Oracle-L: RE: Too many db calls

From: Cary Millsap <cary.millsap_at_hotsos.com>
Date: Fri, 15 Nov 2002 16:13:29 -0800
Message-ID: <F001.00504DCD.20021115161329@fatcity.com>

Greg,

I believe that the cultural root cause of the excessive LIO problem is the conception that physical I/O is what makes databases slow. Disk I/O certainly *can* make a system slow, but in about 598 of 600 cases we've seen in the past three years, it hasn't. ["Why you should focus on LIOs instead of PIOs" at www.hotsos.com/catalog]

The fixation on PIO of course focuses people's attention on the database buffer cache hit ratio (BCHR) metric for evaluating efficiency. The problem is that the BCHR is a metric of INSTANCE efficiency, not SQL efficiency. However, many people mistakenly apply it as a metric of SQL efficiency anyway.

Of course, if one's radar equates SQL efficiency with the BCHR's proximity to 100%, then a lot of really bad SQL is going to show up on your radar wrongly identified as really good SQL. ["Why a 99% buffer cache hit ratio is not okay" at www.hotsos.com/catalog]

One "classic" result is that people go on search and destroy missions for all full-table scans. They end up producing more execution plans that look like this than they should have:

  NESTED LOOPS
    TABLE ACCESS BY INDEX ROWID
      INDEX RANGE SCAN
    TABLE ACCESS BY INDEX ROWID
      INDEX RANGE SCAN This kind of plan produces great hit ratios because it tends to revisit the same small set of blocks over and over again. This kind of plan is of course appropriate in many cases. But sometimes it is actually less work in the database to use full-table scans. ["When to use an index" at www.hotsos.com/catalog.]

Cary Millsap
Hotsos Enterprises, Ltd.
http://www.hotsos.com

Upcoming events:

- Hotsos Clinic, Dec 9-11 Honolulu
- 2003 Hotsos Symposium on OracleR System Performance, Feb 9-12 Dallas
- Jonathan Lewis' Optimising Oracle, Nov 19-21 Dallas

-----Original Message-----
Sent: Friday, November 15, 2002 4:39 PM
To: Multiple recipients of list ORACLE-L

A while back someone mentioned that the two main causes of slow SQL are excesive LIO's and excesscive database calls, which show up as excessive CPU
use and excessive network traffic, respectively.

Regarding the database calls, is there a "classic" reason for this problem?

My best guess is it's caused by an SQL statement in a PL/SQL loop, which could be rewritten as a single SQL statement. But is this the single, commonly seen cause for this problem, or are there other common ways this
inefficiency is introduced?

Thanks in advance for help in understanding this.

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Author: Greg Moore
INET: sqlgreg_at_pacbell.net

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Author: Cary Millsap
INET: cary.millsap_at_hotsos.com

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