From: | Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> |
---|---|
To: | vojtech(at)hafro(dot)is |
Cc: | pgsql-hackers(at)postgresql(dot)org |
Subject: | Re: not reducing same values in IN |
Date: | 2003-09-15 04:22:20 |
Message-ID: | 3246.1063599740@sss.pgh.pa.us |
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Lists: | pgsql-hackers |
vojtech(at)hafro(dot)is writes:
> I'd like to ask some pg developer how complicated task would
> be to patch query processing (most likely parser) to allow
> for the following non-sql change. For a certain scientific
> application where a bootstrapping of data is needed I would
> like to select from a database "with replacement", i.e.
> to be able to send "SELECT col FROM tab WHERE col IN (5,5)"
> and receive two line of output (two fives) where only one
> is stored in the database.
This does not seem like an easy change at all. According to the
SQL semantics, WHERE is a filter condition that indicates whether
to pass on the current row (if TRUE) or not (if not TRUE).
Rejiggering the behavior as you suggest would mean a fundamental
alteration in semantic assumptions --- I don't even want to think
about what it might break.
> One way is to workaround by
> with SELECT ... col IN (5) UNION ALL SELECT ... col IN (5),
> for complex queries however, I run into difficulties.
Fixing your app to generate the UNION ALL seems like a much more
promising approach ...
regards, tom lane
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