Seagate ST43401N/ND Guia de Instalação descarregar pdf (Página 71)

CHAPTER 6 Implement: Making Database Administration Changes

Optimizer Changes

ASE 11.9.2 introduced major changes in the way the server keeps statistics and

optimizes queries. These changes were aimed at increasing granularity in the

statistics kept by the server, improving query plans, and producing better

performance. Changes introduced in 11.9.2 included:

• Two new system tables,

systabstats, which maintains information on

objects such as tables and rows, and

sysstatistics which stores data

distribution statistics.

• New optimizer statistics and added functionality to the

update

statistics

command.

•The

optdiag utility, which simplifies the task of looking at optimizer

statistics. This utility also allows you to model query performance by

running queries against a test database with simulated table, index, and

column statistics.

These and other query processing changes for 11.9.2 are reviewed in the

chapter Chapter 5, “Implement: Making Required Application Changes” and

in the section Summary of Query Processing Changes in 11.9.2 and 12.0 that

May Affect Performance in the white paper

Technical Issues in ASE 11.9.x and

12.0 Upgrade at http://sybase.com/detail/1,3693,1011767,00.html

For more detailed information on the

systabstats and sysstatistics tables, see the

chapter on “Using Statistics to Improve Performance” in the

Performance and

Tuning Guide at http://manuals.sybase.com:80/onlinebooks/group-

as/asg1250e/perf/@Generic__BookView

. For information on the optdiag utility,

see the

Utility Guide at http://manuals.sybase.com:80/onlinebooks/group-

as/asg1250e/util/@Generic__BookView

Updating Statistics After Upgrade

As part of your upgrade procedure, you need to run:

update statistics table_name

on all tables as soon as possible following upgrade. Statistics that are copied

from the old distribution page by the upgrade are not as accurate as those

created by update statistics, because the weights are average, not actual.

Running this command updates default statistics on the leading columns of

your indexes.

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