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From: KES <[email protected]>
To: Tom Lane <[email protected]>
To: Bruce Momjian <[email protected]>
Cc: PostgreSQL-development <[email protected]>
Subject: Re: Typo in doc or wrong EXCLUDE implementation
Date: Fri, 10 Aug 2018 12:10:47 +0300
Message-ID: <[email protected]> (raw)
In-Reply-To: <[email protected]>
References: <[email protected]>
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huh, maybe you are right, I missread that. English is not my native language.
Actually I come there from FK constraints. 

Would it be sufficient for FK require not UNIQUEs, but **allow** "EXCLUDE with operators that act like equality"?

09.08.2018, 22:31, "Tom Lane" <[email protected]>:
> 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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