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Fixed bug lp:800679

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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.
This commit is contained in:
unknown 2011-06-28 15:48:44 +03:00
commit 78fa331eaa
3 changed files with 45 additions and 2 deletions
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23
mysql-test/t/subselect_mat_cost_bugs.test
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@ -326,3 +326,26 @@ select c1 from t1 where c1 in (select kp1 from t2 where kp2 = 10 and c2 = 4) or
select c1 from t1 where c1 in (select kp1 from t2 where kp2 = 10 and c2 = 4) or c1 > 7;
drop table t1, t2;
--echo #
--echo # LP BUG#800679: Assertion `outer_join->table_count > 0' failed in
--echo # JOIN::choose_subquery_plan() with materialization=on,semijoin=off
--echo #
CREATE TABLE t1 ( f1 int);
insert into t1 values (1),(2);
CREATE TABLE t2 ( f1 int);
insert into t2 values (1),(2);
SET @@optimizer_switch='materialization=on,semijoin=off';
EXPLAIN
SELECT * FROM t1
WHERE (f1) IN (SELECT f1 FROM t2)
LIMIT 0;
SELECT * FROM t1
WHERE (f1) IN (SELECT f1 FROM t2)
LIMIT 0;
drop table t1, t2;