Bug#10446 Illegal mix of collations:

item_strfunc.h, item_strfunc.cc, item.cc:
  Try to convert a const item into destination 
  character set. If conversion happens without
  data loss, then cache the converted value
  and return it during val_str().
  Otherwise, if conversion loses data, return
  Illeral mix of collations error, as it happened
  previously.
ctype_recoding.result, ctype_recoding.test:
  Fixing tests accordingly.


sql/item.cc:
  Bug#10446 Illegal mix of collations
  Try to convert a const item into destination 
  character set. If conversion happens without
  data loss, then cache the converted value
  and return it during val_str().
  Otherwise, if conversion loses data, return
  Illeral mix of collations error, as it happened
  previously.
sql/item_strfunc.cc:
  Return cached value when it's possible.
mysql-test/t/ctype_recoding.test:
  Fixing tests accordingly.
mysql-test/r/ctype_recoding.result:
  Fixing tests accordingly.

1 files changed, 32 insertions, 2 deletions

diff --git a/sql/item_strfunc.h b/sql/item_strfunc.h
index 87aee9ac25c..c4505fce248 100644
--- a/sql/item_strfunc.h
+++ b/sql/item_strfunc.h
@@ -614,10 +614,40 @@ public:
 
 class Item_func_conv_charset :public Item_str_func
 {
+  bool use_cached_value;
 public:
+  bool safe;
   CHARSET_INFO *conv_charset; // keep it public
-  Item_func_conv_charset(Item *a, CHARSET_INFO *cs) :Item_str_func(a)
-  { conv_charset=cs; }
+  Item_func_conv_charset(Item *a, CHARSET_INFO *cs) :Item_str_func(a) 
+  { conv_charset= cs; use_cached_value= 0; safe= 0; }
+  Item_func_conv_charset(Item *a, CHARSET_INFO *cs, bool cache_if_const) 
+    :Item_str_func(a) 
+  {
+    DBUG_ASSERT(args[0]->fixed);
+    conv_charset= cs;
+    if (cache_if_const && args[0]->const_item())
+    {
+      uint errors= 0;
+      String tmp, *str= args[0]->val_str(&tmp);
+      if (!str || str_value.copy(str->ptr(), str->length(),
+                                 str->charset(), conv_charset, &errors))
+        null_value= 1;
+      use_cached_value= 1;
+      safe= (errors == 0);
+    }
+    else
+    {
+      use_cached_value= 0;
+      /*
+        Conversion from and to "binary" is safe.
+        Conversion to Unicode is safe.
+        Other kind of conversions are potentially lossy.
+      */
+      safe= (args[0]->collation.collation == &my_charset_bin ||
+             cs == &my_charset_bin ||
+             (cs->state & MY_CS_UNICODE));
+    }
+  }
   String *val_str(String *);
   void fix_length_and_dec();
   const char *func_name() const { return "convert"; }