Explain and time execution 
Author Message
 Explain and time execution
    Hello i have two queries, the first have 24729 timerons as time
execution estimated by explain tools, and the second have 137646 timerons,
but the first query execute two time faster than the second one, i don't
understand the the difference, is their any rules between execution time
evaluated by explain and real time execution, is the raison of this
difference is the fact of using DB2 resources (memory for example), because
for me the first one execute 6 time faster.

Sat, 08 Mar 2003 07:22:19 GMT
 Explain and time execution

Hi (again :-),

These numbers are estimates only. The quality of the estimate depends
among other things on your statistics.
The fresher they are, the more likely it is that the optimizer's guesses
are accurate.
One common reason to give inaccurate estimates are correlated values in
teh tables.
E.g. DOB of an employee is weakly correlated to the employees salary.
Statistics don't caputure this fact as of today.

Visit the DB2 UDB and DB2 Connect Online Support site at:

Sat, 08 Mar 2003 03:00:00 GMT
 [ 2 post ] 

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