Re: not reducing same values in IN

vojtech@hafro.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