select.result, select.test:

  Added a test case for bug #5333.
null_key.result, key_primary.result:
  Made covering index usable for const tables.
sql_select.cc:
  Made covering index usable for const tables:
  downported the corresponding code from 4.1.
  Simultaneously fixed bug #5333 reported for 4.1.
  The bug was due to the fact that field index in join
  structures was always set to 0 for const tables.

mysql-test/r/key_primary.result

@@ -13,7 +13,7 @@ t1
 AB%
 describe select * from t1 where t1="ABC";
 table	type	possible_keys	key	key_len	ref	rows	Extra
-t1	const	PRIMARY	PRIMARY	3	const	1	
+t1	const	PRIMARY	PRIMARY	3	const	1	Using index
 describe select * from t1 where t1="ABCD";
 Comment
 Impossible WHERE noticed after reading const tables

mysql-test/r/null_key.result

@@ -12,7 +12,7 @@ table	type	possible_keys	key	key_len	ref	rows	Extra
 t1	ref	a,b	a	9	const,const	1	Using where; Using index
 explain select * from t1 where a=2 and b = 2;
 table	type	possible_keys	key	key_len	ref	rows	Extra
-t1	const	a,b	a	9	const,const	1	
+t1	const	a,b	a	9	const,const	1	Using index
 explain select * from t1 where a<=>b limit 2;
 table	type	possible_keys	key	key_len	ref	rows	Extra
 t1	index	NULL	a	9	NULL	12	Using where; Using index

mysql-test/r/select.result

@@ -2327,3 +2327,19 @@ select * from t2,t3 where t2.s = t3.s;
 s	s
 two		two	
 drop table t1, t2, t3;
+CREATE TABLE t1 (
+i int(11) NOT NULL default '0',
+c char(10) NOT NULL default '',
+PRIMARY KEY  (i),
+UNIQUE KEY c (c)
+) TYPE=MyISAM;
+INSERT INTO t1 VALUES (1,'a');
+INSERT INTO t1 VALUES (2,'b');
+INSERT INTO t1 VALUES (3,'c');
+EXPLAIN SELECT i FROM t1 WHERE i=1;
+table	type	possible_keys	key	key_len	ref	rows	Extra
+t1	const	PRIMARY	PRIMARY	4	const	1	Using index
+EXPLAIN SELECT i FROM t1 WHERE i=1;
+table	type	possible_keys	key	key_len	ref	rows	Extra
+t1	const	PRIMARY	PRIMARY	4	const	1	Using index
+DROP TABLE t1;

mysql-test/t/select.test

@@ -1874,3 +1874,24 @@ select * from t3 where s = 'one';
 select * from t1,t2 where t1.s = t2.s;
 select * from t2,t3 where t2.s = t3.s;
 drop table t1, t2, t3;
+
+#
+# Covering index is mentioned in EXPLAIN output for const tables (bug #5333)
+#
+
+CREATE TABLE t1 (
+  i int(11) NOT NULL default '0',
+  c char(10) NOT NULL default '',
+  PRIMARY KEY  (i),
+  UNIQUE KEY c (c)
+) TYPE=MyISAM;
+
+INSERT INTO t1 VALUES (1,'a');
+INSERT INTO t1 VALUES (2,'b');
+INSERT INTO t1 VALUES (3,'c');
+
+EXPLAIN SELECT i FROM t1 WHERE i=1;
+
+EXPLAIN SELECT i FROM t1 WHERE i=1;
+
+DROP TABLE t1;
\ No newline at end of file

sql/sql_select.cc

@@ -4901,6 +4901,15 @@ join_read_const_table(JOIN_TAB *tab, POSITION *pos)
   }
   else
   {
+    if (!table->key_read &&
+        (table->used_keys & ((key_map) 1 << tab->ref.key)) &&
+    	!table->no_keyread &&
+        (int) table->reginfo.lock_type <= (int) TL_READ_HIGH_PRIORITY)
+    {
+      table->key_read=1;
+      table->file->extra(HA_EXTRA_KEYREAD);
+      tab->index= tab->ref.key;
+    }
     if ((error=join_read_const(tab)))
     {
       tab->info="unique row not found";
@@ -7617,7 +7626,7 @@ static void select_describe(JOIN *join, bool need_tmp_table, bool need_order,
       sprintf(buff3,"%.0f",join->best_positions[i].records_read);
       item_list.push_back(new Item_string(buff3,strlen(buff3)));
       my_bool key_read=table->key_read;
-      if (tab->type == JT_NEXT &&
+      if ((tab->type == JT_NEXT || tab->type == JT_CONST) &&
 	  ((table->used_keys & ((key_map) 1 << tab->index))))
 	key_read=1;