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Cost Based Optimizer (CBO)

Former Member

‎2005 Mar 17 1:04 PM

613

not sure if this is a daft question or what. but i am trying to find out where exactly it exists.

i know, when performing ST05 and viewing the execution plan, we see what the CBO has used, but is the CBO purely performed at the database server, and not at the SAP Application.

When updating the statistics, are these passed to the database server, and once again, the CBO utilizes them for the execution plan, or do the database statistics actually reside in the database server.

finally, in viewing the execution plan, the statement "execution costs = xxx" (xxx being a numeric value). what exactly is xxx. maybe an internal index used to compare execution plans, or maybe the number of blocks required to read the "estimated #rows".

anyone ??

thanks

glen

2 REPLIES 2

Former Member

‎2005 Mar 17 1:50 PM

131

Hello Glen,

So far as my knowledge is concerned, the statistics are actually located on the database server. That is what appears to be more logical too. what is the use of maintaining the access paths on tha application server ? Most of the modern database servers are equipped with the CBO functionality. And Cost-Based-Optimizing is dependent on the database.

Here's what the documentation says:

<i>You can update statistics on the Oracle database using the Computing Center Management System (CCMS). The transactions to be used are DB20 and DB21.

By running update statistics regularly, you make sure that the database statistics are up-to-date, so improving database performance. The Oracle cost-based optimizer (CBO) uses the statistics to optimize access paths when retrieving data for queries. If the statistics are out-of-date, the CBO might generate inappropriate access paths (such as using the wrong index), resulting in poor performance.

From Release 4.0, the CBO is a standard part of the SAP System. If statistics are available for a table, the database system uses the cost-based optimizer. Otherwise, it uses the rule-based optimizer.</i>

Regards,

Anand Mandalika.

Former Member
In response to Former Member

‎2005 Mar 17 2:04 PM

0 Kudos
131

hi anand

do i take it that the statistics, ie. accesses, indexes, etc are maintained on the sap box, then transferred to the database box using the "update statistics" mechanisms. if so, do you know where they are held in sap.

it is starting to make sense.

any idea what the "estimated cost" figure unit is ??

thanks

glen