Seagate ST43401N/ND Guide d'installation télécharger pdf (Page 55)

CHAPTER 5 Implement: Making Required Application Changes

• Table/Index Level Statistics

• Changes to the update statistics Command

Column Level

Statistics

The sysstatistics table contains statistics related to the data in a column.

Column level statistics describe the distribution of values in the column (and in

the case of multicolumn density values, a set of columns). These are the

“distribution” statistics formerly stored in the single distribution page of each

index; they include the histogram and the density values. Also stored in

sysstatistics is the date and time of the last modification of the column statistics.

Column statistics are now an attribute of a column, not an index. This is a

fundamental change to the statistics. Because statistics are now an attribute of

a column they can be placed on any column, whether it is part of an index or

not.

The number of steps (cells) to use in the histogram can be specified in the

create index or update statistics commands.

Table/Index Level

Statistics

The systabstats table contains statistics related to objects: the table and index

level statistics. Statistics related to tables and indexes are now stored in a single

table and new statistics are also available. Table and index statistics are

maintained dynamically by ASE. They should not be written directly; they will

be quickly overwritten. Table and index statistics cannot be written with

optdiag.

The statistics stored in

systabstats are dynamic. An in-memory copy is kept

and modified as changes occur. This in-memory copy is then flushed to

systabstats in a number of ways, such as by housekeeper, update statistics,

optdiag, checkpoint, shutdown, and sp_flushstats.

Changes to the

update statistics

Command

The update statistics command was given some new extensions and

functionality. These include the ability to use

update statistics to build or update

statistics on an individual column, on all columns of an index or indexes or on

all columns of a table. On DOL tables

update statistics will use isolation level

0 (dirty reads) and avoid locking the table while running.

As in previous versions, running update statistics table_name [index_name] will

update the statistics of the leading column (major attribute) of all indexes on

the table or of the specified index. See the section Updating Statistics After

Upgrade in Chapter 6of this manual for guidelines for running this command

after upgrade.

See the

Performance and Tuning Guide at

http://manuals.sybase.com:80/onlinebooks/group-

as/asg1200e/aseperf/@Generic__BookView

for information on histogram cells.

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Seagate ST43401N/ND Guide d'installation Page 55

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