Comments on: How many ways ?

By: Meaningless Keys « Oracle Scratchpad

Meaningless Keys « Oracle Scratchpad — Fri, 29 Dec 2006 18:21:50 +0000

[...] Pragmatic solution – put a stack of hints into the SQL. [...]

By: in praise of CBO » Andy C

in praise of CBO » Andy C — Mon, 11 Dec 2006 21:49:14 +0000

[...] Jonathan Lewis poses a very interesting question: Have you ever wondered how hard the optimizer has to work to produce a plan ? [...]

By: Jonathan Lewis

Jonathan Lewis — Mon, 11 Dec 2006 16:48:05 +0000

Jason, I can't say. But I suspect the idea is that bitmap conversion should make and-equal redundant, since (a) it isn't restricted to equality on single-column indexes and (b) it isn't restricted to the arbitrary limit that and-equal has of five indexes. Unfortunately, bitmap indexes and the bitmap conversion are not available if you're running Standard Edition.

By: Jason Bucata

Jason Bucata — Mon, 11 Dec 2006 16:14:15 +0000

So what replaces AND_EQUAL in 10g+??

By: Jonathan Lewis

Jonathan Lewis — Mon, 11 Dec 2006 10:16:51 +0000

Joachim, if you check carefully you will see that the initial specification said: "where each table has a few b-tree indexes" deliberately excluding bitmaps. Then the join mechanism I mentioned was: "and bitmap conversion - for a total of 6". You are perhaps not familiar with the fact that the optimizer can do a btree/bitmap conversion even in the complete absence of any bitmap indexes.

By: Joachim Rupik

Joachim Rupik — Mon, 11 Dec 2006 09:02:24 +0000

Jonathan, Yes I know, but please notice when bitmaps are not available won’t be taken into consideration any bitmap operation – that is a kind of a clever mechanisms for minimizing the work.

By: Jonathan Lewis

Jonathan Lewis — Mon, 11 Dec 2006 08:09:27 +0000

Joachim, On the contrary, nothing is naturally excluded. Things are excluded because the optimizer does the arithmetic that excludes them and, as I pointed out, it "has all sorts of clever mechanisms for minimising the work". See Chapter 14 of Cost Based Oracle Fundamentals for a few examples of the clever mechanisms.

By: Joachim Rupik

Joachim Rupik — Mon, 11 Dec 2006 07:49:59 +0000

Jonathan, please consider that not all cases are available in real life, some combinations are naturally excluded by remaining parts of sql statement, object configuration and available statistics, what makes a number of permutations much smaller, however big enough to cost much.

By: Jonathan Lewis

Jonathan Lewis — Sun, 10 Dec 2006 15:05:43 +0000

Jaffar, I had to be running the 10053 to determine how many join orders the optimizer examined – so I did check that the parameter was set to 2,000 at the same time.

By: Jaffar

Jaffar — Sun, 10 Dec 2006 14:53:29 +0000

Jonathan, if I remember correctly, when we enable 10053 event, the trace gives the list parameters and their values used by the Optimizer and you see this parameter as well.
Have you try to enable the 10053 event on your example to find out what is the value for this parameter?
I am just curious becuase its a special case.

Jaffar