From: Tom Lane <tgl@sss.pgh.pa.us>
To: Wells Oliver <wells.oliver@gmail.com>
cc: pgsql-admin <pgsql-admin@postgresql.org>
Subject: Re: Creating an index on a timestamp with time zone cast to a date--
 possible?
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Comments: In-reply-to Wells Oliver <wells.oliver@gmail.com>
	message dated "Mon, 24 May 2021 11:19:05 -0700"
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Date: Mon, 24 May 2021 14:24:47 -0400
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Wells Oliver <wells.oliver@gmail.com> writes:
> Seems you can't do this:
> create index on someschema.sometable ((modified_at::date));
> If modified_at is a timestamp with a time zone:
> ERROR:  functions in index expression must be marked IMMUTABLE
> But you can if it's a timestamp without a time zone. Anyway, any way of
> casting that as to use it as an index?

The problem is that casting a timestamptz to a date (or a timestamp
for that matter) depends on your current timezone setting.

You could do something like

create index on sometable (((modified_at at time zone 'UTC')::date));

			regards, tom lane