Bug#33546: Slowdown on re-evaluation of constant expressions.

Constant expressions in WHERE/HAVING/ON clauses aren't cached and evaluated
for each row. This causes slowdown of query execution especially if constant
UDF/SP function are used.
      
Now WHERE/HAVING/ON expressions are analyzed in the top-bottom direction with
help of the compile function. When analyzer meets a constant item it
sets a flag for the tree transformer to cache the item and doesn't allow tree
walker to go deeper. Thus, the topmost item of a constant expression if
cached. This is done after all other optimizations were applied to
WHERE/HAVING/ON expressions
      
A helper function called cache_const_exprs is added to the JOIN class.
It calls compile method with caching analyzer and transformer on WHERE,
HAVING, ON expressions if they're present.
The cache_const_expr_analyzer and cache_const_expr_transformer functions are
added to the Item class. The first one check if the item can be cached and
the second caches it if so.
A new Item_cache_datetime class is derived from the Item_cache class.
It caches both int and string values of the underlying item independently to
avoid DATETIME aware int-to-string conversion. Thus it completely relies on
the ability of the underlying item to correctly convert DATETIME value from
int to string and vice versa.


mysql-test/r/func_like.result:
  A test case result is corrected after fixing bug#33546.
mysql-test/r/func_time.result:
  A test case result is corrected after fixing bug#33546.
mysql-test/r/select.result:
  Added a test case for the bug#33546.
mysql-test/r/subselect.result:
  A test case result is corrected after fixing bug#33546.
mysql-test/r/udf.result:
  Added a test case for the bug#33546.
mysql-test/t/select.test:
  Added a test case for the bug#33546.
mysql-test/t/udf.test:
  Added a test case for the bug#33546.
sql/item.cc:
  Bug#33546: Slowdown on re-evaluation of constant expressions.
  The cache_const_expr_analyzer and cache_const_expr_transformer functions are
  added to the Item class. The first one check if the item can be cached and
  the second caches it if so.
  Item_cache_datetime class implementation is added.
sql/item.h:
  Bug#33546: Slowdown on re-evaluation of constant expressions.
  Item_ref and Item_cache classes now returns basic_const_item
  from underlying item.
  The cache_const_expr_analyzer and cache_const_expr_transformer functions are
  added to the Item class.
sql/sql_select.cc:
  Bug#33546: Slowdown on re-evaluation of constant expressions.
          
  A helper function called cache_const_exprs is added to the JOIN class.
  It calls compile method with caching analyzer and transformer on WHERE,
  HAVING, ON expressions if they're present.
sql/sql_select.h:
  Bug#33546: Slowdown on re-evaluation of constant expressions.
  A helper function called cache_const_exprs is added to the JOIN class.

sql/sql_select.cc

@@ -1076,6 +1076,10 @@ JOIN::optimize()
   {
     conds=new Item_int((longlong) 0,1);	// Always false
   }
+
+  /* Cache constant expressions in WHERE, HAVING, ON clauses. */
+  cache_const_exprs();
+
   if (make_join_select(this, select, conds))
   {
     zero_result_cause=
@@ -17141,6 +17145,41 @@ bool JOIN::change_result(select_result *res)
   DBUG_RETURN(FALSE);
 }
 
+/**
+  Cache constant expressions in WHERE, HAVING, ON conditions.
+*/
+
+void JOIN::cache_const_exprs()
+{
+  bool cache_flag= FALSE;
+  bool *analyzer_arg= &cache_flag;
+
+  /* No need in cache if all tables are constant. */
+  if (const_tables == tables)
+    return;
+
+  if (conds)
+    conds->compile(&Item::cache_const_expr_analyzer, (uchar **)&analyzer_arg,
+                  &Item::cache_const_expr_transformer, (uchar *)&cache_flag);
+  cache_flag= FALSE;
+  if (having)
+    having->compile(&Item::cache_const_expr_analyzer, (uchar **)&analyzer_arg,
+                    &Item::cache_const_expr_transformer, (uchar *)&cache_flag);
+
+  for (JOIN_TAB *tab= join_tab + const_tables; tab < join_tab + tables ; tab++)
+  {
+    if (*tab->on_expr_ref)
+    {
+      cache_flag= FALSE;
+      (*tab->on_expr_ref)->compile(&Item::cache_const_expr_analyzer,
+                                 (uchar **)&analyzer_arg,
+                                 &Item::cache_const_expr_transformer,
+                                 (uchar *)&cache_flag);
+    }
+  }
+}
+
+
 /**
   @} (end of group Query_Optimizer)
 */