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Analysis:
When the method JOIN::choose_subquery_plan() decided to apply
the IN-TO-EXISTS strategy, it set the unit and select_lex
uncacheable flag to UNCACHEABLE_DEPENDENT_INJECTED unconditionally.
As result, even if IN-TO-EXISTS injected non-correlated predicates,
the subquery was still treated as correlated.
Solution:
Set the subquery as correlated only if the injected predicate(s) depend
on the outer query.
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in EXPLAIN as select_type==MATERIALIZED.
Before, we had select_type==SUBQUERY and it was difficult to tell materialized
subqueries from uncorrelated scalar-context subqueries.
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and 'derived_with_keys'. Now they are set on by default.
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Analysis:
The bug is a result of an incomplete fix for bug lp:869036.
That fix didn't take into account that there may be a case
when ther are no NULLs in the materialized subquery, however
all columns without NULLs may not be grouped in the only
non-null index. This is the case when the left subquery expression
has nullable columns.
Solution:
The patch handles two missing sub-cases of the case when there are
no value (non-null matches) for any outer expression, and there are
both NULLs and non-NUll values in the outer reference.
a) If the materialized subquery contains no NULLs there cannot be a
partial match, because there are no NULLs in those columns where
the outer reference has no NULLs.
b) If the materialized subquery contains NULLs, but there exists a
column, such that its corresponding outer expression has no NULL,
and this column also has no NULL. Then there cannot be a partial
match either.
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Apart from the fix, the patch also adds few more unrelated test
cases for partial matching, and fixes few typos.
Analysis:
This bug uncovered that partial matching via rowid intersection
didn't handle the case when:
- the left IN argument has some NULLs,
- there are no non-null value matches, and there is no non-null
column,
- the subquery columns that are not covered with the NULLs in
the left IN argument contain at least one row, such that it
has NULL values in all columns where the left IN operand has
no NULLs.
In this case there is a partial match.
In addition the analysis of the related code uncovered incorrect
handling of few other related cases.
Solution:
The solution for the bug is to check if there exists a row with
NULLs in all columns other than the ones having NULL in the
let IN operand.
The check is implemented via checking whether the bitmaps that
store NULL information in class Ordered_key have a non-empty
intersection for the relevant columns.
The intersection itself is implemented via the function
bitmap_exists_intersection() in my_bitmap.c.
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Analysis:
The cause of the bug was that the method
subselect_rowid_merge_engine::partial_match()
was not designed for re-execution within the
same query. Specifically, it didn't cleanup
the bitmap of matching keys after completion.
The test query requires double execution of
the IN predicate because it first checks the
predicate as a constant condition. The second
execution during regular execution used the bitmap
of matching keys produced by the first execution
instead of starting with a clean one.
Solution:
Cleanup the bitmap of matching keys at the end of
the partial matching procedure.
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off by default.
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- Added an initial set of feature-specific test cases
- Handled the special case where the materialized subquery of an
IN predicates consists of only NULL values.
- Fixed a bug where making Item_in_subselect a constant,
didn't respect its null_value value.
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Analysis:
This is a bug in MWL#68, where it was incorrectly assumed
that if there is a match in the only non-null key, then
if there is a covering NULL row on all remaining NULL-able
columns there is a partial match. However, this is not the case,
because even if there is such a null-only sub-row, it is not
guaranteed to be part of the matched sub-row. The matched sub-row
and the NULL-only sub-row may be parts of different rows.
In fact there are two cases:
- there is a complete row with only NULL values, and
- all nullable columns contain only NULL values.
These two cases were incorrectly mixed up in the class member
subselect_partial_match_engine::covering_null_row_width.
Solution:
The solution is to:
- split covering_null_row_width into two members:
has_covering_null_row, and has_covering_null_columns, and
- take into account each state during initialization and
execution.
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In addition to the bug fix explained below, the patch performs
few renames, and adds some comments to avoid similar problems.
Analysis:
The failed assert was due to a bug in MWL#68, where it was
incorrectly assumed that the size of the bitmap
subselect_rowid_merge_engine::null_only_columns should be
the same as the size of the array of Ordered_keys.
The bitmap null_only_columns contains bits to mark columns
that contain only NULLs. Therefore the indexes of the bits
to be set in null_only_columns are different from the indexes
of the Ordered_keys. If there is a NULL-only column that appears
in a table after the last partial match column with Ordered_key,
this NULL-only column would require setting a bit with index
bigger than the size of the bitmap null_only_columns.
Accessing such a bit caused the failed assert.
Solution:
Upon analysis, it turns out that null_only_columns is not needed
at all, because we are looking for partial matches, and having
such columns guarantees that there is a partial match for any
corresponding outer value.
Therefore the patch removes
subselect_rowid_merge_engine::null_only_columns.
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Resolved all conflicts and failures.
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|/ / |
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|/ /
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Resolved all conflicts, bad merges and fixed a few minor bugs in the code.
Commented out the queries from multi_update, view, subselect_sj, func_str,
derived_view, view_grant that failed either with crashes in ps-protocol or
with wrong results.
The failures are clear indications of some bugs in the code and these bugs
are to be fixed.
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Analysis:
The send_data method of the result sink class used to collect
data statistics about materialized subqueries incorrectly assumed
that duplicate rows are removed prior to calling send_data. As
a result the collected statistics was wrong, which resulted in
an incorrect maximal number of keys in the Ordered_key buffer.
Solution:
Try to insert each row into the materialized temp table before
collecting statistics, and if the insertion results in a duplicate
row, do not count the current row.
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The set of Ordered keys of a rowid merge engine is dense. Thus when
we decide not to create a key for a column that has only NULLs, this
column shouldn't be counted.
Notice that the caller has already precomputed the correct total
number of keys that should be created.
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The cause for this bug is that MariaDB 5.3 still processes derived tables
(subqueries in the FROM clause) by fully executing them during the parse
phase. This will be remedied by MWL#106 once merged into the main 5.3.
The assert statement is triggered when MATERIALIZATION is ON for EXPLAIN
EXTENDED for derived tables with an IN subquery as follows:
- mysql_parse calls JOIN::exec for the derived table as if it is regular
execution (not explain).
- When materialization is ON, this call goes all the way to
subselect_hash_sj_engine::exec, which creates a partial match engine
because of NULL presence.
- In order to proceed with normal execution, the hash_sj engine substitutes
itself with the created partial match engine.
- After the parse phase it turns out that this execution was part of
EXPLAIN EXTENDED, which in turn calls
Item_cond::print -> ... -> Item_subselect::print,
which calls engine->print().
Since subselect_hash_sj_engine::exec substituted the current
Item_subselect engine with a partial match engine, eventually we call
its ::print() method. However the partial match engines are designed only
for execution, hence there is no implementation of this print() method.
The fix temporarily removes the assert, until this code is merged with
MWL#106.
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The bug is a result of the following change by Monty:
Revision Id: monty@askmonty.org-20100716073301-gstby2062nqd42qv
Timestamp: Fri 2010-07-16 10:33:01 +0300
Where Monty changed the queues interface and implementation.
The fix adjusts the queue_remove call to the new interface.
mysql-test/r/subselect_partial_match.result:
Added new file for tests related to MWL#89.
mysql-test/t/subselect_partial_match.test:
Added new file for tests related to MWL#89.
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