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SAP Note 927295 - FAQ: Oracle system statistics

Note Language: English Version: 19 Validity: Valid Since 26.08.2009

Summary

Symptom

1. What are system statistics?

2. What is the difference between system statistics and regular CBO

statistics?

3. What are WORKLOAD and NOWORKLOAD system statistics?

4. How can I create system statistics?

5. How can I delete system statistics?

6. Where do I find the current values of the system statistics?

7. Where do I find the values of former system statistics?

8. Do you recommend system statistics in the SAP environment?

9. Which advantages do system statistics have?

10. How are system statistics included in CBO cost calculation?

11. Why does the cost of a full table scan decrease when IOSEEKTIM

increases?

12. Which restrictions do I have to consider with respect to system

statistics?

13. Where can I find more information about system statistics?

Other terms

Frequently asked questions

Reason and Prerequisites

Solution

1. What are system statistics?

System statistics contain information on system-specific performance

data such as CPU rate or average duration of a hard disk access.

Consequently, they represent a supplement of the regular CBO

statistics (Note 588668).

2. What is the difference between system statistics and regular CBO

statistics?

You can distinguish system statistics and regular CBO statistics asfollows:

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o Regular CBO statistics

- These are table, column and index-related.

- They contain information on the storage requirement (for

example, number of the table blocks) and of the data

constellation (for example, number of the instances of a

column).

- They serve as a basis to estimate block accesses.

o System statistics

- These are system-specific.

- They contain information such as CPU rate, average duration of

single block and multi block read accesses, and seek and

transfer times.

- They are used to include CPU times and to weight single block

read times, multi block read times, and CPU times.

3. What are WORKLOAD and NOWORKLOAD system statistics?

o NOWORKLOAD statistics contain information which Oracle considers

workload-independent. Internal default values are used with Oracle

9i, while the following values as from 10g are determined

system-dependently:

- CPUSPEEDNW: CPU speed (in instructions per microsecond) when

the system is not under load

- IOSEEKTIM: Seek time for block read accesses (in milliseconds)

- IOTFRSPEED: Block transfer time (in bytes per millisecond)

o WORKLOAD statistics contain additional key figures which may depend

on the system load:

- SREADTIM: Average access time (in milliseconds) for a single

block read (for example, "DB file sequential read", index

access)

- MREADTIM: Average access time (in milliseconds) for a multi

block read ("DB file scattered read", full table scan)

- CPUSPEED: CPU speed (in millions of instructions per second)

- MBRC: Average number of blocks read for a multi block read

("multi block read count")

Since WORKLOAD statistics are load-dependent, it is important to

generate these statistics during a characteristic workload phase.

4. How can I create system statistics?

You can use Oracle tools to create system statistics with the

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DBMS_STATS.GATHER_SYSTEM_STATS command:

o NOWORKLOAD statistics:

EXEC DBMS_STATS.GATHER_SYSTEM_STATS;

o WORKLOAD statistics:

EXEC DBMS_STATS.GATHER_SYSTEM_STATS('INTERVAL', <minutes>);

<minutes>: Number of minutes during which you are to determine the

WORKLOAD statistics.

In the SAP environment, BRCONNECT permits the creation of system

statistics with the aid of the "-f stats -t system_stats [-i

<minutes>]" options. For more information, see Note 601395.

5. How can I delete system statistics?

You can delete WORKLOAD statistics with the aid of the

DELETE_SYSTEM_STATS function of the DBMS_STATS package:

EXEC DBMS_STATS.DELETE_SYSTEM_STATS;

NOWORKLOAD statistics are retained in spite of the execution of this

command.

6. Where do I find the current values of the system statistics?

In the Oracle Data Dictionary, the system statistics are stored in the

AUX_STATS$ table. With the following query, you can receive the

present values of the system statistics:

SELECT PNAME, PVAL1 FROM SYS.AUX_STATS$

WHERE SNAME = 'SYSSTATS_MAIN';

You can determine the start date and the end date of the system

statistic creation as follows:

SELECT

PNAME,

SUBSTR(PVAL2, 1, 40) "DATE"

FROM SYS.AUX_STATS$

WHERE SNAME = 'SYSSTATS_INFO' AND PNAME IN ('DSTART', 'DSTOP');

In the SAP system, you can access this information from transaction

ST04N / DBACOCKPIT under "Additional Functions -> System Statistics

for CBO". You must implement the views from Note 706927 before you can

use this function.

7. Where do I find the values of former system statistics?

As of Oracle 10g, you can find historical system statistics in the

WRI$_OPTSTAT_AUX_HISTORY Oracle DDIC table. You can determine the

historical values for a key figure <param> (for example, "MBRC") as

follows:

SELECT SAVTIME, PVAL1

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FROM WRI$_OPTSTAT_AUX_HISTORY

WHERE PNAME = '<param>'

ORDER BY SAVTIME ASC;

Also as of Oracle 10g, the DBA_OPTSTAT_OPERATIONS view exists, which

stores the executed commands concerning system statistics:

8. Do you recommend system statistics in the SAP environment?

With Oracle 9i, SAP does not recommend the creation of system

statistics in the standard. In individual cases, however, a creation

might be useful.

