cleanup: Item_func_case

reorder items in args[] array. Instead of

  when1,then1,when2,then2,...[,case][,else]

sort them as

  [case,]when1,when2,...,then1,then2,...[,else]

in this case all items used for comparison take a continuous part
of the array and can be aggregated directly. and all items that
can be returned take a continuous part of the array and can be
aggregated directly. Old code had to copy them to a temporary
array before aggreation, and then copy back (thd->change_item_tree)
everything that was changed.

sql/item_cmpfunc.cc

@@ -3003,11 +3003,12 @@ Item_func_case::Item_func_case(THD *thd, List<Item> &list,
   Item_func_hybrid_field_type(thd), first_expr_num(-1), else_expr_num(-1),
   left_cmp_type(INT_RESULT), case_item(0), m_found_types(0)
 {
-  ncases= list.elements;
+  DBUG_ASSERT(list.elements % 2 == 0);
+  nwhens= list.elements / 2;
   if (first_expr_arg)
   {
-    first_expr_num= list.elements;
-    list.push_back(first_expr_arg, thd->mem_root);
+    first_expr_num= 0;
+    list.push_front(first_expr_arg, thd->mem_root);
   }
   if (else_expr_arg)
   {
@@ -3015,6 +3016,22 @@ Item_func_case::Item_func_case(THD *thd, List<Item> &list,
     list.push_back(else_expr_arg, thd->mem_root);
   }
   set_arguments(thd, list);
+
+  /*
+    Reorder args, to have at first the optional CASE expression, then all WHEN
+    expressions, then all THEN expressions.  And the optional ELSE expression
+    at the end.
+  */
+  const size_t size= sizeof(Item*)*nwhens*2;
+  Item **arg_buffer= (Item **)my_safe_alloca(size);
+  memcpy(arg_buffer, args + first_expr_num + 1, size);
+  for (uint i= 0; i < nwhens ; i++)
+  {
+    args[first_expr_num + 1 + i]= arg_buffer[i*2];
+    args[first_expr_num + 1 + i + nwhens] = arg_buffer[i*2 + 1];
+  }
+  my_safe_afree(arg_buffer, size);
+
   bzero(&cmp_items, sizeof(cmp_items));
 }
 
@@ -3045,18 +3062,17 @@ Item *Item_func_case::find_item(String *str)
 
   if (first_expr_num == -1)
   {
-    for (uint i=0 ; i < ncases ; i+=2)
+    for (uint i=0 ; i < nwhens ; i++)
     {
       // No expression between CASE and the first WHEN
       if (args[i]->val_bool())
-	return args[i+1];
-      continue;
+	return args[i+nwhens];
     }
   }
   else
   {
     /* Compare every WHEN argument with it and return the first match */
-    for (uint i=0 ; i < ncases ; i+=2)
+    for (uint i=1 ; i <= nwhens; i++)
     {
       if (args[i]->real_item()->type() == NULL_ITEM)
         continue;
@@ -3065,13 +3081,13 @@ Item *Item_func_case::find_item(String *str)
       DBUG_ASSERT(cmp_items[(uint)cmp_type]);
       if (!(value_added_map & (1U << (uint)cmp_type)))
       {
-        cmp_items[(uint)cmp_type]->store_value(args[first_expr_num]);
-        if ((null_value=args[first_expr_num]->null_value))
+        cmp_items[(uint)cmp_type]->store_value(args[0]);
+        if ((null_value= args[0]->null_value))
           return else_expr_num != -1 ? args[else_expr_num] : 0;
         value_added_map|= 1U << (uint)cmp_type;
       }
       if (cmp_items[(uint)cmp_type]->cmp(args[i]) == FALSE)
-        return args[i + 1];
+        return args[i + nwhens];
     }
   }
   // No, WHEN clauses all missed, return ELSE expression
@@ -3174,9 +3190,6 @@ bool Item_func_case::fix_fields(THD *thd, Item **ref)
   */
   uchar buff[MAX_FIELD_WIDTH*2+sizeof(String)*2+sizeof(String*)*2+sizeof(double)*2+sizeof(longlong)*2];
 
-  if (!(arg_buffer= (Item**) thd->alloc(sizeof(Item*)*(ncases+1))))
-    return TRUE;
-
   bool res= Item_func::fix_fields(thd, ref);
   /*
     Call check_stack_overrun after fix_fields to be sure that stack variable
@@ -3191,31 +3204,17 @@ bool Item_func_case::fix_fields(THD *thd, Item **ref)
 /**
   Check if (*place) and new_value points to different Items and call
   THD::change_item_tree() if needed.
-
-  This function is a workaround for implementation deficiency in
-  Item_func_case. The problem there is that the 'args' attribute contains
-  Items from different expressions.
- 
-  The function must not be used elsewhere and will be remove eventually.
 */
 
