RE: ** ordered use_nl question

  • From: "Jonathan Lewis" <jonathan@xxxxxxxxxxxxxxxxxx>
  • To: <oracle-l@xxxxxxxxxxxxx>
  • Date: Tue, 20 Jun 2006 07:41:32 +0100



Wolfgang,

The comma in use_nl (a,b,c,d) is allowed
(at least in some versions of Oracle) even
though it may not be expected.

The interpretation of the hint is that it is
simply a shorthand for
   use_nl(a)
   use_nl(b)
   use_nl(c)
   use_nl(d)

I think it would be quite helpful if the OP
posted the original SQL, showing us the
actual use of hint. There is no reason for
Oracle to ignore something like
/*+
     ordered
     use_nl(t1, t4, t2, t3)
*/
if its use is valid.

See you in Zurich this evening.

Regards

Jonathan Lewis
http://www.oracle.com/technology/community/oracle_ace/ace1.html#lewis

The Co-operative Oracle Users' FAQ
http://www.jlcomp.demon.co.uk/faq/ind_faq.html

Cost Based Oracle: Fundamentals
http://www.jlcomp.demon.co.uk/cbo_book/ind_book.html



Date: Mon, 19 Jun 2006 12:03:15 -0700
From: Wolfgang Breitling <breitliw@xxxxxxxxxxxxx>
Subject: RE: ** ordered use_nl question

The syntax is still not right. Read the documentation (as I did): No comma in
the list within the use_nl hint. Also, according to the documentation, the table
(s) listed in the hint "is the name or alias of a table to be used as the inner
table of a nested loops join". I know that a list is legal, I just don't know
what that is supposed to mean in terms of being the inner table in an NL join.
I always use a single alias in the use_nl hint, except I hardly use such hints
at all. If the selectivity of the predicate used to select the driving table in
the current plan, maybe a better approach would be to collect a histogram so
that the optimizer "knows" that.



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