The patch differs from the original MySQL patch as follows:
- All test case differences have been reviewed one by one, and
care has been taken to restore the original plan so that each
test case executes the code path it was designed for.
- A bug was found and fixed in MariaDB 5.3 in
Item_allany_subselect::cleanup().
- ORDER BY is not removed because we are unsure of all effects,
and it would prevent enabling ORDER BY ... LIMIT subqueries.
- ref_pointer_array.m_size is not adjusted because we don't do
array bounds checking, and because it looks risky.
Original comment by Jorgen Loland:
-------------------------------------------------------------
WL#5953 - Optimize away useless subquery clauses
For IN/ALL/ANY/SOME/EXISTS subqueries, the following clauses are
meaningless:
* ORDER BY (since we don't support LIMIT in these subqueries)
* DISTINCT
* GROUP BY if there is no HAVING clause and no aggregate
functions
This WL detects and optimizes away these useless parts of the
query during JOIN::prepare()
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.
This bug in the function Loose_scan_opt::check_ref_access_part1 could lead
to choosing an invalid execution plan employing a loose scan access to a
semi-join table even in the cases when such access could not be used at all.
This could result in wrong answers for some queries with IN subqueries.
Analysis:
The cause of the bug was the changed meaning of
subselect_partial_match_engine::has_covering_null_row.
Previously it meant that there is row with NULLs in
all nullable fields of the materialized subquery table.
Later it was changed to mean a row with NULLs in all
fields of this table.
At the same time there was a shortcut in
subselect_rowid_merge_engine::partial_match() that
detected a special case where:
- there is no match in any of the columns with NULLs, and
- there is no NULL-only row that covers all columns with
NULLs.
With the change in the meaning of has_covering_null_row,
the condition that detected this special case was incomplete.
This resulted in an incorrect FALSE, when the result was a
partial match.
Solution:
Expand the condition that detected the special case with the
correct test for the existence of a row with NULL values in
all columns that contain NULLs (a kind of parially covering
NULL-row).
of the 5.3 code line after a merge with 5.2 on 2010-10-28
in order not to allow the cost to access a joined table to be equal
to 0 ever.
Expanded data sets for many test cases to get the same execution plans
as before.
Analysis:
In the test query semi-join merges the inner-most subquery
into the outer subquery, and the optimization of the merged
subquery finds some new index access methods. Later the
IN-EXISTS transformation is applied to the unmerged subquery.
Since the optimizer is instructed to not consider
materialization, it reoptimizes the plan in-place to take into
account the new IN-EXISTS conditions. Just before reoptimization
JOIN::choose_subquery_plan resets the query plan, which also
resets the access methods found during the semi-join merge.
Then reoptimization discovers there are no new access methods,
but it leaves the query plan in its reset state. Later semi-join
crashes because it assumes these access methods are present.
Solution:
When reoptimizing in-place, reset the query plan only after new
access methods were discovered. If no new access methods were
discovered, leave the current plan as it was.
Analysis:
The failed assert ensured that the choice of subquery strategy
is performed only for queries with at least one table. If there
is a LIMIT 0 clause all tables are removed, and the subquery is
neither optimized, nor executed during actual optimization. However,
if the query is EXPLAIN-ed, the EXPLAIN execution path doesn't remove
the query tables if there is a LIMIT 0 clause. As a result, the
subquery optimization code is called, which violates the ASSERT
condition.
Solution:
Transform the assert into a condition, and if the outer query
has no tables assume that there will be at most one subquery
execution.
There is potentially a better solution by reengineering the
EXPLAIN/optimize code, so that subquery optimization is not
done if not needed. Such a solution would be a lot bigger and
more complex than a bug fix.
mysql-test/r/subselect4.result:
Moved test case for LP BUG#718593 into the correct test file subselect_mat_cost_bugs.test.
mysql-test/t/subselect4.test:
Moved test case for LP BUG#718593 into the correct test file subselect_mat_cost_bugs.test.
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.
Analysis:
Build_equal_items_for_cond() rewrites the WHERE clause in such a way,
that it may merge the list join->cond_equal->current_level with the
list of child Items in an AND condition of the WHERE clause.
The place where this is done is:
static COND *build_equal_items_for_cond(THD *thd, COND *cond,
COND_EQUAL *inherited)
{
...
if (and_level)
{
args->concat(&eq_list);
args->concat((List<Item> *)&cond_equal.current_level);
}
...
}
As a result, later transformations on the WHERE clause may change the
structure of the list join->cond_equal->current_level without knowing this.
Specifically in this bug, Item_in_subselect::inject_in_to_exists_cond
creates a new AND of the old WHERE clause and the IN->EXISTS conditions.
It then calls fix_fields() for the new AND. Among other things, fix_fields
flattens all nested ANDs into one by merging the AND argument lists.
When there is a cond_equal for the JOIN, its list of Item_equal objects
is attached to the end of the original AND. When a lower-level AND is
merged into the top-level one, the argument list of the lower-level AND
is concatenated to the list of multiple equalities in the upper-level AND.
As a result, when substitute_for_best_equal_field processes the
multiple equalities, it turns out that the multiple equality list contains
the Items from the lower-level AND which were concatenated to the end of
the join->cond_equal->current_level list. This results in a crash because
this list must not contain any other Items except for the previously found
Item_equal ones.
Solution:
When performing IN->EXIST predicate injection, and the where clause is an
AND, detach the list of Item_equal objects before calling fix_fields on
the injected where clause.
After fix_fields is done, reattach back the multiple equalities list to
the end of the argument list of the new AND.
Split the tests for MWL#89 into two parts - one for bugs
(currently active), and one for functionality tets
(currently in progress, and thus disabled).
Disable the test for LP BUG#718593.
