Connor - Congratulations
Stephane,
I'm in the camp of "don't put hints in when not necessary". My reasons for
this include (some have been mentioned already):
1) You are limiting functionality when the database is upgraded - I have
seen several examples where Oracle went from 7 to 8 and noone looks at
every SQL statement to reevaluate the validity of every hint.
2) Changes in data volume or distribution can invalidate the benefits of
the hint. The classic example would be a developer coding hints based on
anything other than a perfect copy of production data.
3) Index renaming can cause real grief. In a test environment recently I
had to create a subset of data and in my rush / laziness to get the job
done asap I renamed the existing table (we needed the data back later on)
and created a new table and new indexes. The hints of course then failed
and performance died until I realised the problem.
Like I said - my motto is that if it doesn't require a hint then don't add
one. Some people wisely mentioned that there are categories of hints -
those which add a requirement (like append, first_rows and to a lesser
degree parallel) are pretty safe, whereas those which dictate execution
(like index and use_hash) are a bit more risky. Finally, you can sometimes
use hints (like index_desc and index_asc, combined with rownum = 1) to get
a minimum or maximum value without using a sort operation - and these can
give tremendous performance gains for relatively little risk.
Regards,
Mark.
"Stephane Paquette"
<[EMAIL PROTECTED] To: Multiple recipients of list
ORACLE-L <[EMAIL PROTECTED]>
rdlife.ca> cc:
Sent by: [EMAIL PROTECTED] Subject: RE: Oracle position
on hints
11/03/2003 08:15
Please respond to
ORACLE-L
Congratulations !
I had never thought that my question on hints would have generated so much
trafic.
Stephane
-----Original Message-----
McDonald
Sent: Saturday, March 08, 2003 8:54 PM
To: Multiple recipients of list ORACLE-L
Just to add my meaningless diatribe to the discussion,
I think that there are "good" hints and "bad" hints.
For example, first_rows(n) is a good hint, in that you
are giving the optimizer more information as to what
your motivations are for this query, without stifling
it into a choice that may not be appropriate down the
track.
Hints such as index(tab,idx) are "bad" hints, because
you are more exposed to grief. If the index is
renamed, then its no longer a hint. If the data
distribution changes, or the index is rebuilt in
reverse or compressed, then it may no longer be a good
option, etc.
In terms of the vendor speak of "hints imply weak
optimizer" I think thats nonsense. Having worked with
DB2 in the past, nothing used to be more irritating
then coming up with bizarre convolutions on an
otherwise simple SQL to ensure that certain indexes
were or were not chosen.
Cheers
Connor
PS - Its probably a little sad that I'm chit-chatting
on hints on the morning of my wedding day, but hey,
you gotta pass the time somehow whilst the other half
is getting the hair and makeup done :-)
--- "Khedr, Waleed" <[EMAIL PROTECTED]> wrote: >
It's like the Auto-pilot
>
> -----Original Message-----
> To: Multiple recipients of list ORACLE-L
> Sent: 3/8/03 2:13 PM
>
> Stephane
> I don't see how Oracle could ever have an
> official position on hints.
> If
> they say hints are going away, many developers
> panic. If they say hints
> are
> great, then Oracle competitors will claim Oracle has
> a bad optimizer.
> A key political issue in some shops is "Using
> hints makes our code
> Oracle-specific". My reply is that hints are only
> comments that another
> database will ignore.
>
> Dennis Williams
> DBA, 40%OCP, 100% DBA
> Lifetouch, Inc.
> [EMAIL PROTECTED]
>
>
> -----Original Message-----
> Sent: Friday, March 07, 2003 12:19 PM
> To: Multiple recipients of list ORACLE-L
>
>
> Hi,
>
> I have to write a SQL and PL/SQL guide for our
> developpers.
> Some are good but most of them can use help.
>
> I'm not a big fan of hints except for insert append,
> I used them only if
> I
> really need to.
>
> In last october I went to a 3 days DB2 UDB course,
> unless I miss
> something
> there are no hint with DB2 UDB.
> So I was thinking that when the Oracle optimiser
> would be more mature
> maybe
> hints would go away.
>
> I just want to know what is Oracle direction with
> hints.
>
> TIA
>
> Stephane
>
>
> -----Original Message-----
> Robert - IL
> Sent: Friday, March 07, 2003 12:01 PM
> To: Multiple recipients of list ORACLE-L
>
>
> This is facinating, where does this come from? This
> is the second time
> in a
> week that I've heard this statement being made by
> someone. Hints will
> never
> go away IMHO, and Oracle continues to add more and
> more of them. I was
> told
> by someone at a client site that they were told by
> an Oracle instructor
> that
> they should not use hints in 9i....
>
> RF
>
> -----Original Message-----
> To: Multiple recipients of list ORACLE-L
> Sent: 3/7/2003 10:04 AM
>
> Hi,
>
> Does Oracle have an official position on hints ?
> Will they go away as the optimiser is becoming
> bettre or they are there
> to stay ?
>
> TIA
>
>
>
> Stephane Paquette
>
>
> Administrateur de bases de donnees
>
> Database Administrator
>
> Standard Life
>
> www.standardlife.ca
>
> Tél. (514) 925-7187
>
> [EMAIL PROTECTED]
> <mailto:[EMAIL PROTECTED]>
>
>
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=====
Connor McDonald
web: http://www.oracledba.co.uk
web: http://www.oaktable.net
email: [EMAIL PROTECTED]
"GIVE a man a fish and he will eat for a day. But TEACH him how to fish,
and...he will sit in a boat and drink beer all day"
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