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Re: Which of these selects is faster?

From: Scott Silvey <scott_at_baygate.bayarea.net>
Date: 15 Apr 1999 19:45:56 GMT
Message-ID: <7f5flk$9rm$1@news.bayarea.net>


Frank van Bortel <f.van.bortel_at_vnl.nl> wrote:
:>>If the where clause has ANDs then it is evaluated from the end of the query
:>>towards the beginning, but if it has ORs it is evaluated starting closest to
:>>the FROM clause and working its way downward.
:>
:> Thanks, it's helpful to know that things are sufficiently complex
:> that it seems oracle did not intend for users to predict the order
:> of evaluation. (That sucks of course, and I'd like to see them fix
:> it so that you could at least have the option to force a certain
:> order.)
:>
:> How did you come by this information? Trial and error? Oracle tech
:> support? Actual documentation? Source code?
:
:It quite logical, and documented.

Documented? WHERE?!

:For the OR's, there's no point in evaluating, after a TRUE condition
:is found (TRUE or FALSE still is TRUE)- so this way of evaluating will
:actually speed up things.

Obviously. Check the beginning of this thread and you'll see this is the whole point of the thread. The problem is, to exploit this optimization fully, you need to know ahead of time what the order of evaluation is. Furthermore, you need to know that this convention is deterministic and consistent across all versions.

If Oracle has documented the order of evaluation, then they have a public contract with their developers and are obligated to guarantee that their engine behaves accoringly.

THIS is what I seek. But few people seem to have a solid idea what goes on underneath and up to now nobody has said anything about documentation.

:As for the ANDs, I think it's been there since V4, and I _know_
:it's still there because of V6 backward compatibility.

IT's still there? What do you mean by "IT"?

:I would need to check, but my best guess is the concepts manual is
:the place to find the documentation.

I would probably have to use Oracle's online documentation. Can you provide an exact-word phrase from a subsection heading in the general document so I can do a search and perhaps find it?

Thanks,

Scott Received on Thu Apr 15 1999 - 14:45:56 CDT

Original text of this message

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