Hi All! Happy Wednesday!
I have been reading lately (on the list and in other materials)
about the venerable Cache Hit Ratio not really being a good
measure of performance. I agree, but there's something I can't
quite grasp.
I have this performance report from one of my data marts (the
report queries the Statspack table STATS$SYSSTAT):
BUFFER
SNAPSHOT DB BLOCK CONSISTENT PHYSICAL CACHE
TIME GETS GETS READS HIT%
----------- ---------- ---------- ---------- -------
05/01 12:01 3082 3940345 77913 98.02
05/01 12:16 10030 114062688 1972731 98.27
05/01 12:31 8494 126757798 1488216 98.83
05/01 12:47 6967 145269614 948446 99.35
05/01 13:02 5702 126382287 1166288 99.08
05/01 13:17 6325 96525651 2145440 97.78
05/01 13:33 6312 109787906 2295739 97.91
05/01 13:48 6065 135562099 2491945 98.16
05/01 14:04 5948 129659382 2299102 98.23
I know (KNOW!) that there have been a lot of one-off queries that
use fixed values (as opposed to bind variables) in the WHERE
clause executed on this instance all morning (from COGNOS cube
builds). I know (KNOW!) that most of these queries have done
full table scans of multi-gigabyte tables which would have to
result in flushing and re-filling the buffer cache. The buffer
cache is 1.2G - not enough to cache all of any single table.
I still do not understand why that result does not show up in this
query. I think the CHR in this report should be in the 10-20%
range at best. Can anybody help educate me?
Thanks,
Mike
P.S. And NO...I did not run up the CHR with Connor's script!
---
===========================================================================
Michael P. Vergara
Oracle DBA
Guidant Corporation
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Author: Vergara, Michael (TEM)
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