Fix bug #14016 date_format() 2nd parameter was compared using case insensitive

collation

By default constant strings in second parameter of date_time() have case
insensitive collation. Because of this expressions date_format(f,'%m') and 
date_format(f,'%M') wrongly becomes equal, which results in choosing wrong 
column to sort by.

Now if second parameter of date_format() is constant then it's collation is 
changed to case sensitive.

mysql-test/r/date_formats.result

@@ -456,3 +456,11 @@ f1	f2
 Warnings:
 Warning	1292	Truncated incorrect date value: '2003-04-05  g'
 Warning	1292	Truncated incorrect datetime value: '2003-04-05 10:11:12.101010234567'
+create table t1 (f1 datetime);
+insert into t1 (f1) values ("2005-01-01");
+insert into t1 (f1) values ("2005-02-01");
+select date_format(f1, "%m") as d1, date_format(f1, "%M") as d2 from t1 order by date_format(f1, "%M");
+d1	d2
+02	February
+01	January
+drop table t1;

mysql-test/t/date_formats.test

@@ -260,4 +260,12 @@ select str_to_date("2003-04-05  g", "%Y-%m-%d") as f1,
        str_to_date("2003-04-05 10:11:12.101010234567", "%Y-%m-%d %H:%i:%S.%f") as f2;
 --enable_ps_protocol
 
+#
+# Bug #14016 
+#
+create table t1 (f1 datetime);
+insert into t1 (f1) values ("2005-01-01");
+insert into t1 (f1) values ("2005-02-01");
+select date_format(f1, "%m") as d1, date_format(f1, "%M") as d2 from t1 order by date_format(f1, "%M");
+drop table t1;
 # End of 4.1 tests

sql/item_timefunc.cc

@@ -1528,6 +1528,16 @@ void Item_func_date_format::fix_length_and_dec()
   if (args[1]->type() == STRING_ITEM)
   {						// Optimize the normal case
     fixed_length=1;
+
+    /*
+      Force case sensitive collation on format string.
+      This needed because format modifiers with different case,
+      for example %m and %M, have different meaning. Thus eq()
+      will distinguish them.
+    */
+    args[1]->collation.set(
+        get_charset_by_csname(args[1]->collation.collation->csname,
+                              MY_CS_BINSORT,MYF(0)), DERIVATION_COERCIBLE);
     /*
       The result is a binary string (no reason to use collation->mbmaxlen
       This is becasue make_date_time() only returns binary strings