Sample size in ANALYZE command

Posted on 2001-06-29
Last Modified: 2012-05-04
Is there a recommended rule of thumb for choosing what
percentage of a table to analyse ?

I have some large tables (20 million rows) which I want
to analyse on a regular basis.

I don't want to analyse too small a number of rows, as
the stat's probably won't be accurate, and could make
things worse if the wrong execution path is chosen as
a result of inaccurate stat's.

But I don't want to analyse the whole table as this will
take too long.

What percentage will give me a good balance ? 10 ? 20 ?

Does Oracle recommend a suitable figure ?

Any accompanying explanations or URL's would be most

Question by:myerpz
LVL 35

Expert Comment

by:Mark Geerlings
ID: 6239166
Rather than a percentage, I've seen the number 1024 recommended as the number of rows to count when you want analyze to do a quicker job.  I used that figure in the past when I was working on a system with some multi-million row tables, with good results as far as I could tell.

Author Comment

ID: 6239177
I think the default on the ANALYZE command is 1604.
I just wonder if there's any official word from Oracle
on it, or whether they leave it up to the analyst.

Expert Comment

ID: 6239195

Specify the amount of data from the analyzed object Oracle should sample to estimate statistics. If you omit this parameter, Oracle samples 1064 rows.
The default sample value is adequate for tables up to a few thousand rows. If your tables are larger, specify a higher value for SAMPLE. If you specify more than half of the data, Oracle reads all the data and computes the statistics.
ROWS causes Oracle to sample integer rows of the table or cluster or integer entries from the index. The integer must be at least 1.

PERCENT causes Oracle to sample integer percent of the rows from the table or cluster or integer percent of the index entries. The integer can range from 1 to 99.
I think 20% of the rows is a good figure estimate the statistics.
PRTG Network Monitor: Intuitive Network Monitoring

Network Monitoring is essential to ensure that computer systems and network devices are running. Use PRTG to monitor LANs, servers, websites, applications and devices, bandwidth, virtual environments, remote systems, IoT, and many more. PRTG is easy to set up & use.

LVL 35

Accepted Solution

Mark Geerlings earned 100 total points
ID: 6239269
The others are correct, the number is 1064, not 1024 as I said in my earlier comment.  I don't know what is magical about this number, but unless you know or suspect that the first 1,000 or so records in the table are not representative of the majority of records, it is likely much faster for analyze to do this number of rows than a fixed (even small) percentage of a large table.

Expert Comment

ID: 6239297
I have used this script for a number of years an it seems to do the trick. It is base on the size of the table not the number of rows.

REM *******************************************************
REM File: C:\Dba\analytab.SQL
REM ******************************************************
REM  This script will produce and run the script
REM  ANALYTB2.SQL to analyze
REM  all tables and create the listing analytab.rpt
REM ******************************************************

set echo off
set heading off
set pagesize 50000
set feedback off
spool c:\dba\analytb2.sql
SELECT 'ANALYZE TABLE ' || RTRIM( owner ) || '.' ||
          segment_name || ' COMPUTE STATISTICS ;'
     FROM sys.dba_segments
     WHERE segment_type = 'TABLE'
          AND owner NOT IN ('SYS','SYSTEM','VOSER')
          AND bytes < 5000000
SELECT 'ANALYZE TABLE ' || RTRIM( owner ) || '.' ||
          segment_name || ' ESTIMATE STATISTICS SAMPLE 30 PERCENT;'
     FROM sys.dba_segments
     WHERE segment_type = 'TABLE'
          AND owner NOT IN ('SYS','SYSTEM','VOSER')
          AND bytes between 5000001 AND 30000000
SELECT 'ANALYZE TABLE ' || RTRIM( owner ) || '.' ||
          segment_name || ' ESTIMATE STATISTICS SAMPLE 10 PERCENT;'
     FROM sys.dba_segments
     WHERE segment_type = 'TABLE'
          AND owner NOT IN ('SYS','SYSTEM','VOSER')
          AND bytes between 30000001 AND 50000000
SELECT 'ANALYZE TABLE ' || RTRIM( owner ) || '.' ||
          segment_name || ' ESTIMATE STATISTICS SAMPLE 5 PERCENT;'
     FROM sys.dba_segments
     WHERE segment_type = 'TABLE'
          AND owner IN ('SYS','SYSTEM')
          AND bytes > 50000000

spool off

set echo on
set feedback on



Expert Comment

ID: 6242125
I'm not experienced with analyze schema.

But dbrower, are you sure, that you always used THIS script for a number of years?

What if a table is > 50000000 Bytes?

Ooops, is there perhaps a "NOT" missing ;-)?

Good luck.

Expert Comment

ID: 6243956

Why not test different samples percentage for both time and statistics accuracy ?


Featured Post

Is Your Active Directory as Secure as You Think?

More than 75% of all records are compromised because of the loss or theft of a privileged credential. Experts have been exploring Active Directory infrastructure to identify key threats and establish best practices for keeping data safe. Attend this month’s webinar to learn more.

Question has a verified solution.

If you are experiencing a similar issue, please ask a related question

Suggested Solutions

How to Create User-Defined Aggregates in Oracle Before we begin creating these things, what are user-defined aggregates?  They are a feature introduced in Oracle 9i that allows a developer to create his or her own functions like "SUM", "AVG", and…
Using SQL Scripts we can save all the SQL queries as files that we use very frequently on our database later point of time. This is one of the feature present under SQL Workshop in Oracle Application Express.
This video shows syntax for various backup options while discussing how the different basic backup types work.  It explains how to take full backups, incremental level 0 backups, incremental level 1 backups in both differential and cumulative mode a…
This video shows how to Export data from an Oracle database using the Datapump Export Utility.  The corresponding Datapump Import utility is also discussed and demonstrated.

920 members asked questions and received personalized solutions in the past 7 days.

Join the community of 500,000 technology professionals and ask your questions.

Join & Ask a Question

Need Help in Real-Time?

Connect with top rated Experts

16 Experts available now in Live!

Get 1:1 Help Now