Planner fails to eliminate outer join on unique keys

[fixdb]

Bug Report

1. Minimal reproduce step (Required)

CREATE TABLE `customer` (
  `C_CUSTKEY` bigint(20) NOT NULL,
  `C_NAME` varchar(25) NOT NULL,
  `C_ADDRESS` varchar(40) NOT NULL,
  `C_NATIONKEY` bigint(20) NOT NULL,
  `C_PHONE` char(15) NOT NULL,
  `C_ACCTBAL` decimal(15,2) NOT NULL,
  `C_MKTSEGMENT` char(10) NOT NULL,
  `C_COMMENT` varchar(117) NOT NULL,
  PRIMARY KEY (`C_CUSTKEY`) /*T![clustered_index] CLUSTERED */
);
CREATE TABLE `nation` (
  `N_NATIONKEY` bigint(20) NOT NULL,
  `N_NAME` char(25) NOT NULL,
  `N_REGIONKEY` bigint(20) NOT NULL,
  `N_COMMENT` varchar(152) DEFAULT NULL,
  PRIMARY KEY (`N_NATIONKEY`) /*T![clustered_index] CLUSTERED */
)

2. What did you expect to see? (Required)

Join should be eliminated

3. What did you see instead (Required)

explain select count(*) from customer a left join nation b on a.C_NATIONKEY=b.N_NATIONKEY;
+-------------------------------+----------+-----------+---------------+---------------------------------------------------------------------------------+
| id                            | estRows  | task      | access object | operator info                                                                   |
+-------------------------------+----------+-----------+---------------+---------------------------------------------------------------------------------+
| HashAgg_8                     | 1.00     | root      |               | funcs:count(1)->Column#13                                                       |
| └─HashJoin_19                 | 12500.00 | root      |               | left outer join, equal:[eq(test.customer.c_nationkey, test.nation.n_nationkey)] |
|   ├─TableReader_24(Build)     | 10000.00 | root      |               | data:TableFullScan_23                                                           |
|   │ └─TableFullScan_23        | 10000.00 | cop[tikv] | table:b       | keep order:false, stats:pseudo                                                  |
|   └─TableReader_22(Probe)     | 10000.00 | root      |               | data:TableFullScan_21                                                           |
|     └─TableFullScan_21        | 10000.00 | cop[tikv] | table:a       | keep order:false, stats:pseudo                                                  |
+-------------------------------+----------+-----------+---------------+---------------------------------------------------------------------------------+
6 rows in set (0.00 sec)

4. What is your TiDB version? (Required)

master