RE: Update statement performance

From: Chitale, Hemant K <>
Date: Wed, 26 Feb 2014 13:35:56 +0800
Message-ID: <>

Your UPDATE statement doesn't make sense. If table "a" and "b" are the same table, then "a.mycolumn" is the same as "b.mycolumn" -- meaning that your are updating a value back to itself (for a subset of rows in the table). So for those rows by "anothercol"='someval', you are updating "mycolumn" to itself.  

I presume that either "b" is not the same table as "a" or that the "mycolumn" in b is not the same as "mycolumn" in a.      

Hemant K Chitale    

[] On Behalf Of Praveen Ramanan Sent: Wednesday, February 26, 2014 12:01 PM To:
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 |

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!!!    


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Received on Wed Feb 26 2014 - 06:35:56 CET

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