MDEV-305: SHOW EXPLAIN: ref returned by SHOW EXPLAIN is different from the...

MDEV-305: SHOW EXPLAIN: ref returned by SHOW EXPLAIN is different from the normal EXPLAIN ('const' vs empty string)
- The problem was that create_ref_for_key() would act differently, depending on 
  whether we're running EXPLAIN or the actual query.
- As the first step, fixed the EXPLAIN printout not to depend on actions in create_ref_for_key().

mysql-test/r/show_explain.result

@@ -448,7 +448,7 @@ WHERE pk= (SELECT MAX(pk) FROM t2 WHERE pk = 3)
 show explain for $thr2;
 id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
 1	PRIMARY	t2	ALL	NULL	NULL	NULL	NULL	20	Using where
-2	SUBQUERY	t2	const	PRIMARY	PRIMARY	4		1	Using where
+2	SUBQUERY	t2	const	PRIMARY	PRIMARY	4	const	1	Using where
 3	SUBQUERY	NULL	NULL	NULL	NULL	NULL	NULL	NULL	Select tables optimized away
 Warnings:
 Note	1003	SELECT * FROM t2 WHERE a = 
@@ -521,9 +521,7 @@ INSERT INTO t2 VALUES
 (1,5),(2,4),(3,6),(4,9),(5,2),(6,8),(7,4),(8,8),(9,0),(10,43),
 (11,23),(12,3),(13,45),(14,16),(15,2),(16,33),(17,2),(18,5),(19,9),(20,2);
 set @show_explain_probe_select_id=1;
-set debug='d,show_explain_probe_join_exec_end';
-Warnings:
-Warning	1287	The syntax '@@debug' is deprecated and will be removed in MariaDB 5.6. Please use '@@debug_dbug' instead
+set debug_dbug='d,show_explain_probe_join_exec_end';
 SELECT * FROM t2 WHERE (5, 78) IN (SELECT `a1`, MAX(`a1`) FROM t2 GROUP BY `a1`);
 show explain for $thr2;
 id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
@@ -531,8 +529,25 @@ id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
 Warnings:
 Note	1003	SELECT * FROM t2 WHERE (5, 78) IN (SELECT `a1`, MAX(`a1`) FROM t2 GROUP BY `a1`)
 pk	a1
-set debug='';
-Warnings:
-Warning	1287	The syntax '@@debug' is deprecated and will be removed in MariaDB 5.6. Please use '@@debug_dbug' instead
+set debug_dbug='';
 DROP TABLE t2;
-drop table t0,t1;
+DROP TABLE t1;
+#
+# MDEV-305: SHOW EXPLAIN: ref returned by SHOW EXPLAIN is different from the normal EXPLAIN ('const' vs empty string)
+#
+CREATE TABLE t1(a INT, KEY(a));
+INSERT INTO t1 VALUES (3),(1),(5),(1);
+set @show_explain_probe_select_id=1;
+set debug_dbug='d,show_explain_probe_join_exec_start';
+SELECT 'test' FROM t1 WHERE a=1;
+show explain for $thr2;
+id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
+1	SIMPLE	t1	ref	a	a	5	const	1	Using index
+Warnings:
+Note	1003	SELECT 'test' FROM t1 WHERE a=1
+test
+test
+test
+set debug_dbug='';
+DROP TABLE t1;
+drop table t0;

mysql-test/t/show_explain.test

@@ -508,7 +508,7 @@ INSERT INTO t2 VALUES
   (11,23),(12,3),(13,45),(14,16),(15,2),(16,33),(17,2),(18,5),(19,9),(20,2);
 
 set @show_explain_probe_select_id=1;
-set debug='d,show_explain_probe_join_exec_end';
+set debug_dbug='d,show_explain_probe_join_exec_end';
 send
   SELECT * FROM t2 WHERE (5, 78) IN (SELECT `a1`, MAX(`a1`) FROM t2 GROUP BY `a1`);
 connection default;
@@ -516,10 +516,33 @@ connection default;
 evalp show explain for $thr2;
 connection con1;
 reap;
-set debug='';
+set debug_dbug='';
 
 DROP TABLE t2;
 
+DROP TABLE t1;
+
+--echo #
+--echo # MDEV-305: SHOW EXPLAIN: ref returned by SHOW EXPLAIN is different from the normal EXPLAIN ('const' vs empty string)
+--echo #
+CREATE TABLE t1(a INT, KEY(a));
+INSERT INTO t1 VALUES (3),(1),(5),(1);
+
+set @show_explain_probe_select_id=1;
+set debug_dbug='d,show_explain_probe_join_exec_start';
+
+send SELECT 'test' FROM t1 WHERE a=1;
+connection default;
+--source include/wait_condition.inc
+evalp show explain for $thr2;
+connection con1;
+reap;
+set debug_dbug='';
+
+DROP TABLE t1;
+
+
+
 
 ## TODO: Test this: have several SHOW EXPLAIN requests be queued up for a
 ##       thread and served together.
@@ -527,4 +550,4 @@ DROP TABLE t2;
 ## TODO: SHOW EXPLAIN while the primary query is running EXPLAIN EXTENDED/PARTITIONS
 ##
 
-drop table t0,t1;
+drop table t0;

sql/sql_select.cc

@@ -7905,6 +7905,7 @@ static bool create_ref_for_key(JOIN *join, JOIN_TAB *j,
   j->ref.null_rejecting= 0;
   j->ref.disable_cache= FALSE;
   j->ref.null_ref_part= NO_REF_PART;
+  j->ref.const_ref_part_map= 0;
   keyuse=org_keyuse;
 
   store_key **ref_key= j->ref.key_copy;
@@ -7952,6 +7953,7 @@ static bool create_ref_for_key(JOIN *join, JOIN_TAB *j,
 	if (thd->is_fatal_error)
 	  DBUG_RETURN(TRUE);
 	tmp.copy();
+        j->ref.const_ref_part_map |= key_part_map(1) << i ;
       }
       else
 	*ref_key++= get_store_key(thd,
@@ -21521,13 +21523,21 @@ int JOIN::print_explain(select_result_sink *result, uint8 explain_flags,
         length= (longlong10_to_str(key_len, keylen_str_buf, 10) - 
                  keylen_str_buf);
         tmp3.append(keylen_str_buf, length, cs);
-        if (tab->ref.key_parts)
+        if (tab->ref.key_parts && tab_type != JT_FT)
 	{
-	  for (store_key **ref=tab->ref.key_copy ; *ref ; ref++)
+          store_key **ref=tab->ref.key_copy;
+          for (uint kp= 0; kp < tab->ref.key_parts; kp++)
 	  {
 	    if (tmp4.length())
 	      tmp4.append(',');
-	    tmp4.append((*ref)->name(), strlen((*ref)->name()), cs);
+
+            if ((key_part_map(1) << kp) & tab->ref.const_ref_part_map)
+              tmp4.append("const");
+            else
+            {
+              tmp4.append((*ref)->name(), strlen((*ref)->name()), cs);
+              ref++;
+            }
           }
         }
       }

sql/sql_select.h

@@ -101,6 +101,13 @@ typedef struct st_table_ref
   uchar         *key_buff;                ///< value to look for with key
   uchar         *key_buff2;               ///< key_buff+key_length
   store_key     **key_copy;               //
+
+  /*
+    Bitmap of key parts which refer to constants. key_copy only has copiers for
+    non-const key parts.
+  */
+  key_part_map  const_ref_part_map;
+
   Item          **items;                  ///< val()'s for each keypart
   /*  
     Array of pointers to trigger variables. Some/all of the pointers may be