From: Jorgen Loland
Date: December 1 2010 11:32am
Subject: Re: bzr commit into mysql-trunk branch (roy.lyseng:3287) Bug#57525
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On 12/01/2010 12:22 PM, Roy Lyseng wrote:
> Hi Jørgen,
>
> On 01.12.10 09.41, Jorgen Loland wrote:
>> Roy,
>>
>> thank you for the patch. I agree to the logic of setting
>> OUTER_REF_TABLE_BIT in
>> all selects that have subselects with outer dependencies. However, I
>> wonder if
>> this is the correct place to fix the bug. See comment below.
>>
>> > === modified file 'sql/item.cc'
>> > --- a/sql/item.cc 2010-11-14 18:09:32 +0000
>> > +++ b/sql/item.cc 2010-11-18 09:47:04 +0000
>> > @@ -2359,17 +2366,16 @@ void Item_field::fix_after_pullout(st_se
>> > */
>> > st_select_lex *child_select= context->select_lex;
>> >
>> > - if (child_select->outer_select() != depended_from)
>> > + while (child_select->outer_select() != depended_from)
>> > {
>> > /*
>> > The subquery on this level is outer-correlated with respect to the
>> field
>> > */
>> > Item_subselect *subq_predicate= child_select->master_unit()->item;
>> > - subq_predicate->used_tables_cache|= OUTER_REF_TABLE_BIT;
>> > - }
>> >
>> > - while (child_select->outer_select() != depended_from)
>> > + subq_predicate->used_tables_cache|= OUTER_REF_TABLE_BIT;
>> > child_select= child_select->outer_select();
>> > + }
>>
>> AFAIU, the problem in this bug was that SELECT C did not have ORTB set
>> after
>> semijoin transformation but with the patch it does:
>>
>> Before semijoin
>> ---------------
>> A - t1 <- OUTER_REF_TABLE_BIT not set
>> \
>> B - t2 <- OUTER_REF_TABLE_BIT not set
>> \
>> C - t3 <- OUTER_REF_TABLE_BIT not set
>> \
>> D - t4 <- ORTB set, depended_from points to A
>> where t1.x=t4.y
>>
>>
>> After semijoining B into A
>> --------------------------
>> A - t1, t2 <- OUTER_REF_TABLE_BIT not set
>> \
>> C - t3 <- pre-patch: OUTER_REF_TABLE_BIT not set
>> \ post-patch: OUTER_REF_TABLE_BIT set
>> D - t4 <- ORTB set, depended_from points to A
>> where t1.x=t4.y
>>
>> With the logic that SELECT C should have ORTB set because it contains a
>> subselect that has a dependency to ot1, wouldn't it be more correct to
>> set this
>> bit both for SELECT B and C during the prepare stage rather than
>> during semijoin
>> transformation?
>>
>
> Sorry for confusing you - the ORTB is indeed set for subquery predicates
> in SELECT B and C after initial resolver stage. So my patch just tries
> to preserve that information during fix_after_pullout() calls.

Aahh... in that case: patch approved.

-- 
Jørgen Løland | Senior Software Engineer | +47 73842138
Oracle MySQL
Trondheim, Norway