Optimizer convert hash semi join with nulleq condition to cross semi join with other condition

[windtalker]

Bug Report

Please answer these questions before submitting your issue. Thanks! In TiDB, it will convert intersect/except into a hash join with nulleq join condition:

mysql> create table t1(id int, v1 int, v2 int, v3 int);
Query OK, 0 rows affected (1.47 sec)

mysql> create table t2(id int, v1 int, v2 int, v3 int);
Query OK, 0 rows affected (1.19 sec)
mysql> explain select id from t1 intersect select id from t1;
+------------------------------+----------+-----------+---------------+-------------------------------------------------------------+
| id                           | estRows  | task      | access object | operator info                                               |
+------------------------------+----------+-----------+---------------+-------------------------------------------------------------+
| HashJoin_7                   | 6400.00  | root      |               | semi join, equal:[nulleq(test.t1.id, test.t1.id)]           |
| ├─TableReader_18(Build)      | 10000.00 | root      |               | data:TableFullScan_17                                       |
| │ └─TableFullScan_17         | 10000.00 | cop[tikv] | table:t1      | keep order:false, stats:pseudo                              |
| └─HashAgg_12(Probe)          | 8000.00  | root      |               | group by:test.t1.id, funcs:firstrow(test.t1.id)->test.t1.id |
|   └─TableReader_13           | 8000.00  | root      |               | data:HashAgg_8                                              |
|     └─HashAgg_8              | 8000.00  | cop[tikv] |               | group by:test.t1.id,                                        |
|       └─TableFullScan_11     | 10000.00 | cop[tikv] | table:t1      | keep order:false, stats:pseudo                              |
+------------------------------+----------+-----------+---------------+-------------------------------------------------------------+

As you can see, HashJoin_7 is a hash join with nulleq join condition However, if the query is a little bit complex, TiDB optimizer will generate a CARTESIAN semi join

mysql> explain select t1.id from t1 join t2 on t1.v1 = t2.v2 intersect select t1.id from t1 join t2 on t1.v1 = t2.v2;
+----------------------------------+----------+-----------+---------------+----------------------------------------------------------------+
| id                               | estRows  | task      | access object | operator info                                                  |
+----------------------------------+----------+-----------+---------------+----------------------------------------------------------------+
| HashJoin_15                      | 6393.60  | root      |               | CARTESIAN semi join, other cond:nulleq(test.t1.id, test.t1.id) |
| ├─HashJoin_26(Build)             | 12487.50 | root      |               | inner join, equal:[eq(test.t1.v1, test.t2.v2)]                 |
| │ ├─TableReader_33(Build)        | 9990.00  | root      |               | data:Selection_32                                              |
| │ │ └─Selection_32               | 9990.00  | cop[tikv] |               | not(isnull(test.t2.v2))                                        |
| │ │   └─TableFullScan_31         | 10000.00 | cop[tikv] | table:t2      | keep order:false, stats:pseudo                                 |
| │ └─TableReader_30(Probe)        | 9990.00  | root      |               | data:Selection_29                                              |
| │   └─Selection_29               | 9990.00  | cop[tikv] |               | not(isnull(test.t1.v1))                                        |
| │     └─TableFullScan_28         | 10000.00 | cop[tikv] | table:t1      | keep order:false, stats:pseudo                                 |
| └─HashAgg_16(Probe)              | 7992.00  | root      |               | group by:test.t1.id, funcs:firstrow(test.t1.id)->test.t1.id    |
|   └─HashJoin_17                  | 12487.50 | root      |               | inner join, equal:[eq(test.t1.v1, test.t2.v2)]                 |
|     ├─TableReader_24(Build)      | 9990.00  | root      |               | data:Selection_23                                              |
|     │ └─Selection_23             | 9990.00  | cop[tikv] |               | not(isnull(test.t2.v2))                                        |
|     │   └─TableFullScan_22       | 10000.00 | cop[tikv] | table:t2      | keep order:false, stats:pseudo                                 |
|     └─TableReader_21(Probe)      | 9990.00  | root      |               | data:Selection_20                                              |
|       └─Selection_20             | 9990.00  | cop[tikv] |               | not(isnull(test.t1.v1))                                        |
|         └─TableFullScan_19       | 10000.00 | cop[tikv] | table:t1      | keep order:false, stats:pseudo                                 |
+----------------------------------+----------+-----------+---------------+----------------------------------------------------------------+

The direct cause seems in https://github.com/pingcap/tidb/tree/c091dba89718599dc1b3d3e45f9b0308a8d49ef0/pkg/planner/core/operator/logicalop/logical_join.go#L200

For inner/semi join, it will collect all join's condition together with the predicate, and call LogicalJoin::extractOnCondition to re-extract the condition, and in https://github.com/pingcap/tidb/tree/c091dba89718599dc1b3d3e45f9b0308a8d49ef0/pkg/planner/core/operator/logicalop/logical_join.go#L1214 only eq is treated as equal condition: https://github.com/pingcap/tidb/tree/c091dba89718599dc1b3d3e45f9b0308a8d49ef0/pkg/planner/core/operator/logicalop/logical_join.go#L1257-L1261

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