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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
parent 95cba1043e
commit 78fa331eaa
3 changed files with 45 additions and 2 deletions
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3
sql/opt_subselect.cc
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@@ -4360,7 +4360,7 @@ bool JOIN::choose_subquery_plan(table_map join_tables)
by the IN predicate.
*/
outer_join= unit->outer_select() ? unit->outer_select()->join : NULL;
if (outer_join)
if (outer_join && outer_join->table_count > 0)
{
/*
The index of the last JOIN_TAB in the outer JOIN where in_subs is
@@ -4373,7 +4373,6 @@ bool JOIN::choose_subquery_plan(table_map join_tables)
JOIN_TAB, and their join_tab_idx remains MAX_TABLES. Such predicates
are evaluated for each complete row of the outer join.
*/
DBUG_ASSERT(outer_join->table_count > 0);
max_outer_join_tab_idx= (in_subs->get_join_tab_idx() == MAX_TABLES) ?
outer_join->table_count - 1:
in_subs->get_join_tab_idx();