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Do not need to push down ColumnRef <compare_operator> null which is always false #51446

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Lloyd-Pottiger opened this issue Mar 1, 2024 · 2 comments · May be fixed by #53206
Open

Do not need to push down ColumnRef <compare_operator> null which is always false #51446

Lloyd-Pottiger opened this issue Mar 1, 2024 · 2 comments · May be fixed by #53206
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@King-Dylan
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sig/planner SIG: Planner type/enhancement

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@Lloyd-Pottiger
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Enhancement

mysql> create table t (a int, f float);
Query OK, 0 rows affected (0.09 sec)

mysql> insert into t values (1, 0.1), (2, 0.2);
Query OK, 2 rows affected (0.01 sec)
Records: 2  Duplicates: 0  Warnings: 0

mysql> explain select * from t where f != null;
+-------------------------+----------+-----------+---------------+--------------------------------+
| id                      | estRows  | task      | access object | operator info                  |
+-------------------------+----------+-----------+---------------+--------------------------------+
| TableReader_7           | 0.00     | root      |               | data:Selection_6               |
| └─Selection_6           | 0.00     | cop[tikv] |               | ne(test.t.f, NULL)             |
|   └─TableFullScan_5     | 10000.00 | cop[tikv] | table:t       | keep order:false, stats:pseudo |
+-------------------------+----------+-----------+---------------+--------------------------------+
3 rows in set (0.00 sec)

mysql> explain select * from t where f > null;
+-------------------------+----------+-----------+---------------+--------------------------------+
| id                      | estRows  | task      | access object | operator info                  |
+-------------------------+----------+-----------+---------------+--------------------------------+
| TableReader_7           | 0.00     | root      |               | data:Selection_6               |
| └─Selection_6           | 0.00     | cop[tikv] |               | gt(test.t.f, NULL)             |
|   └─TableFullScan_5     | 10000.00 | cop[tikv] | table:t       | keep order:false, stats:pseudo |
+-------------------------+----------+-----------+---------------+--------------------------------+
3 rows in set (0.00 sec)

mysql> explain select * from t where f >= null;
+-------------------------+----------+-----------+---------------+--------------------------------+
| id                      | estRows  | task      | access object | operator info                  |
+-------------------------+----------+-----------+---------------+--------------------------------+
| TableReader_7           | 0.00     | root      |               | data:Selection_6               |
| └─Selection_6           | 0.00     | cop[tikv] |               | ge(test.t.f, NULL)             |
|   └─TableFullScan_5     | 10000.00 | cop[tikv] | table:t       | keep order:false, stats:pseudo |
+-------------------------+----------+-----------+---------------+--------------------------------+
3 rows in set (0.00 sec)

mysql> explain select * from t where f < null;
+-------------------------+----------+-----------+---------------+--------------------------------+
| id                      | estRows  | task      | access object | operator info                  |
+-------------------------+----------+-----------+---------------+--------------------------------+
| TableReader_7           | 0.00     | root      |               | data:Selection_6               |
| └─Selection_6           | 0.00     | cop[tikv] |               | lt(test.t.f, NULL)             |
|   └─TableFullScan_5     | 10000.00 | cop[tikv] | table:t       | keep order:false, stats:pseudo |
+-------------------------+----------+-----------+---------------+--------------------------------+
3 rows in set (0.00 sec)

mysql> explain select * from t where f <= null;
+-------------------------+----------+-----------+---------------+--------------------------------+
| id                      | estRows  | task      | access object | operator info                  |
+-------------------------+----------+-----------+---------------+--------------------------------+
| TableReader_7           | 0.00     | root      |               | data:Selection_6               |
| └─Selection_6           | 0.00     | cop[tikv] |               | le(test.t.f, NULL)             |
|   └─TableFullScan_5     | 10000.00 | cop[tikv] | table:t       | keep order:false, stats:pseudo |
+-------------------------+----------+-----------+---------------+--------------------------------+
3 rows in set (0.00 sec)

mysql> explain select * from t where f = null;
+-------------+---------+------+---------------+---------------+
| id          | estRows | task | access object | operator info |
+-------------+---------+------+---------------+---------------+
| TableDual_5 | 0.00    | root |               | rows:0        |
+-------------+---------+------+---------------+---------------+
1 row in set (0.00 sec)
@Lloyd-Pottiger Lloyd-Pottiger changed the title Do not need to push down ColumnRed <compare_operator> null which is always false Do not need to push down ColumnRef <compare_operator> null which is always false Mar 1, 2024
@elsa0520 elsa0520 added the sig/planner SIG: Planner label Mar 1, 2024
@elsa0520
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elsa0520 commented Mar 1, 2024

Also , we have some predicate evaluate logic when build selection operator.
img_v3_028i_655feb32-2650-49da-93cd-f5bde1f6bacg

But the bad thing is that we cannot evaluate x!=null to false.

By the way, I think maybe the predicate evaluate feature should be a single rewrite rule in the optimizer to handle this kind of predicate (always false).

@King-Dylan
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/assign @King-Dylan

@King-Dylan King-Dylan linked a pull request May 12, 2024 that will close this issue
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planner: Use dual plan when comparing with null values | tidb-test=pr/2328 King-Dylan/tidb
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@King-Dylan @Lloyd-Pottiger @elsa0520