Re: Prevent FTS

From: Stephane Faroult <sfaroult_at_oriole.com>
Date: Mon, 06 Jan 2003 11:24:29 -0800
Message-ID: <F001.00527D9C.20030106112429@fatcity.com>

manoj.gurnani_at_orbitech.co.in wrote:
>
> Hi All,
> I've 3 tables table x,y,z
> pk of x is (col_1,col_2),pk of y is (col_1,col_2,col_4,col_5),pk of z is
> (col_1,col_2)
>
> where clause is :
>
> where x.col_1 = y.col_1
> and x.col_2 = y.col_2
> and x.col_1 = z.col_1
> and x.col_2 = z.col_2
> and y.col_1 = z.col_1
> and y.col_2 = z.col_2
> and x.col_3 in ('val1','val2','val3')
> and.....
> ..
>
> Explain plan shows that there is a FTS of table z .
> what can be the reason for this and how to prevent this.
>
> Thanks
> Manoj
>
> --

Manoj,

    Look at the way you have written your conditions : you join x to z and y to z too. A bit redundant since you also have the x to y join. Can confuse Oracle. ASSUMING THAT THE CONDITION ON X.COL_3 IS SELECTIVE add /*+ ORDERED */ after SELECT and list

              from x, y, z
in this order. And remove the unnecessary join between x and z (just keep x to y and y to z). As Dan remarked, don't forget that FTS are not necessarily bad, and even are sometimes much more efficient than something else (especially when used in a hash join, typically). The proof of the pudding is in

     SET TIMING ON
     SET AUTOTRACE TRACEONLY

and compare the number of logical reads in both cases (and of course elapsed time, but run each query twice to avoid measuring the effect of parsing and bringing data into the cache)..  

-- 
Regards,

Stephane Faroult
Oriole Software
-- 
Please see the official ORACLE-L FAQ: http://www.orafaq.net
-- 
Author: Stephane Faroult
  INET: sfaroult_at_oriole.com

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Received on Mon Jan 06 2003 - 13:24:29 CST