Comments on: What is a hint ?

By: Defy Logic – the Cost-Based Optimizer does Not Select the Lowest Cost Plan – Implicit Data Type Conversion « Charles Hooper's Oracle Notes

Fri, 28 May 2010 06:02:10 +0000

[...] Lewis stated here (a blog article from 2006) that: “It’s a great shame that Oracle Corp. decided to use the name “hints” for its [...]

By: Ignoring Hints – 2 « Oracle Scratchpad

Ignoring Hints – 2 « Oracle Scratchpad — Thu, 11 Feb 2010 19:29:54 +0000

[...] one of them “ignore” the use_hash() hint and do a nested loop ? Remember: the optimizer does not ignore hints (with the usual caveats about legality, spelling, syntax and [...]

By: Quiz Night « Oracle Scratchpad

Quiz Night « Oracle Scratchpad — Tue, 15 Dec 2009 21:34:28 +0000

[...] postings on hints: Hints – again Ignoring Hints What is a hint ? Taking a hint Comments [...]

By: Functions « Oracle Scratchpad

Functions « Oracle Scratchpad — Sat, 07 Jul 2007 20:41:31 +0000

[...] from the function to access the entire data set and produce a result table – either with a no_merge view, or a materialized factored subquery – is a good starting step – then join this result into [...]

By: Jonathan Lewis

Jonathan Lewis — Thu, 15 Feb 2007 15:43:05 +0000

SP, this isn’t AskTom – I don’t answer arbitrary questions, especially when a proper answer would require a significant extract from the manuals. Here’s a quick clue, though: (1) yes, you can use materialized views in OLTP, but I’d be very careful about exactly how and wouldn’t expect to see them often. (2) materialized views are completely different from views – so cannot be compared in any sensible way. Materialized views require you to have a (potentially expensive) supporting infrastructure – which you have to pay for at some point.

By: sp

sp — Wed, 14 Feb 2007 23:05:33 +0000

Hi,
I have question regarding View and MView.

1.Can i use Mview in OLTP application ?
2.Does Mview is better than View ?
I mean when Appliction do query againest View its on the fly query again query where MView is stored in db so we can take advantage of better plan. Please Explain

Thanks,
SP

By: Jonathan Lewis

Jonathan Lewis — Thu, 04 Jan 2007 10:27:51 +0000

Hrishy, Email the trace file to me, and I’ll see if I can explain what’s going on.

My email address is jonathan@jlcomp.demon.co.uk.

By the way, one obvious example of why the /*+ use_hash() */ hint would be “ignored” – if the join condition wasn’t an equality, a hash join is not possible, so the hint would be illegal.

Updated 5th Jan following an email from Hrishy. My comment about the hash joins and equalities happened to be the appropriate explanation. The critical join condition in his case was a “between” clause.

By: Hrishy

Hrishy — Thu, 04 Jan 2007 10:07:42 +0000

Hi Jonathan

I can also provide a detailed 10053 trace and post it here in case you need to see exactly what was happening behind the scenes and my hint was repeatedly ignored.

regards
Hrishy

By: Hrishy

Hrishy — Thu, 04 Jan 2007 09:41:57 +0000

Hi Jonathan

Well i have seen a well written hint is ignored by the oracle optimizer.

I vaguely remeber the situation like this the query was returning zero rows and the optimizer knew it from the stats but the query developer had put in a hint to use a hash join and oracle always used a Nested Loop Join no matter what.

regards
Hrishy

By: Robert

Robert — Tue, 14 Nov 2006 04:54:30 +0000

>> Telling the optimizer what not to do is a lot easier than telling it to do only what you want.

AMEN to that !!!