Friday, 6 March 2009

deadlocks: get rid of the bitmap indexes

This one is for a certain Peter, he is a DBA at one of my customers.

We had a problem: Deadlocks happening. Bitmap index.

Users: Complaining, deadlock errors ... ???
we: That bitmap index is causing the deadlocks.
Duhvelopers: But we NEED that index for job xyz, and others Might use it too.
we: Sure, but that index is causing your deadlocks
Users: still complaining!

Yes, we know there is more to Deadlocks, but this was the Bitmap!
Peter is the DBA who found Bug 6175584.
And in the lenghty process of convincing metalink to see that bug, he learned more about Ora-00060 then you will ever need - until version 11.2 comes along at least.

Anyway, this bitmap index...

SQL> Alter index monitor usage ;
Showed the index was indeed being used. But by whom ?
Only by the one intended job or also by other more user-relevant queries?

Peter, who is the local onsite-DBA where I am only a passing consultant, finally managed to convince the "architecs" and other title-bearers that the index should go.

It went, and with it went the deadlock-error.
General Performance also felt better.

Apparently nobody but the batch job was using that index.
Or if they did, it was not helping them.
Problem Solved.

Then Anjo Kolk, over a nice meal, mentioned he had seen something similar, and had it fixed while on his hand-free carphone.
And I should have realized how simple it is to "prove" this case to the Architects.

A Query like this:
SQL> select sql_id, object_name
from v$sql_plan
where object_name like '%STATUS_BMP';

Or a similar qry against the WRH$ table will show you immediately which statements have used a particular index !

Peter is off to check a few other indexes straight away.
Kuddos to Anjo for pointing it out. Sometimes a Simple solution is right under your nose. But it takes a nice "incident" to learn even simple lessons.

As for the developers and architects who concoct it all: No hard feelings. After all, the Funtastic manual says bitmpas are for low-cardinality colums. And at least their tests had been really fast using "this bitmap thing".
They had no bad intentions, and where would we be without their complex solutions and indexes eh ?

1 comment:

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