Manual merge

mysql-test/r/select_found.result:
  Auto merged

mysql-test/r/select_found.result

@@ -246,3 +246,11 @@ SELECT FOUND_ROWS();
 FOUND_ROWS()
 0
 DROP TABLE t1;
+CREATE TABLE t1 (a int, b int);
+INSERT INTO t1 VALUES (1,2), (1,3), (1,4), (1,5);
+SELECT SQL_CALC_FOUND_ROWS DISTINCT 'a' FROM t1 GROUP BY b LIMIT 2;
+a
+a
+SELECT FOUND_ROWS();
+FOUND_ROWS()
+1

mysql-test/t/select_found.test

@@ -166,3 +166,12 @@ INSERT INTO t1 VALUES (0), (0), (1), (2);
 SELECT SQL_CALC_FOUND_ROWS * FROM t1 WHERE a = 0 GROUP BY a HAVING a > 10;
 SELECT FOUND_ROWS();
 DROP TABLE t1;
+
+#
+# Bug #7945: group by + distinct with constant expression + limit 
+#
+
+CREATE TABLE t1 (a int, b int);
+INSERT INTO t1 VALUES (1,2), (1,3), (1,4), (1,5);
+SELECT SQL_CALC_FOUND_ROWS DISTINCT 'a' FROM t1 GROUP BY b LIMIT 2;
+SELECT FOUND_ROWS();

sql/sql_select.cc

@@ -4473,9 +4473,15 @@ find_best(JOIN *join,table_map rest_tables,uint idx,double record_count,
 		    x = used key parts (1 <= x <= c)
 		  */
 		  double rec_per_key;
+#if 0
 		  if (!(rec_per_key=(double)
 			keyinfo->rec_per_key[keyinfo->key_parts-1]))
 		    rec_per_key=(double) s->records/rec+1;
+#else
+                  rec_per_key= keyinfo->rec_per_key[keyinfo->key_parts-1] ?
+		    (double) keyinfo->rec_per_key[keyinfo->key_parts-1] :
+		    (double) s->records/rec+1;   
+#endif
 
 		  if (!s->records)
 		    tmp=0;
@@ -10765,13 +10771,14 @@ remove_duplicates(JOIN *join, TABLE *entry,List<Item> &fields, Item *having)
       field_count++;
   }
 
-  if (!field_count)
-  {						// only const items
+  if (!field_count && !(join->select_options & OPTION_FOUND_ROWS)) 
+  {                    // only const items with no OPTION_FOUND_ROWS
     join->unit->select_limit_cnt= 1;		// Only send first row
     DBUG_RETURN(0);
   }
   Field **first_field=entry->field+entry->s->fields - field_count;
-  offset=entry->field[entry->s->fields - field_count]->offset();
+  offset= field_count ? 
+          entry->field[entry->s->fields - field_count]->offset() : 0;
   reclength=entry->s->reclength-offset;
 
   free_io_cache(entry);				// Safety