planner: should use TableScan when use index() has no arguments

[francis0407]

Bug Report

Please answer these questions before submitting your issue. Thanks!

1. What did you do?
   If possible, provide a recipe for reproducing the error.

mysql> create table t(a int, b int primary key, index idx_a(a));
Query OK, 0 rows affected (0.03 sec)

mysql> explain select a from t use index();

2. What did you expect to see?
   In MySQL, the query above chooses TableScan.

mysql> explain select a from t use index();
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+-------+
| id | select_type | table | partitions | type | possible_keys | key  | key_len | ref  | rows | filtered | Extra |
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+-------+
|  1 | SIMPLE      | t     | NULL       | ALL  | NULL          | NULL | NULL    | NULL |    1 |   100.00 | NULL  |
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+-------+
1 row in set, 1 warning (0.00 sec)

mysql> explain select a from t;
+----+-------------+-------+------------+-------+---------------+-------+---------+------+------+----------+-------------+
| id | select_type | table | partitions | type  | possible_keys | key   | key_len | ref  | rows | filtered | Extra       |
+----+-------------+-------+------------+-------+---------------+-------+---------+------+------+----------+-------------+
|  1 | SIMPLE      | t     | NULL       | index | NULL          | idx_a | 5       | NULL |    1 |   100.00 | Using index |
+----+-------------+-------+------------+-------+---------------+-------+---------+------+------+----------+-------------+
1 row in set, 1 warning (0.01 sec)

3. What did you see instead?

mysql> explain select a from t use index();
+-------------------+----------+------+---------------------------------------------------------------------+
| id                | count    | task | operator info                                                       |
+-------------------+----------+------+---------------------------------------------------------------------+
| IndexReader_7     | 10000.00 | root | index:IndexScan_6                                                   |
| └─IndexScan_6     | 10000.00 | cop  | table:t, index:a, range:[NULL,+inf], keep order:false, stats:pseudo |
+-------------------+----------+------+---------------------------------------------------------------------+

4. What version of TiDB are you using (tidb-server -V or run select tidb_version(); on TiDB)?

mysql> select tidb_version();
+-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| tidb_version()                                                                                                                                                                                                                                                                                                                            |
+-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| Release Version: v4.0.0-alpha-198-g8810a9c
Git Commit Hash: 8810a9c167ad71c9a51e82e2a0411405d1862512
Git Branch: improve_skylineprune
UTC Build Time: 2019-09-04 10:23:18
GoVersion: go version go1.12.7 linux/amd64
Race Enabled: false
TiKV Min Version: v3.0.0-60965b006877ca7234adaced7890d7b029ed1306
Check Table Before Drop: false |
+-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
1 row in set (0.00 sec)