Re: constant vs function param differs in performance

Dear Tom,

Thanks for your early response.

An addition: the nastier difference increased by adding an index (it was an
essential index for this query):

  func with param improved from 2700ms to 2300ms
  func with constant improved from 400ms to 31ms
  inline query improved from 390ms to 2ms

So am I reading correct and it is completely normal and can't be helped?
(couldn't have tried 7.4 yet)

In case it reveals something:

------------------------------- cut here -------------------------------
SELECT field FROM
(SELECT field, sum(something)=0 AS boolvalue
 FROM
 (SELECT * FROM subselect1 NATURAL LEFT JOIN subselect2
  UNION
  SELECT * FROM subselect3 NATURAL LEFT JOIN subselect4
 ) AS u
 GROUP BY field) AS t
WHERE not boolvalue
ORDER BY simple_sql_func_returns_bool(field) DESC
LIMIT 1;
------------------------------- cut here -------------------------------

G.
----- Original Message -----
From: "Tom Lane" <tgl@sss.pgh.pa.us>
Sent: Friday, November 14, 2003 9:59 PM


> "=?iso-8859-2?B?U1rbQ1MgR+Fib3I=?=" <surrano@mailbox.hu> writes:
> > I have two SQL function that produce different times and I can't
understand
> > why.
>
> The planner often produces different plans when there are constants in
> WHERE clauses than when there are variables, because it can get more
> accurate ideas of how many rows will be retrieved.
>
> regards, tom lane
>