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.
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.
