From: Tom Lane <tgl@sss.pgh.pa.us>
To: chris+postgresql@qwirx.com
cc: pgsql-docs@lists.postgresql.org
Subject: Re: Why can NUMERIC columns not use deduplication?
In-reply-to: <164216867903.23277.10737033258186320533@wrigleys.postgresql.org>
References: <164216867903.23277.10737033258186320533@wrigleys.postgresql.org>
Comments: In-reply-to PG Doc comments form <noreply@postgresql.org>
	message dated "Fri, 14 Jan 2022 13:57:59 +0000"
MIME-Version: 1.0
Content-Type: text/plain; charset="us-ascii"
Content-ID: <3379156.1642173281.1@sss.pgh.pa.us>
Date: Fri, 14 Jan 2022 10:14:41 -0500
Message-ID: <3379157.1642173281@sss.pgh.pa.us>
Archived-At: 
 <https://www.postgresql.org/message-id/3379157.1642173281%40sss.pgh.pa.us>
Precedence: bulk

PG Doc comments form <noreply@postgresql.org> writes:
> "64.4.3. Deduplication" says:
> "numeric cannot use deduplication. Numeric display scale must be preserved
> among equal datums."

> However I don't understand this. An equal datum is an equal datum regardless
> of display scale, isn't it?

No.  For example:

regression=# select 1.0, 1.00;
 ?column? | ?column? 
----------+----------
      1.0 |     1.00
(1 row)

regression=# select 1.0 = 1.00;
 ?column? 
----------
 t
(1 row)

These values are "equal" per the datatype's equality operator,
but there are some operations (not only output) that can distinguish
them, so the index can't lump them together.

There are similar issues in some other datatypes, for example
zero and minus zero in the float types.

			regards, tom lane