From: Tom Lane <tgl@sss.pgh.pa.us>
To: Bruce Momjian <bruce@momjian.us>
cc: KES <kes-kes@yandex.ru>,
        PostgreSQL-development <pgsql-hackers@postgresql.org>
Subject: Re: Typo in doc or wrong EXCLUDE implementation
In-reply-to: <20180809190913.GB14011@momjian.us>
References: <153121527691.1408.5686988620817799073@wrigleys.postgresql.org>
 <20180807173659.GD7297@momjian.us>
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 <20180808130040.GA2611@momjian.us> <12107.1533736288@sss.pgh.pa.us>
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 <20180809190913.GB14011@momjian.us>
Comments: In-reply-to Bruce Momjian <bruce@momjian.us>
	message dated "Thu, 09 Aug 2018 15:09:13 -0400"
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Date: Thu, 09 Aug 2018 15:31:09 -0400
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Bruce Momjian <bruce@momjian.us> writes:
> On Thu, Aug 9, 2018 at 01:11:05PM +0300, KES wrote:
>> Why surprising? It is
>> [documented](https://www.postgresql.org/docs/current/static/sql-create
>> table.html#sql-createtable-exclude):
>>> If all of the specified operators test for equality, this is
>>> equivalent to a UNIQUE constraint, although an ordinary unique
>>> constraint will be faster.

>> Thus the UNIQUE constraint is just particular case of exclusion
>> constraint, is not?

> Well, for me a UNIQUE constraint guarantees each discrete value is
> unique, while exclusion constraint says discrete or ranges or geometric
> types don't overlap.  I realize equality is a special case of discrete,
> but having such cases be marked as UNIQUE seems too confusing.

I think the OP is reading "equivalent" literally, as meaning that
an EXCLUDE with operators that act like equality is treated as being
the same as UNIQUE for *every* purpose.  We're not going there, IMO,
so probably we need to tweak the doc wording a little.  Perhaps
writing "functionally equivalent" would be better?  Or instead of
"is equivalent to", write "imposes the same restriction as"?

			regards, tom lane