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From: Tom Lane <[email protected]>
To: Bruce Momjian <[email protected]>
Cc: KES <[email protected]>
Cc: PostgreSQL-development <[email protected]>
Subject: Re: Typo in doc or wrong EXCLUDE implementation
Date: Thu, 09 Aug 2018 15:31:09 -0400
Message-ID: <[email protected]> (raw)
In-Reply-To: <[email protected]>
References: <[email protected]>
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Bruce Momjian <[email protected]> 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

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