-static void change_item_tree_if_needed(THD *thd,
-                                       Item **place,
-                                       Item *new_value)
+static void change_item_tree_if_needed(THD *thd, Item **place, Item *new_value)
 {
-  if (*place == new_value)
-    return;
-
+  if (new_value && *place != new_value)
     thd->change_item_tree(place, new_value);
 }
 
 
 void Item_func_case::fix_length_and_dec()
 {
-  Item **agg= arg_buffer;
-  uint nagg;
-  THD *thd= current_thd;
-
   m_found_types= 0;
   if (else_expr_num == -1 || args[else_expr_num]->maybe_null)
     maybe_null= 1;
@@ -3224,33 +3223,17 @@ void Item_func_case::fix_length_and_dec()
     Aggregate all THEN and ELSE expression types
     and collations when string result
   */
-  
-  for (nagg= 0 ; nagg < ncases/2 ; nagg++)
-    agg[nagg]= args[nagg*2+1];
-  
-  if (else_expr_num != -1)
-    agg[nagg++]= args[else_expr_num];
-  
-  set_handler_by_field_type(agg_field_type(agg, nagg, true));
+  Item **rets= args + first_expr_num + 1 + nwhens;
+  uint nrets= nwhens + (else_expr_num != -1);
+  set_handler_by_field_type(agg_field_type(rets, nrets, true));
 
   if (Item_func_case::result_type() == STRING_RESULT)
   {
-    if (count_string_result_length(Item_func_case::field_type(), agg, nagg))
+    if (count_string_result_length(Item_func_case::field_type(), rets, nrets))
       return;
-    /*
-      Copy all THEN and ELSE items back to args[] array.
-      Some of the items might have been changed to Item_func_conv_charset.
-    */
-    for (nagg= 0 ; nagg < ncases / 2 ; nagg++)
-      change_item_tree_if_needed(thd, &args[nagg * 2 + 1], agg[nagg]);
-
-    if (else_expr_num != -1)
-      change_item_tree_if_needed(thd, &args[else_expr_num], agg[nagg++]);
   }
   else
-  {
-    fix_attributes(agg, nagg);
-  }
+    fix_attributes(rets, nrets);
   
   /*
     Aggregate first expression and all WHEN expression types
@@ -3258,25 +3241,14 @@ void Item_func_case::fix_length_and_dec()
   */
   if (first_expr_num != -1)
   {
-    uint i;
-    agg[0]= args[first_expr_num];
-    left_cmp_type= agg[0]->cmp_type();
+    left_cmp_type= args[0]->cmp_type();
 
-    /*
-      As the first expression and WHEN expressions
-      are intermixed in args[] array THEN and ELSE items,
-      extract the first expression and all WHEN expressions into 
-      a temporary array, to process them easier.
-    */
-    for (nagg= 0; nagg < ncases/2 ; nagg++)
-      agg[nagg+1]= args[nagg*2];
-    nagg++;
-    if (!(m_found_types= collect_cmp_types(agg, nagg)))
+    if (!(m_found_types= collect_cmp_types(args, nwhens + 1)))
       return;
 
     Item *date_arg= 0;
     if (m_found_types & (1U << TIME_RESULT))
-      date_arg= find_date_time_item(args, arg_count, 0);
+      date_arg= find_date_time_item(args, nwhens + 1, 0);
 
     if (m_found_types & (1U << STRING_RESULT))
     {
@@ -3304,25 +3276,15 @@ void Item_func_case::fix_length_and_dec()
 
            CASE utf16_item WHEN CONVERT(latin1_item USING utf16) THEN ... END
       */
-      if (agg_arg_charsets_for_comparison(cmp_collation, agg, nagg))
+      if (agg_arg_charsets_for_comparison(cmp_collation, args, nwhens + 1))
         return;
-      /*
-        Now copy first expression and all WHEN expressions back to args[]
-        arrray, because some of the items might have been changed to converters
-        (e.g. Item_func_conv_charset, or Item_string for constants).
-      */
-      change_item_tree_if_needed(thd, &args[first_expr_num], agg[0]);
-
-      for (nagg= 0; nagg < ncases / 2; nagg++)
-        change_item_tree_if_needed(thd, &args[nagg * 2], agg[nagg + 1]);
     }
 
-    for (i= 0; i <= (uint)TIME_RESULT; i++)
+    for (uint i= 0; i <= (uint)TIME_RESULT; i++)
     {
       if (m_found_types & (1U << i) && !cmp_items[i])
       {
         DBUG_ASSERT((Item_result)i != ROW_RESULT);
-
         if (!(cmp_items[i]=
             cmp_item::get_comparator((Item_result)i, date_arg,
                                      cmp_collation.collation)))
@@ -3342,19 +3304,9 @@ Item* Item_func_case::propagate_equal_fields(THD *thd, const Context &ctx, COND_
     return this;
   }
 
-  for (uint i= 0; i < arg_count; i++)
-  {
-    /*
-      Even "i" values cover items that are in a comparison context:
-        CASE x0 WHEN x1 .. WHEN x2 .. WHEN x3 ..
-      Odd "i" values cover items that are not in comparison:
-        CASE ... THEN y1 ... THEN y2 ... THEN y3 ... ELSE y4 END
-    */
-    Item *new_item= 0;
-    if ((int) i == first_expr_num) // Then CASE (the switch) argument
-    {
   /*
-        Cannot replace the CASE (the switch) argument if
+    First, replace CASE expression.
+    We cannot replace the CASE (the switch) argument if
     there are multiple comparison types were found, or found a single
     comparison type that is not equal to args[0]->cmp_type().
 
