RE: oracle can ignore hints
- From: "Lex de Haan" <lex.de.haan@xxxxxxxxxxxxxx>
- To: <oracle-l@xxxxxxxxxxxxx>
- Date: Fri, 12 Mar 2004 12:45:18 +0100
excellent explanation of Oracle hints. the only exception I know of is the
STAR TRANSFORMATION hint; there are some built-in heuristics that must be
met to open up that join optimization -- but even there you could argue that
if the method is not even considered in the first place, the hint will not
change that behavior; the CBO will only do so when the start transformation
method is initially considered but rejected.
cheers,
Lex.
-----Original Message-----
From: oracle-l-bounce@xxxxxxxxxxxxx
[mailto:oracle-l-bounce@xxxxxxxxxxxxx]On Behalf Of Niall Litchfield
Sent: vrijdag 12 maart 2004 12:06
To: oracle-l@xxxxxxxxxxxxx
Subject: RE: oracle can ignore hints
Comments embedded - catching up on 400 messages after 2 days away :(
> -----Original Message-----
> From: Jared.Still@xxxxxxxxxxx
> Sent: 10 March 2004 13:22
> To: Jared.Still@xxxxxxxxxxx; oracle-l@xxxxxxxxxxxxx
> Subject: Re: oracle can ignore hints
>
>
> Correct, so did Oracle 'ignore' the hint?
>
> I guess the point is this: does Oracle ignore your hints,
> or are they merely unusable?
I don't think that the experiment goes quite far enough.
I have repeated it with the following modification
Run the statement without the hint and c1 is null
Run the statement with the hint and c1 is null.
Next
Run the statement without the hint and c1 is not null
Run the statement with the hint and c1 is not null.
I.E. First see what oracle does on its own, then see what adding the hint
does.
Output each of these to a separate 10053 trace file and compare.
In the null case Oracle *never* considers the index (as you should expect).
In the not null case Oracle considers the index and rejects it without the
hint in favour of the FTS, with the hint it only considers the index as an
access path into t.
In other words I think this demonstrates quite well that hints (at least
access path hints) don't ADD new access paths for consideration, but
restrict the available access paths. Thus hinting an index that cannot be
used* and so will never be considered is not (IMO) a case of "ignoring" the
hint, but a case of supplying an invalid hint (just as if I'd supplied
select /*+ index (idx t) */ to the query). In general I find this way of
thinking about hints (as restricting the available choices for the
optimizer) as quite helpful. It enables you when the CBO is 'ignoring' your
hint to ask questions like - is the access path that I want actually
available, have I restricted the optimizer's choices enough - as well as
giving a hint (sorry for the pun) as to why bright people often look down on
hints or treat them as a quick fix/ workaround. If you view every hint you
give as handicapping Oracle in some way you tend to avoid sprinkling them
liberally thorughout your code.
> You could also try this with a bitmapped index.
>
> Jared
And actually there is a prediction that can be made from the above for this
case. Bitmapped indexes index nulls, therefore the bitmapped index will be
available to the optimizer. Thus I predict that the bitmapped index will be
*considered* without the hint (i.e it will show in a 10053 trace), and will
be *chosen* with the hint.
Niall Litchfield
Oracle DBA
Audit Commission
+44 117 975 7805
* NB it is perfectly possible to hint Oracle to use an index that is marked
'UNUSABLE' I think of this as a special case especially as the unhinted
optimizer will also choose an 'UNUSABLE' index. I regard this as a bug - to
me Oracle knows the index is unusable so it should not consider it at all.
You can look at it as a feature (reducing invalidations of execution plans
when tables move/indexes are set unusable).
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