From: Tom Lane <tgl@sss.pgh.pa.us>
To: Mark Steben <mark.steben@drivedominion.com>
cc: pgsql-admin <pgsql-admin@postgresql.org>
Subject: Re: creating index with a cast statement in the predicate
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Comments: In-reply-to Mark Steben <mark.steben@drivedominion.com>
	message dated "Mon, 31 Jul 2017 13:33:43 -0400"
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Date: Mon, 31 Jul 2017 14:12:13 -0400
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Mark Steben <mark.steben@drivedominion.com> writes:
>     I would like to create an index on a column named value: defined as
> varchar(255)
> Values in this column can be many, including both dates and non-dates.
> The developers run a query that first uses a regular expression to filter
> non-dates out
> then, ANDS it with the following
>  *and cast(value as date) >= date_trunc('MONTH', current_date) - interval '
> 2 Month '*
>     When I attempt to create an index using this statement in the
> predicate, I get the error that states that functions in an index predicate
> must be marked as Immutable.

I think it's unhappy about the varchar-to-date cast.  That is not
considered immutable because the behavior of date_in() depends on
the setting of DateStyle.

			regards, tom lane


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