@@ -3381,14 +3333,12 @@ Item* Item_func_case::propagate_equal_fields(THD *thd, const Context &ctx, COND_
               ELSE FALSE END;
   */
   if (m_found_types == (1UL << left_cmp_type))
-        new_item= args[i]->propagate_equal_fields(thd,
-                                                  Context(
-                                                    ANY_SUBST,
-                                                    left_cmp_type,
+    change_item_tree_if_needed(thd, args,
+      args[0]->propagate_equal_fields(thd, Context(ANY_SUBST, left_cmp_type,
                                              cmp_collation.collation),
-                                                  cond);
-    }
-    else if ((i % 2) == 0) // WHEN arguments
+                                      cond));
+  uint i= 1;
+  for (; i <= nwhens ; i++) // WHEN expressions
   {
     /*
       These arguments are in comparison.
@@ -3398,19 +3348,15 @@ Item* Item_func_case::propagate_equal_fields(THD *thd, const Context &ctx, COND_
       Such a change for WHEN arguments would require rebuilding cmp_items.
     */
     Item_result tmp_cmp_type= item_cmp_type(args[first_expr_num], args[i]);
-      new_item= args[i]->propagate_equal_fields(thd,
-                                                Context(
-                                                  ANY_SUBST,
-                                                  tmp_cmp_type,
+    change_item_tree_if_needed(thd, args + i,
+      args[i]->propagate_equal_fields(thd, Context(ANY_SUBST, tmp_cmp_type,
                                              cmp_collation.collation),
-                                                cond);
+                                      cond));
   }
-    else // THEN and ELSE arguments (they are not in comparison)
+  for (; i < arg_count ; i++) // THEN expressions and optional ELSE expression
   {
-      new_item= args[i]->propagate_equal_fields(thd, Context_identity(), cond);
-    }
-    if (new_item && new_item != args[i])
-      thd->change_item_tree(&args[i], new_item);
+    change_item_tree_if_needed(thd, args + i,
+      args[i]->propagate_equal_fields(thd, Context_identity(), cond));
   }
   return this;
 }
@@ -3419,11 +3365,8 @@ Item* Item_func_case::propagate_equal_fields(THD *thd, const Context &ctx, COND_
 uint Item_func_case::decimal_precision() const
 {
   int max_int_part=0;
-  for (uint i=0 ; i < ncases ; i+=2)
-    set_if_bigger(max_int_part, args[i+1]->decimal_int_part());
-
-  if (else_expr_num != -1) 
-    set_if_bigger(max_int_part, args[else_expr_num]->decimal_int_part());
+  for (uint i=first_expr_num + 1 + nwhens ; i < arg_count; i++)
+    set_if_bigger(max_int_part, args[i]->decimal_int_part());
   return MY_MIN(max_int_part + decimals, DECIMAL_MAX_PRECISION);
 }
 
@@ -3438,15 +3381,15 @@ void Item_func_case::print(String *str, enum_query_type query_type)
   str->append(STRING_WITH_LEN("case "));
   if (first_expr_num != -1)
   {
-    args[first_expr_num]->print_parenthesised(str, query_type, precedence());
+    args[0]->print_parenthesised(str, query_type, precedence());
     str->append(' ');
   }
-  for (uint i=0 ; i < ncases ; i+=2)
+  for (uint i= first_expr_num + 1 ; i < nwhens + first_expr_num + 1; i++)
   {
     str->append(STRING_WITH_LEN("when "));
     args[i]->print_parenthesised(str, query_type, precedence());
     str->append(STRING_WITH_LEN(" then "));
-    args[i+1]->print_parenthesised(str, query_type, precedence());
+    args[i+nwhens]->print_parenthesised(str, query_type, precedence());
     str->append(' ');
   }
   if (else_expr_num != -1)

sql/item_cmpfunc.h

@@ -1558,12 +1558,11 @@ class Item_func_case :public Item_func_hybrid_field_type
   int first_expr_num, else_expr_num;
   enum Item_result left_cmp_type;
   String tmp_value;
-  uint ncases;
+  uint nwhens;
   Item_result cmp_type;
   DTCollation cmp_collation;
   cmp_item *cmp_items[6]; /* For all result types */
   cmp_item *case_item;
-  Item **arg_buffer;
   uint m_found_types;
 public:
   Item_func_case(THD *thd, List<Item> &list, Item *first_expr_arg,
@@ -1593,7 +1592,6 @@ class Item_func_case :public Item_func_hybrid_field_type
     if (clone)
     {
       clone->case_item= 0;
-      clone->arg_buffer= 0;
       bzero(&clone->cmp_items, sizeof(cmp_items));
     }
     return clone;