODY_SECTION"><div><div><blockquote style=3D"margin:0 0 0 40px; bo=
rder:none; padding:0px;"><div><br></div></blockquote></div></div></span></bo=
dy></html>

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