collect statistics - estimate_percent
Hi guys,.
My DB: 10.2.0.5.
Recently, I have a problem with the auto_sample_size.
When you use the sample size car with one of the table, it takes only 2 percent.
For example, some of my references SQLs this table went slow.
When I tried to generate the statistics with 50%, it went very well. (picking up a more optimized plan).
I'm trying to understand (with examples) you guys (experts), on:
1. How does oracle choose the percent when auto_sample_size is specified.
2. a good example why a bad plan will be selected at 2%. and a better plan will be chosen with stats of 50%.
Thank you very much.
I can't seem to find an example on that.
2. a good example why a bad plan may be generated when 2%. and a better plan will be generated with stats of 50%. (when the data are asymmetric)
Can anyone help to provide a simple example to understand?
Of course - have you ever played or watched a basketball game? Have you noticed how REALLY, REALLY big, some of the players in a team are still other players are really short?
1. you are a tailor
2. the coach of the basketball team wants you to uniforms for its 10 players.
3. the coach only will you more than 5 of its 10 players for you can measure uniforms
4. do you prefer a 20% sample or a sample of 50%?
5. what sample size will generate a better set of uniforms ("plan")?
When the data is really uneven (as if it were in this example), you should still a "scan to complete basketball team. Your uniforms will not go well at all if two players get you to "sample" are the shortest (or higher) on the team?
Moral: the information more and better, you have the best decision you can make. It is not only the quantity of statistics but the quality that can make the difference.
How this example does not work for you?
Tags: Database
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52 million lines?
and how to change every day or be inserted?
It is possible that you can never bring his stats again
and everything will probably continue to work as expected
read the guide to setting performance to understand why you need to collect statistics and when
https://docs.Oracle.com/CD/E11882_01/server.112/e41573/stats.htm#PFGRF94714
After reading this guide, you'll be ready for any question, you get all that against this team
Sometimes you need to say 'no' and why the answer is 'no '.
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SQL > exec dbms_stats.gather_SCHEMA_stats (ownname = > 'WMWHSE1', estimate_percent = > dbms_stats.auto_sample_size, method_opt = > 'FOR ALL THE COLUMNS SIZE AUTO', cascade = > TRUE);
BEGIN dbms_stats.gather_SCHEMA_stats (ownname = > 'WMWHSE1', estimate_percent = > dbms_stats.auto_sample_size, method_opt = > 'FOR ALL THE COLUMNS SIZE AUTO', cascade = > TRUE); END;
*
ERROR on line 1:
ORA-03113: end of file on communication channel
Process ID: 24820
Session ID: 1038 serial number: 34509
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ERROR on line 1:
ORA-03113: end of file on communication channel
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I intend to run the query to collect the statistics below. The table has 50 million records.
COUNT (*)
----------
51364617
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My doubts specified below.
1. can I use the also estimate_percent parameter to collect statistics?
2 should. how much we indicate for the estimate_percent parameter?
3. what difference it will make if I use the estimate_percent parameter?
Thanks in advance
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1. can you use estimate_percent? Sure! Go ahead.
2. what percentage of use? Why not let the data decide with auto_sample_size? Various "rules of thumb" were thrown around over the years, usually about 10 to 20%.
3. what difference it will make? Very little, no doubt. Occasionally, you can see where a small sample makes a difference, but in general that it's perfectly ok to estimate its stats.Maybe something like this:
BEGIN dbms_stats.gather_table_stats(ownname => user, tabname => 'MY_TABLE', estimate_percent => dbms_stats.auto_sample_size, method_opt=>'for all columns size auto', cascade=>true,degree=>8); END; -
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GRANT ALL ON DETAIL_VALUE TO SCHEMA1
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