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From: Durgamahesh Manne <maheshpostgres9@gmail.com>
Date: Fri, 11 Oct 2024 19:01:50 +0530
Message-ID: <CAJCZkoKEA4ZcYq0ereB2vFUQ+RQ8o2CEAUu5a4p-_ijuUvFL0g@mail.gmail.com>
Subject: Fwd: Inefficient use of index scan on 2nd column of composite index
 during concurrent activity
To: PostgreSQL mailing lists <pgsql-general@postgresql.org>, Greg Sabino Mullane <htamfids@gmail.com>
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---------- Forwarded message ---------
From: Durgamahesh Manne <maheshpostgres9@gmail.com>
Date: Mon, Oct 7, 2024 at 10:01=E2=80=AFAM
Subject: Inefficient use of index scan on 2nd column of composite index
during concurrent activity
To: <pgsql-in-general@postgresql.org>


Hi team

Second column of composite index not in use effectively with index scan
when using second column at where clause

I have composite index on (placedon,id) of test
When quering  select * from test where id =3D '4234';
Value of id changes and during concurrent activity and cpu utilization
increased toomuch  that i have observed which means query plan changed why

I could see index scan with explain for it

Is there any way to keep index scan for it during even on concurrency
rather than seperate index on second column of composite index ?

Hope everyone understand this

Regards,
Durga Mahesh


Hi Greg

you mentioned that below

(please start a new thread in the future rather than replying to an
existing one)

You cannot query on b and use an index on (a,b) as you observed. However,
you can have two indexes:

index1(a)
index2(b)

Postgres will be able to combine those when needed in the case where your
WHERE clause needs to filter by both columns. So then you no longer need
the two-column index.


Hi Greg ,

Here not using composite index on ordinary table.
Composite index that i use on partitioned table is mandatory for use to
replicate data to target using pglogical  (sorry this is not mentioned
earlier)

composite key (placedon,id)
In concurrent mode if i use id at where clause then query plan for that id
column changes

How to mitigate it rather than use seperate index for id to continue
without change in query plan (index scan) during concurrent activity

I hope you understand this

Regards,
Durga Mahesh


Cheers,
Greg

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<div dir=3D"ltr"><br><br><br><div><div class=3D"gmail_quote"><div dir=3D"lt=
r" class=3D"gmail_attr">---------- Forwarded message ---------<br>From: <st=
rong class=3D"gmail_sendername" dir=3D"auto">Durgamahesh Manne</strong> <sp=
an dir=3D"auto">&lt;<a href=3D"mailto:maheshpostgres9@gmail.com">maheshpost=
gres9@gmail.com</a>&gt;</span><br>Date: Mon, Oct 7, 2024 at 10:01=E2=80=AFA=
M<br>Subject: Inefficient use of index scan on 2nd column of composite inde=
x during concurrent activity<br>To:  &lt;<a href=3D"mailto:pgsql-in-general=
@postgresql.org">pgsql-in-general@postgresql.org</a>&gt;<br></div><br><br><=
div dir=3D"auto">Hi team=C2=A0<div dir=3D"auto"><br></div><div dir=3D"auto"=
>Second column of composite index not in use effectively with index scan=C2=
=A0 when using second column at where clause</div><div dir=3D"auto"><br></d=
iv><div dir=3D"auto">I have composite index on (placedon,id) of test=C2=A0<=
/div><div dir=3D"auto">When quering=C2=A0 select * from test where id =3D &=
#39;4234&#39;;</div><div dir=3D"auto">Value of id changes and during concur=
rent activity and cpu utilization increased toomuch=C2=A0 that i have obser=
ved which means query plan changed why</div><div dir=3D"auto"><br></div><di=
v dir=3D"auto">I could see index scan with explain for it=C2=A0</div><div d=
ir=3D"auto"><br></div><div dir=3D"auto">Is there any way to keep index scan=
 for it during even on concurrency rather than seperate index on second col=
umn of composite index ?</div><div dir=3D"auto"><br></div><div dir=3D"auto"=
>Hope everyone understand this=C2=A0</div><div dir=3D"auto"><br></div><div =
dir=3D"auto">Regards,</div><div dir=3D"auto">Durga Mahesh=C2=A0</div><div d=
ir=3D"auto"><br></div><div dir=3D"auto"><br></div><div dir=3D"auto">Hi Greg=
=C2=A0</div><div dir=3D"auto"><br></div><div dir=3D"auto">you mentioned tha=
t below<br><div><br></div><div>(please start a new thread in the future rat=
her than replying to an existing one)<div><br></div><div>You cannot query o=
n b and use an index on (a,b) as you observed. However, you can have two in=
dexes:</div><div><br></div><div>index1(a)</div><div>index2(b)</div><div><br=
></div><div>Postgres will be able to combine those when needed in the case =
where your WHERE clause needs to filter by both columns. So then you no lon=
ger need the two-column index.</div><div><br></div><div><br></div><div><br>=
</div><div>Hi Greg ,</div><div><br></div><div>Here not using composite inde=
x on ordinary table.</div><div>Composite index that i use on partitioned ta=
ble is mandatory for use to replicate data to target using pglogical=C2=A0 =
(sorry this is not mentioned earlier)</div><div><br></div><div>composite ke=
y (placedon,id)=C2=A0</div><div>In concurrent mode if i use id at where cla=
use then query plan for that id column changes=C2=A0</div><div><br></div><d=
iv>How to mitigate it rather than use seperate index for id to continue wit=
hout change in query plan (index scan) during concurrent activity=C2=A0</di=
v><div><br></div><div>I hope you understand this=C2=A0</div><div><br></div>=
<div>Regards,</div><div>Durga Mahesh=C2=A0</div><div><br></div><div><br></d=
iv><div><br></div><div><br></div><div><br></div><div>Cheers,</div><div>Greg=
</div></div></div></div>
</div></div></div>

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