BUG#18068: SELECT DISTINCT (with duplicates and covering index)

When converting DISTINCT to GROUP BY where the columns are from the covering
index and they are quoted twice in the SELECT list the optimizer is creating
improper processing sequence. This is because of the fact that the columns
of the covering index are not recognized as such and treated as non-index
columns.

Generally speaking duplicate columns can safely be removed from the GROUP
BY/DISTINCT list because this will not add or remove new rows in the
resulting set. Duplicates can be removed even if they are not consecutive
(as is the case for ORDER BY, where the duplicate columns can be removed
only if they are consecutive).

So we can safely transform "SELECT DISTINCT a,a FROM ... ORDER BY a" to
"SELECT a,a FROM ... GROUP BY a ORDER BY a" instead of 
"SELECT a,a FROM .. GROUP BY a,a ORDER BY a". We can even transform 
"SELECT DISTINCT a,b,a FROM ... ORDER BY a,b" to
"SELECT a,b,a FROM ... GROUP BY a,b ORDER BY a,b".

The fix to this bug consists of checking for duplicate columns in the SELECT
list when constructing the GROUP BY list in transforming DISTINCT to GROUP
BY and skipping the ones that are already in.

1 files changed, 16 insertions, 0 deletions

diff --git a/mysql-test/t/distinct.test b/mysql-test/t/distinct.test
index 45bd0c7a51c..09f07c2852f 100644
--- a/mysql-test/t/distinct.test
+++ b/mysql-test/t/distinct.test
@@ -382,3 +382,19 @@ INSERT INTO t1 VALUES
 select count(distinct x,y) from t1;
 select count(distinct concat(x,y)) from t1;
 drop table t1;
+
+#
+# Bug #18068: SELECT DISTINCT
+#
+CREATE TABLE t1 (a INT, b INT, PRIMARY KEY (a,b));
+
+INSERT INTO t1 VALUES (1, 101);
+INSERT INTO t1 SELECT a + 1, a + 101 FROM t1;
+INSERT INTO t1 SELECT a + 2, a + 102 FROM t1;
+INSERT INTO t1 SELECT a + 4, a + 104 FROM t1;
+INSERT INTO t1 SELECT a + 8, a + 108 FROM t1;
+
+EXPLAIN SELECT DISTINCT a,a FROM t1 WHERE b < 12 ORDER BY a;
+SELECT DISTINCT a,a FROM t1 WHERE b < 12 ORDER BY a;
+
+DROP TABLE t1;