On Oct 11, 2006, at 3:00 PM, Andrew Sullivan wrote:
On Wed, Oct 11, 2006 at 12:40:42PM -0400, Gregory Stark wrote:
poorly written query. In fact Oracle is going in the opposite
direction of
even relying on hints internally. Its plan stability feature
depends on
generating and storing hints internally associated with every query.
But IBM, whose DB2 planner and optimiser is generally regarded as way
better than Oracle's (at least by anyone I know who's used both),
doesn't like hints. The IBM people all say the same thing Tom has
said before: that the work to design the thing correctly is better
spent making the planner and optimiser parts smarter and cheaper,
because out of that work you also manage not to have the DBA
accidentally mess things up by simple-minded rule-based hints. (Note
that I'm not trying to wade into the actual argument; I'm just
pointing out that even the biggest industry people don't agree on
this point.)
DBAs can mess things up already if they misuse the tools they are
provided. Like 'rm'. Which is there, but should _RARELY_ be used on
database datafiles. The argument that people _could_ use them in a
bad way is silly. Of course, they could use them in a bad way,
that's not an _argument_. Everyone agrees people can be stupid.
However, the planner will never be perfect. I would like to see 1
out of every 500,000 queries actually benefit from a hint system
(which means that 499,999 of the queries were planned perfectly fine
by the planner). To fix my one query, that is crucially important to
my business, it is a much more sane approach to hint the system to
change its plan than it is to have to upgrade my binaries.
// Theo Schlossnagle
// CTO -- http://www.omniti.com/~jesus/
// OmniTI Computer Consulting, Inc. -- http://www.omniti.com/
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