Archives of the TeradataForum
Message Posted: Fri, 23 Aug 2002 @ 08:00:07 GMT
After my last posting, some people gave me advices. I also did further experiments and figured out:
* If I didn't collect stats on selection columns, the optimizer seems to choose 10% of total records as default estimated # of rows selected.
(The optimizer knows the exact number of total records if I collect stats on PI, or it does random sampling if there is no stats on PI)
* If I collect stats on the selection column, the optimizer seems to choose
* Even if I select a value which actually exists in the column, somehow the optimizer occasionally falls back to the default.
In my test data, with 83 out of 143 different values the optimizer estimates # of selected rows almost accurately, so it seems to use
stats on that column. But with other 60 values, it falls back to the default:
Those 60 values seem to be values with small # of records, but not the 60 values with least records. It seems that those values are choosed somewhat randomly (depending on its PI?).
I'll be happy if the optimizer will make a bit better guess than
|Copyright 2016 - All Rights Reserved|
|Last Modified: 27 Dec 2016|