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From: John Scalia <jayknowsunix@gmail.com>
Mime-Version: 1.0 (1.0)
Subject: Re: Explain plan on a Select query
Date: Mon, 16 Mar 2020 20:38:45 -0400
Message-Id: <62998F18-D770-4521-9CE0-C11A565825FE@gmail.com>
References: 
 <CAMkU=1yQ11TNr9mmUiaW6SkmSL92JCLrC6XZ1AFGD97oyQXvPg@mail.gmail.com>
Cc: Pgsql-admin <pgsql-admin@postgresql.org>
In-Reply-To: 
 <CAMkU=1yQ11TNr9mmUiaW6SkmSL92JCLrC6XZ1AFGD97oyQXvPg@mail.gmail.com>
To: Jeff Janes <jeff.janes@gmail.com>
Precedence: bulk


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Well, after rebuilding the statistics on the table at least three separate t=
imes, the errant messages finally disappeared. BTW, this was on an AWS RDS i=
nstance, which I know is not the favorite of any development team member. It=
 just threw me a bit, as the first analyze did not clear the issue.

Sent from my iPad

> On Mar 16, 2020, at 6:43 PM, Jeff Janes <jeff.janes@gmail.com> wrote:
>=20
> =EF=BB=BF
>> On Mon, Mar 16, 2020 at 12:26 PM John Scalia <jayknowsunix@gmail.com> wro=
te:
>=20
>> I=E2=80=99m a bit confused on an explain plan I just generated for a sele=
ct statement. The plan=E2=80=99s first line begins with the word =E2=80=9CAp=
pend=E2=80=9D followed by the usual (cost....) data. What in the world is an=
 Append doing here? I=E2=80=99ve recently performed  another analyze on the t=
able, btw.=20
>=20
> A top level Append would generally be used when you use a UNION ALL, or yo=
u have inheritance.
> =20
>>=20
>> Also, the query does not seem to be using an index, as the explain shows a=
 pair of sequential scans, speaking of which, the second sequential scan sho=
ws it=E2=80=99s on a table, called <table>_test. Which is a little strange, c=
onsidering that does not exist.
>=20
> Is that the name of the table, or the alias for that table?
>=20
>   "->  Seq Scan on pgbench_accounts pa_alias  (cost=3D0.00..527869.00 rows=
=3D20000000 width=3D97)"
>=20
> "pgbench_accounts" is the actual table, "pa_alias" is the alias for it.=20=

>=20
>=20
> PostgreSQL will create alias for table when it needs to, but I've never se=
en it do so by adding "_test" to the end.  Usually it adds "_" and an intege=
r.
>=20
> It would really help to be able to see either the plan or the query, or bo=
th.
>=20
> Cheers,
>=20
> Jeff

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<html><head><meta http-equiv=3D"content-type" content=3D"text/html; charset=3D=
utf-8"></head><body dir=3D"auto">Well, after rebuilding the statistics on th=
e table at least three separate times, the errant messages finally disappear=
ed. BTW, this was on an AWS RDS instance, which I know is not the favorite o=
f any development team member. It just threw me a bit, as the first analyze d=
id not clear the issue.<br><br><div dir=3D"ltr">Sent from my iPad</div><div d=
ir=3D"ltr"><br><blockquote type=3D"cite">On Mar 16, 2020, at 6:43 PM, Jeff J=
anes &lt;jeff.janes@gmail.com&gt; wrote:<br><br></blockquote></div><blockquo=
te type=3D"cite"><div dir=3D"ltr">=EF=BB=BF<div dir=3D"ltr"><div dir=3D"ltr"=
>On Mon, Mar 16, 2020 at 12:26 PM John Scalia &lt;<a href=3D"mailto:jayknows=
unix@gmail.com">jayknowsunix@gmail.com</a>&gt; wrote:<br></div><div class=3D=
"gmail_quote"><blockquote class=3D"gmail_quote" style=3D"margin:0px 0px 0px 0=
.8ex;border-left:1px solid rgb(204,204,204);padding-left:1ex">I=E2=80=99m a b=
it confused on an explain plan I just generated for a select statement. The p=
lan=E2=80=99s first line begins with the word =E2=80=9CAppend=E2=80=9D follo=
wed by the usual (cost....) data. What in the world is an Append doing here?=
 I=E2=80=99ve recently performed&nbsp; another analyze on the table, btw. <b=
r></blockquote><div><br></div><div>A top level Append would generally be use=
d when you use a UNION ALL, or you have inheritance.</div><div>&nbsp;</div><=
blockquote class=3D"gmail_quote" style=3D"margin:0px 0px 0px 0.8ex;border-le=
ft:1px solid rgb(204,204,204);padding-left:1ex">
<br>
Also, the query does not seem to be using an index, as the explain shows a p=
air of sequential scans, speaking of which, the second sequential scan shows=
 it=E2=80=99s on a table, called &lt;table&gt;_test. Which is a little stran=
ge, considering that does not exist.<br></blockquote><div><br></div><div>Is t=
hat the name of the table, or the alias for that table?</div><div><br></div>=
<div>&nbsp; "-&gt; &nbsp;Seq Scan on pgbench_accounts pa_alias &nbsp;(cost=3D=
0.00..527869.00 rows=3D20000000 width=3D97)"<br></div><div><br></div><div>"p=
gbench_accounts" is the actual table, "pa_alias" is the alias for it.&nbsp;<=
br></div><div><br></div><div><br></div><div>PostgreSQL will create alias for=
 table when it needs to, but I've never seen it do so by adding "_test" to t=
he end.&nbsp; Usually it adds "_" and an integer.</div><div><br></div><div>I=
t would really help to be able to see either the plan or the query, or both.=
</div><div><br></div><div>Cheers,</div><div><br></div><div>Jeff</div></div><=
/div>
</div></blockquote></body></html>=

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