splittable derived
If one of joined tables of the processed query is a materialized derived
table (or view or CTE) with GROUP BY clause then under some conditions it
can be subject to split optimization. With this optimization new equalities
are injected into the WHERE condition of the SELECT that specifies this
derived table. The injected equalities are generated for all join orders
with which the split optimization can employed. After the best join order
has been chosen only certain of this equalities are really needed. The
others can be safely removed. If it's not done and some of injected
equalities involve expressions over semi-joins with look-up access then
the query may return a wrong result set.
This patch effectively removes equalities injected for split optimization
that are needed only at the optimization stage and not needed for execution.
Approved by serg@mariadb.com
This bug could cause a crash for any query that used a derived table/view/CTE
whose specification was a SELECT with a GROUP BY clause and a FROM list
containing 32 or more table references.
The problem appeared only in the cases when the splitting optimization
could be applied to such derived table/view/CTE.
Do not materialize a semi-join nest if it contains a materialized derived
table /view that potentially can be subject to the split optimization.
Splitting of materialization of such nest would help, but currently there
is no code to support this technique.
The bug manifested itself when executing a query with materialized
view/derived/CTE whose specification was a SELECT query contained
another materialized derived and impossible WHERE/HAVING condition
was detected for this SELECT.
As soon as such condition is detected the join structures of all
derived tables used in the SELECT are destroyed. So optimization
of the queries specifying these derived tables is impossible. Besides
it's not needed.
In 10.3 optimization of a materialized derived table is performed before
detection of impossible WHERE/HAVING condition in the embedding SELECT.
in the tree bb-10.4-mdev7486
The crash was caused because of the similar problem as in mdev-16765:
Item_cond::excl_dep_on_group_fields_for_having_pushdown() was missing.
Condition can be pushed from the HAVING clause into the WHERE clause
if it depends only on the fields that are used in the GROUP BY list
or depends on the fields that are equal to grouping fields.
Aggregate functions can't be pushed down.
How the pushdown is performed on the example:
SELECT t1.a,MAX(t1.b)
FROM t1
GROUP BY t1.a
HAVING (t1.a>2) AND (MAX(c)>12);
=>
SELECT t1.a,MAX(t1.b)
FROM t1
WHERE (t1.a>2)
GROUP BY t1.a
HAVING (MAX(c)>12);
The implementation scheme:
1. Extract the most restrictive condition cond from the HAVING clause of
the select that depends only on the fields that are used in the GROUP BY
list of the select (directly or indirectly through equalities)
2. Save cond as a condition that can be pushed into the WHERE clause
of the select
3. Remove cond from the HAVING clause if it is possible
The optimization is implemented in the function
st_select_lex::pushdown_from_having_into_where().
New test file having_cond_pushdown.test is created.
main.derived_cond_pushdown: Move all 10.3 tests to the end,
trim trailing white space, and add an "End of 10.3 tests" marker.
Add --sorted_result to tests where the ordering is not deterministic.
main.win_percentile: Add --sorted_result to tests where the
ordering is no longer deterministic.
The bug was in the in the code of JOIN::check_for_splittable_materialized()
where the structures describing the fields of a materialized derived
table that potentially could be used in split optimization were build.
As a result of this bug some fields that were not usable for splitting
were detected as usable. This could trigger crashes further in
st_join_table::choose_best_splitting().
upon select with view and subqueries
This bug occurred when a splittable materialized derived/view
were used inside another splittable materialized derived/view.
The bug happened because the function JOIN::fix_all_splittings_in_plan()
was called at the very beginning of the optimization phase 2 at
the moment when the plan structure of the embedding derived/view
were not valid. The proper position for this call is the very
end of the optimization phase 1.
The crash happened because JOIN::check_for_splittable_materialized()
called by mistake the function JOIN_TAB::is_inner_table_of_outer_join()
instead of the function TABLE_LIST::is_inner_table_of_outer_join().
The former cannot be called before the call of make_outerjoin_info().