Optimizing a Simple 'select count(*)'
- From: Fred Habash <fmhabash@xxxxxxxxx>
- To: "oracle-l@xxxxxxxxxxxxx" <oracle-l@xxxxxxxxxxxxx>
- Date: Wed, 5 Mar 2014 12:56:49 -0500
select count (*) from my_table where completion_time between
to_date(:"SYS_B_0",:"SYS_B_1") and to_date(:"SYS_B_2",:"SYS_B_3")
so does these variations of it ...
select (my_col) where my_col is PK
select (rowid) ...
select count(1) ...
select /*+ index(my_index) */
This takes 1630769 PIOs, 1630774 LIOs, 165 secs and does an FTS on my_table.
My research led me to the following ...
1) As of 8i, count(*) is the same as count(rowid), count(1), &
count(my_col). This is true conceptually & practically.
2) count(*) should be served via a fast full index scans. Why is it not in
this example?
3) There are options to rely on CBO stats to get row count, but this
happens at the expense of accuracy.
So, why should we live with such costly resource consumptions for such a
simple query?
Thank you ...
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Fred Habash, Database Solutions Architect (Oracle OCP 8i,9i,10g,11g)
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