Move collation aggregation with superset conversion code

from Item_bool_func2 into DTCollation to make it reusable
for other types of items.

sql/item.cc

@@ -265,8 +265,9 @@ CHARSET_INFO *Item::default_charset()
   return current_thd->variables.collation_connection;
 }
 
-bool DTCollation::aggregate(DTCollation &dt)
+bool DTCollation::aggregate(DTCollation &dt, bool superset_conversion)
 {
+  nagg++;
   if (!my_charset_same(collation, dt.collation))
   {
     /* 
@@ -280,15 +281,39 @@ bool DTCollation::aggregate(DTCollation &dt)
       if (derivation <= dt.derivation)
 	; // Do nothing
       else
+      {
 	set(dt); 
+        strong= nagg;
+      }
     }
     else if (dt.collation == &my_charset_bin)
     {
       if (dt.derivation <= derivation)
+      {
         set(dt);
+        strong= nagg;
+      }
       else
        ; // Do nothing
     }
+    else if (superset_conversion)
+    {
+      if (derivation < dt.derivation &&
+          collation->state & MY_CS_UNICODE)
+        ; // Do nothing
+      else if (dt.derivation < derivation &&
+               dt.collation->state & MY_CS_UNICODE)
+      {
+        set(dt);
+        strong= nagg;
+      }
+      else
+      {
+        // Cannot convert to superset
+        set(0, DERIVATION_NONE);
+        return 1;
+      }
+    }
     else
     {
       set(0, DERIVATION_NONE);
@@ -302,6 +327,7 @@ bool DTCollation::aggregate(DTCollation &dt)
   else if (dt.derivation < derivation)
   {
     set(dt);
+    strong= nagg;
   }
   else
   { 

sql/item.h

@@ -41,16 +41,22 @@ class DTCollation {
 public:
   CHARSET_INFO     *collation;
   enum Derivation derivation;
+  uint nagg;    // Total number of aggregated collations.
+  uint strong;  // Number of the strongest collation.
   
   DTCollation()
   {
     collation= &my_charset_bin;
     derivation= DERIVATION_NONE;
+    nagg= 0;
+    strong= 0;
   }
   DTCollation(CHARSET_INFO *collation_arg, Derivation derivation_arg)
   {
     collation= collation_arg;
     derivation= derivation_arg;
+    nagg= 0;
+    strong= 0;
   }
   void set(DTCollation &dt)
   { 
@@ -66,9 +72,9 @@ public:
   { collation= collation_arg; }
   void set(Derivation derivation_arg)
   { derivation= derivation_arg; }
-  bool aggregate(DTCollation &dt);
-  bool set(DTCollation &dt1, DTCollation &dt2)
-  { set(dt1); return aggregate(dt2); }
+  bool aggregate(DTCollation &dt, bool superset_conversion= FALSE);
+  bool set(DTCollation &dt1, DTCollation &dt2, bool superset_conversion= FALSE)
+  { set(dt1); return aggregate(dt2, superset_conversion); }
   const char *derivation_name() const
   {
     switch(derivation)

sql/item_cmpfunc.cc

@@ -188,25 +188,17 @@ void Item_bool_func2::fix_length_and_dec()
   {
     uint strong= 0;
     uint weak= 0;
+    DTCollation coll;
 
-    if ((args[0]->collation.derivation < args[1]->collation.derivation) && 
+    if (args[0]->result_type() == STRING_RESULT &&
+        args[1]->result_type() == STRING_RESULT &&
         !my_charset_same(args[0]->collation.collation,
                          args[1]->collation.collation) &&
-        (args[0]->collation.collation->state & MY_CS_UNICODE))
-    {
-      weak= 1;
-    }
-    else if ((args[1]->collation.derivation < args[0]->collation.derivation) && 
-	     !my_charset_same(args[0]->collation.collation,
-			      args[1]->collation.collation) &&
-             (args[1]->collation.collation->state & MY_CS_UNICODE))
-    {
-      strong= 1;
-    }
-    
-    if (strong || weak)
+        !coll.set(args[0]->collation, args[1]->collation, TRUE))
     {
       Item* conv= 0;
+      strong= coll.strong;
+      weak= strong ? 0 : 1;
       if (args[weak]->type() == STRING_ITEM)
       {
         String tmp, cstr;