From: Tom Lane <tgl@sss.pgh.pa.us>
To: sgarden@club.fr
cc: pgsql-docs@lists.postgresql.org
Subject: Re: the date() function is undocumented while quite necessary
In-reply-to: <158177916388.6746.12385265846339386100@wrigleys.postgresql.org>
References: <158177916388.6746.12385265846339386100@wrigleys.postgresql.org>
Comments: In-reply-to PG Doc comments form <noreply@postgresql.org>
	message dated "Sat, 15 Feb 2020 15:06:03 +0000"
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Date: Sat, 15 Feb 2020 11:36:50 -0500
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PG Doc comments form <noreply@postgresql.org> writes:
> I can't find anywhere the documentation of the SQL DATE() function which
> extracts the date part out of a timestamp.

It's a type cast written in function-like syntax, as documented in

https://www.postgresql.org/docs/current/sql-expressions.html#SQL-SYNTAX-TY=
PE-CASTS

We don't really encourage that notation, because it's confusing
and it doesn't work for every possible type name, which is why
you won't find it mentioned very many places.

> Please note that according to my tests, one can't use the cast operator =
when
> defining expression-based indexes, while the date(column) expression is
> accepted.

You can if you put parentheses around it:

regression=3D# create table t1 (f1 timestamp);
CREATE TABLE
regression=3D# create index on t1 (f1::date);
ERROR:  syntax error at or near "::"
LINE 1: create index on t1 (f1::date);
                              ^
regression=3D# create index on t1 ((f1::date));
CREATE INDEX

This is per the restriction explained in the CREATE INDEX docs, that
any expression-to-be-indexed that doesn't look like a function call
requires parentheses.

https://www.postgresql.org/docs/current/indexes-expressional.html

https://www.postgresql.org/docs/current/sql-createindex.html

			regards, tom lane