Fix LP BUG#680038

Analysis:
Single-row subqueries are not considered expensive and are
evaluated both during EXPLAIN in to detect errors like
"Subquery returns more than 1 row", and during optimization to
perform constant optimization.

The cause for the failed ASSERT is in JOIN::join_free, where we set
  bool full= (!select_lex->uncacheable && !thd->lex->describe);
Thus for EXPLAIN statements full == FALSE, and as a result the call to
JOIN::cleanup doesn't call JOIN_TAB::cleanup which should have
called table->disable_keyread().

Solution:
Consider all kinds of subquery predicates as expensive.

1 files changed, 11 insertions, 2 deletions

diff --git a/sql/item_subselect.h b/sql/item_subselect.h
index 85e0191348e..eae9b0e558c 100644
--- a/sql/item_subselect.h
+++ b/sql/item_subselect.h
@@ -194,6 +194,7 @@ public:
   */
   bool is_evaluated() const;
   bool is_uncacheable() const;
+  bool is_expensive() { return TRUE; }
 
   /*
     Used by max/min subquery to initialize value presence registration
@@ -210,6 +211,16 @@ public:
   {
     return trace_unsupported_by_check_vcol_func_processor("subselect");
   }
+  /**
+    Callback to test if an IN predicate is expensive.
+
+    @notes
+    The return value affects the behavior of make_cond_for_table().
+
+    @retval TRUE  if the predicate is expensive
+    @retval FALSE otherwise
+  */
+  bool is_expensive_processor(uchar *arg) { return TRUE; }
   Item *safe_charset_converter(CHARSET_INFO *tocs);
 
   /**
@@ -484,8 +495,6 @@ public:
   bool init_left_expr_cache();
   /* Inform 'this' that it was computed, and contains a valid result. */
   void set_first_execution() { if (first_execution) first_execution= FALSE; }
-  bool is_expensive_processor(uchar *arg);
-  bool is_expensive() { return TRUE; }
   bool expr_cache_is_needed(THD *thd);
   
   /*