Bug Report
Please answer these questions before submitting your issue. Thanks!
1. Minimal reproduce step (Required)
mysql> create table t(a int, b int, index (a));
Query OK, 0 rows affected (0.01 sec)
SQL is
select * from t t1 inner join t t2 on t1.a = t2.a and t1.b = t2.b;
If Index Join is used, currently t1.a = t2.a is put in the equality condition, and t1.b = t2.b is put in other conditions. If the selectivity of t1.a = t2.a is high, there will be quite a lot of rows that need to be filtered with t1.b = t2.b. This part of the cost will be very large.
mysql> explain select /*+ INL_HASH_JOIN(t1) */ * from t t1 inner join t t2 on t1.a = t2.a and t1.b = t2.b;
+-------------------------------+----------+-----------+----------------------+-------------------------------------------------------------------------------------------------------------+
| id | estRows | task | access object | operator info |
+-------------------------------+----------+-----------+----------------------+-------------------------------------------------------------------------------------------------------------+
| IndexHashJoin_30 | 12475.01 | root | | inner join, inner:IndexLookUp_27, outer key:test.t.a, inner key:test.t.a, other cond:eq(test.t.b, test.t.b) |
| ├─TableReader_41(Build) | 9980.01 | root | | data:Selection_40 |
| │ └─Selection_40 | 9980.01 | cop[tikv] | | not(isnull(test.t.a)), not(isnull(test.t.b)) |
| │ └─TableFullScan_39 | 10000.00 | cop[tikv] | table:t2 | keep order:false, stats:pseudo |
| └─IndexLookUp_27(Probe) | 1.25 | root | | |
| ├─Selection_25(Build) | 1.25 | cop[tikv] | | not(isnull(test.t.a)) |
| │ └─IndexRangeScan_23 | 1.25 | cop[tikv] | table:t1, index:a(a) | range: decided by [eq(test.t.a, test.t.a)], keep order:false, stats:pseudo |
| └─Selection_26(Probe) | 1.25 | cop[tikv] | | not(isnull(test.t.b)) |
| └─TableRowIDScan_24 | 1.25 | cop[tikv] | table:t1 | keep order:false, stats:pseudo |
+-------------------------------+----------+-----------+----------------------+-------------------------------------------------------------------------------------------------------------+
9 rows in set (0.01 sec)
If Hash Join is used, t1.a = t2.a and t1.b = t2.b will be put in equal condition. There is no such problem.
mysql> explain select * from t t1 inner join t t2 on t1.a = t2.a and t1.b = t2.b;
+------------------------------+----------+-----------+---------------+-------------------------------------------------------------------+
| id | estRows | task | access object | operator info |
+------------------------------+----------+-----------+---------------+-------------------------------------------------------------------+
| HashJoin_40 | 12475.01 | root | | inner join, equal:[eq(test.t.a, test.t.a) eq(test.t.b, test.t.b)] |
| ├─TableReader_61(Build) | 9980.01 | root | | data:Selection_60 |
| │ └─Selection_60 | 9980.01 | cop[tikv] | | not(isnull(test.t.a)), not(isnull(test.t.b)) |
| │ └─TableFullScan_59 | 10000.00 | cop[tikv] | table:t2 | keep order:false, stats:pseudo |
| └─TableReader_54(Probe) | 9980.01 | root | | data:Selection_53 |
| └─Selection_53 | 9980.01 | cop[tikv] | | not(isnull(test.t.a)), not(isnull(test.t.b)) |
| └─TableFullScan_52 | 10000.00 | cop[tikv] | table:t1 | keep order:false, stats:pseudo |
+------------------------------+----------+-----------+---------------+-------------------------------------------------------------------+
7 rows in set (0.01 sec)
2. What did you expect to see? (Required)
3. What did you see instead (Required)
4. What is your TiDB version? (Required)
Bug Report
Please answer these questions before submitting your issue. Thanks!
1. Minimal reproduce step (Required)
SQL is
If Index Join is used, currently t1.a = t2.a is put in the equality condition, and t1.b = t2.b is put in other conditions. If the selectivity of t1.a = t2.a is high, there will be quite a lot of rows that need to be filtered with t1.b = t2.b. This part of the cost will be very large.
If Hash Join is used, t1.a = t2.a and t1.b = t2.b will be put in equal condition. There is no such problem.
2. What did you expect to see? (Required)
3. What did you see instead (Required)
4. What is your TiDB version? (Required)