As of Oracle 10g, we recommend that you create NOWORKLOAD system

statistics on a regular basis.

9. Which advantages do system statistics have?

As a standard, the CPU demand is not included in the cost calculationof the CBO. In addition, the weighting of multi block read accesses is

static in comparison with single block read accesses.

With system statistics, you can weight the three components CPU

consumption, single block reads, and multi block reads correctly so

that the CBO has significantly more data available for the actual

resource and time demand and can thus make more optimal decisions.

Furthermore, you can adjust system statistics manually by means of

DBMS_STATS.SET_SYSTEM_STATS if required. For example, you may set MBRC

explicitly in order to achieve a useful separation of I/O behavior

(parameter DB_FILE_MULTIBLOCK_READ_COUNT) and cost calculation (MBRC).

For this, read the particular information from Note 750631.

10. How are system statistics included in CBO cost calculation?

Before system statistics are analyzed, the system determines following

sizes based on the normal CBO statistics and on hard-coded assumptions

regarding the CPU consumption of certain actions:

o #CPUCycles: Number of required CPU cycles

o #SingleBlockReadBlocks: Number of blocks that are read within the

framework of single block read accesses

o #MultiBlockReadBlocks: Number of blocks that are read within the

framework of multi block read accesses

If there are only NOWORKLOAD statistics, the system performs internal

calculations with Oracle 9i. With Oracle 10g, the WORKLOAD statistic

values are determined dynamically as follows:

o MBRC:

- DB_FILE_MULTIBLOCK_READ_COUNT (if explicitly set)

- _DB_FILE_OPTIMIZER_READ_COUNT (otherwise)

o SREADTIM: IOSEEKTIM + DB_BLOCK_SIZE / IOTFRSPEED

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o MREADTIM: IOSEEKTIM + MBRC * DB_BLOCK_SIZE / IOTFRSPEED

o CPUSPEED: CPUSPEEDNW

Based on these values or when WORKLOAD statistics are present, the

costs are now determined roughly in accordance with the following

formula:

Costs = (#SingleBlockReadBlocks +

#MultiBlockReadBlocks * MREADTIM / (MBRC * SREADTIM) +

#CPUCycles / (CPUSPEED * SREADTIM))

Therefore, the single block reads are the unit of the costs - the

other key figures multi block reads and CPU time are scaled onto the

single block reads.

Note that the final costs are still affected by other factors such as

parameterization or parallel processing (Note 750631).

11. Why does the cost of a full table scan decrease when IOSEEKTIM

increases?

At first, it is not clear why an increase in the I/O seek time should

reduce the costs of full table scans. This is explained when we

examine the cost calculation formulas described above, and the

normalization of multi block reads relative to single block reads that

they contain: If IOSEEKTIM increases, the ratio of MREADTIM and

SREADTIM decreases. Multi-accesses are less expensive than

single-block accesses, which reduces costs.

12. Which restrictions do I have to consider with respect to system

statistics?

The following restrictions exist with respect to system statistics:

o WORKLOAD statistics always reflect the behavior of the system in

the interval of the system statistic creation. Hence, it is

important to create WORKLOAD statistics in a time period of

characteristic system activities. However, note that we do not

recommend that you normally create WORKLOAD statistics.

o The system creates temporary help entries in AUX_STATS$ while

creating WORKLOAD statistics. The actual system statistics are onlyvisible upon completion of the creation interval.

o If no sufficient load is applied in the interval of the WORKLOAD

statistics creation, individual statistic values such as SREADTIM,

MBRC or MREADTIM are not determined and are stored with the value

-1 (9i) or with an empty entry (10g) in AUX_STATS$ instead. As a

consequence, the affected part of the system statistics is ignored

and only the rest is applied (for example, CPU-specific

statistics).

o If the system determines a MREADTIM value which is smaller than

SREADTIM , Oracle assumes an error in the statistics and ignoresthe block-related system statistics. Merely the CPU-related

statistics are used.

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13. Where can I find more information about system statistics?

You can find exact details on system statistics as part of the Oracle

online documentation.

Header Data

Release Status: Released for Customer

Released on: 26.08.2009 10:01:47

Master Language: German

Priority: Recommendations/additional info

Category: FAQ

Primary Component: BC-DB-ORA Oracle

The Note is release-independent

Related Notes

Number Short Text

825653 Oracle: Common misconceptions

750631 Approximations for cost calculation of the CBO

706927 Oracle DB-Mon: Installation script for ST04N/DBA Cockpit

601395 Collecting system statistics with BRCONNECT

588668 FAQ: Database statistics

176754 Problems with CBO and RBO

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