RE: Update statement performance

From: Jonathan Lewis <>
Date: Wed, 26 Feb 2014 07:17:48 +0000
Message-ID: <CE70217733273F49A8A162EE074F64D901DE00C1_at_exmbx05.thus.corp>

Look at the access path into the subquery table - it's using an index on "anothercol" which is deemed to be very expensive, and then visiting the table to check "joincolumn" to limit the result set to a single row. It looks as if you don't have a suitable index into mytable to make this subquery approach efficient.

Two possible strategies - use a hash join (either through a MERGE command or by using an UPDATEABLE JOIN VIEW, if you have suitable constraints in place) or create an index that starts with joincolumn and anothercol in some order to avoid visiting the subquery table (or an index on joincolumn, perhaps, to minimise visits to the subquery table)

Jonathan Lewis

From: [] on behalf of Praveen Ramanan [] Sent: 26 February 2014 04:00
Subject: Update statement performance

Hello All,

I am using oracle 11gr2 on windows server 2008. This is a test system.Stats are current and archivelog is turned off temporarily.

I am trying to update a table that has over 1.1 million rows. I have a simple update statement that does the following. (basically updates mycolumn when it is null, to somevalue)

update mytable a set a.mycolumn = (select b.mycolumn from

                                                                  mytable b where
                                                                  a.joincolumn = b.joincolumn and
where a.mycolumn is null;

The number of rows where mycolumn is null is around 1/5th of the total. (225000 rows)

The above update statement seems to be running forever.On checking the explain plan this is what i see.

| Id  | Operation                    | Name                     | Rows  | Bytes | Cost (%CPU)| Time     |
|   0 | UPDATE STATEMENT             |                          |   224K|  2630K|  6454M  (1)|127:19:43 |
|   1 |  UPDATE                      | mytable            |       |       |            |          |

|* 2 | TABLE ACCESS FULL |mytable | 224K| 2630K| 6644 (1)| 00:00:01 |
|* 3 | TABLE ACCESS BY INDEX ROWID| mytable | 1 | 21 | 28752 (1)| 00:00:03 |
|* 4 | INDEX RANGE SCAN | mytable_someval_INDEX | 874K| | 3318 (1)| 00:00:01 |

Predicate Information (identified by operation id):

   2 - filter("a"."mycolumn" IS NULL)
   3 - filter("b"."joincolumn"=:B1)
   4 - access("b"."anothercol"='somecol')

I know i can use CTAS to achieve my task.However i am not sure why updating a .225 million rows would take hours.

Can my update statement be written more efficiently? (perhaps using merge update?)

Maybe there is some other bottleneck that I need to look?

Any thoughts would be very well appreciated. Thanks!!!

Thousands of candles can be lighted from a single candle, and the life of the single candle will not be shortened. Happiness never decreases by being shared.

Received on Wed Feb 26 2014 - 08:17:48 CET

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