Re: Oracle Hint Behavior

From: Michael Austin <>
Date: Wed, 30 Jan 2008 17:03:56 -0600
Message-ID: <Il7oj.9841$>

Mark D Powell wrote:
> On Jan 30, 9:28 am, "Dereck L. Dietz" <> wrote:

>> Oracle
>> Windows 2003 Server
>> What is the behavior of hints in Oracle 10g?  I knew that prior to 10g
>> Oracle would treat them as only "suggestions" but I thought in 10g they
>> would be mostly followed.
>> We have a query which is doing a full table scan even with hints being used
>> to try to get it to use an index.  I can see why it would choose a full
>> table scan considering the percentage of the table being returned but I'd
>> like to be able to explain why the hint is being ignored now.
>> Thanks.

> Hints are directives to the optimizer and if valid will be followed;
> however, there are operations on the SQL such as query transformation
> that can render what is syntaxically a valid hint invalid. There are
> also optimizer decisions that are made prior to the hint being
> considered that can render the hint invalid such as the choice to hash
> join.
> Multiple hints are often necessary to try to force a specific plan and
> you may need to disable the pushing of predicates or sub-queries
> (hints available) to get the plan you want to test.
> To get help for your specific SQL you should post the SQL and the
> explain plan. Otherwise all anyone can provide is general cases.
> HTH -- Mark D Powell --

There is an interesting way to influence the optimizer without the use of hints that is not written in any books and coding it seems - well, ummm, redundant.

I have used this on many different queries where we could not get the optimizer to use a particular index no matter how hard we tried.

This is an over-simplified query, but you will get the point.

select a.a, b.b, c.c
from tablea a, tableb b, tablec c
where b.a = a.a

   and c.a = a.a
   and c.a = a.a !!!<<<<NOTE added a second time ......

The optimizer sees this and says "OH!!! you really did want THAT Index..."

Again this is an over-simplifed example. The optimizer was pulled out of DEC Rdb right after Oracle acquired Rdb back in '94/'95 timeframe. Working with Rdb Engineering, this is something we discovered shortly before the DEC fire sale. Not too long ago, I had a colleague that was having a horrendous time with performance and noticed that there was an index not being used that "should have been". I offered this solution and the query used the index and performance went from ~30 minutes to 2-3 minutes (data warehouse timeframe using CBO). I have more recently had similar results using 10gR2.

Yeah, it's a kludge, but it does work -- and BTW - still works in Rdb as well.

cool part is that this is all ANSI compliant and if you ever have to move to another db engine (heaven forbid (cough,cough)) - it will still execute unchanged. May not get the same performance, but it would still run... Received on Wed Jan 30 2008 - 17:03:56 CST

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