diff --git a/mysql-test/r/error_simulation.result b/mysql-test/r/error_simulation.result
index 6153dad2534..27e51a33112 100644
--- a/mysql-test/r/error_simulation.result
+++ b/mysql-test/r/error_simulation.result
@@ -18,3 +18,26 @@ SELECT MAX(a) FROM t1 GROUP BY a,b;
 ERROR 23000: Can't write; duplicate key in table 'tmp_table'
 set tmp_table_size=default;
 DROP TABLE t1;
+#
+# Bug #50946: fast index creation still seems to copy the table
+#
+CREATE TABLE t1 (a INT(100) NOT NULL);
+INSERT INTO t1 VALUES (1), (0), (2);
+SET SESSION debug='+d,alter_table_only_index_change';
+ALTER TABLE t1 ADD INDEX a(a);
+SET SESSION debug=DEFAULT;
+SHOW CREATE TABLE t1;
+Table	Create Table
+t1	CREATE TABLE `t1` (
+  `a` int(100) NOT NULL,
+  KEY `a` (`a`)
+) ENGINE=MyISAM DEFAULT CHARSET=latin1
+SELECT * FROM t1;
+a
+0
+1
+2
+DROP TABLE t1;
+#
+# End of 5.1 tests
+#
diff --git a/mysql-test/t/error_simulation.test b/mysql-test/t/error_simulation.test
index f730c95208e..7cd16a6bc5a 100644
--- a/mysql-test/t/error_simulation.test
+++ b/mysql-test/t/error_simulation.test
@@ -33,3 +33,19 @@ set tmp_table_size=default;
 DROP TABLE t1;
 
 
+--echo #
+--echo # Bug #50946: fast index creation still seems to copy the table
+--echo #
+CREATE TABLE t1 (a INT(100) NOT NULL);
+INSERT INTO t1 VALUES (1), (0), (2);
+SET SESSION debug='+d,alter_table_only_index_change';
+ALTER TABLE t1 ADD INDEX a(a);
+SET SESSION debug=DEFAULT;
+SHOW CREATE TABLE t1;
+SELECT * FROM t1;
+DROP TABLE t1;
+
+
+--echo #
+--echo # End of 5.1 tests
+--echo #
diff --git a/sql/field.cc b/sql/field.cc
index b6323d7b839..b203a42a918 100644
--- a/sql/field.cc
+++ b/sql/field.cc
@@ -8878,14 +8878,20 @@ bool Field_num::eq_def(Field *field)
 }
 
 
+/**
+  Check whether two numeric fields can be considered 'equal' for table
+  alteration purposes. Fields are equal if they are of the same type
+  and retain the same pack length.
+*/
+
 uint Field_num::is_equal(Create_field *new_field)
 {
   return ((new_field->sql_type == real_type()) &&
-	  ((new_field->flags & UNSIGNED_FLAG) == (uint) (flags &
-							 UNSIGNED_FLAG)) &&
+          ((new_field->flags & UNSIGNED_FLAG) == 
+           (uint) (flags & UNSIGNED_FLAG)) &&
 	  ((new_field->flags & AUTO_INCREMENT_FLAG) ==
 	   (uint) (flags & AUTO_INCREMENT_FLAG)) &&
-	  (new_field->length <= max_display_length()));
+          (new_field->pack_length == pack_length()));
 }
 
 
diff --git a/sql/sql_table.cc b/sql/sql_table.cc
index ad72cab664e..1101be67d5e 100644
--- a/sql/sql_table.cc
+++ b/sql/sql_table.cc
@@ -6922,6 +6922,13 @@ view_err:
                        &candidate_key_count))
       goto err;
    
+    DBUG_EXECUTE_IF("alter_table_only_metadata_change", {
+      if (need_copy_table_res != ALTER_TABLE_METADATA_ONLY)
+        goto err; });
+    DBUG_EXECUTE_IF("alter_table_only_index_change", {
+      if (need_copy_table_res != ALTER_TABLE_INDEX_CHANGED)
+        goto err; });
+   
     if (need_copy_table == ALTER_TABLE_METADATA_ONLY)
       need_copy_table= need_copy_table_res;
   }