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Re: HOW does 8.3 CREATE REPLACe .. FUNCTION ..COST work

From: "Obe, Regina" <robe(dot)dnd(at)cityofboston(dot)gov>
To: "Tom Lane" <tgl(at)sss(dot)pgh(dot)pa(dot)us>
Cc: <pgsql-general(at)postgresql(dot)org>
Subject: Re: HOW does 8.3 CREATE REPLACe .. FUNCTION ..COST work
Date: 2008-02-25 16:28:46
Message-ID: 53F9CF533E1AA14EA1F8C5C08ABC08D2033D2CB3@ZDND.DND.boston.cob (view raw or flat)
Thread:
Lists: pgsql-general
 
> > --Test 2: This works as I would expect - shows that none of the
> > functions are run presumably its going straight for 5 > 2
> > --becuase it recognizes its the cheapest route
> > TRUNCATE TABLE log_call;
> > SELECT foo.value
> > FROM (SELECT (fn_pg_costlyfunction() > 2 OR fn_pg_cheapfunction() >
2 OR
> > 5 > 2 ) as value) as foo

> That's just constant-folding: x OR TRUE is TRUE.  It has exactly
> zero to do with the cost of anything.

> Offhand I think the behavior you are looking for of choosing to run
more
> expensive subexpressions later only occurs for top-level WHERE clauses
>that are combined with AND.

>			regards, tom lane

Tom thanks for the clarification - based on your comment I verified with
these

-- fn_pg_cheapfunction() is the only one run as you predicted
TRUNCATE TABLE log_call;
SELECT true as value 
WHERE (fn_pg_costlyfunction() > 2 AND fn_pg_cheapfunction() > 5 );

-- fn_pg_costlyfunction() is the only one run - again as predicted by
your statement
TRUNCATE TABLE log_call;
SELECT true as value 
WHERE (fn_pg_costlyfunction() > 2 OR fn_pg_cheapfunction() > 2 );

It would be really nice if this worked with OR as well.  Is it just much
harder to deal with the 
OR case in the planner or was there some other reason why the OR case
was left out?

Thanks,
Regina
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Subject: Re: HOW does 8.3 CREATE REPLACe .. FUNCTION ..COST work

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