To: pgsql-sql@postgresql.org
From: Thomas Kellerer <spam_eater@gmx.net>
Subject: Re: Reuse temporary calculation results in an SQL update query
Date: Sat, 29 Sep 2012 16:13:59 +0200
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Message-ID: <k46vmq$p11$1@ger.gmane.org>
References: <43516431.HfO3TYfNBy@hek506>
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In-Reply-To: <43516431.HfO3TYfNBy@hek506>

Matthias Nagel wrote on 29.09.2012 12:49:
> Hello,
>
> is there any way how one can store the result of a time-consuming calculation if this result is needed more
>than once in an SQL update query? This solution might be PostgreSQL specific and not standard SQL compliant.
>  Here is an example of what I want:
>
> UPDATE table1 SET
>     StartTime = 'time consuming calculation 1',
>     StopTime = 'time consuming calculation 2',
>     Duration = 'time consuming calculation 2' - 'time consuming calculation 1'
> WHERE foo;
>
> It would be nice, if I could use the "new" start and stop time to calculate the duration time.
>First of all it would make the SQL statement faster and secondly much more cleaner and easily to understand.


Something like:

with my_calc as (
     select pk,
            time_consuming_calculation_1 as calc1,
            time_consuming_calculation_2 as calc2
     from foo
)
update foo
   set startTime = my_calc.calc1,
       stopTime = my_calc.calc2,
       duration = my_calc.calc2 - calc1
where foo.pk = my_calc.pk;

http://www.postgresql.org/docs/current/static/queries-with.html#QUERIES-WITH-MODIFYING