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Revise the result in the Semi join section in "Explain Statements That Use Subqueries" #7769

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Mar 4, 2022
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Revise the result in the Semi join section in "Explain Statements That Use Subqueries" #7769

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23 changes: 11 additions & 12 deletions explain-subqueries.md
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Original file line number Diff line number Diff line change
Expand Up @@ -106,18 +106,17 @@ EXPLAIN SELECT * FROM t1 WHERE id IN (SELECT t1_id FROM t2 WHERE t1_id != t1.int
```

```sql
+----------------------------------+---------+-----------+-----------------------------+-------------------------------------------------------------------------------------------------------------------------------+
| id | estRows | task | access object | operator info |
+----------------------------------+---------+-----------+-----------------------------+-------------------------------------------------------------------------------------------------------------------------------+
| IndexJoin_14 | 1582.40 | root | | anti semi join, inner:IndexLookUp_13, outer key:test.t3.t1_id, inner key:test.t1.id, equal cond:eq(test.t3.t1_id, test.t1.id) |
| ├─TableReader_35(Build) | 1978.00 | root | | data:TableFullScan_34 |
| │ └─TableFullScan_34 | 1978.00 | cop[tikv] | table:t3 | keep order:false |
| └─IndexLookUp_13(Probe) | 1.00 | root | | |
| ├─IndexRangeScan_10(Build) | 1.00 | cop[tikv] | table:t1, index:PRIMARY(id) | range: decided by [eq(test.t1.id, test.t3.t1_id)], keep order:false |
| └─Selection_12(Probe) | 1.00 | cop[tikv] | | lt(test.t1.int_col, 100) |
| └─TableRowIDScan_11 | 1.00 | cop[tikv] | table:t1 | keep order:false |
+----------------------------------+---------+-----------+-----------------------------+-------------------------------------------------------------------------------------------------------------------------------+
7 rows in set (0.00 sec)
+-----------------------------+-----------+-----------+------------------------------+--------------------------------------------------------------------------------------------------------+
| id | estRows | task | access object | operator info |
+-----------------------------+-----------+-----------+------------------------------+--------------------------------------------------------------------------------------------------------+
| MergeJoin_9 | 45446.40 | root | | semi join, left key:test.t1.id, right key:test.t2.t1_id, other cond:ne(test.t2.t1_id, test.t1.int_col) |
| ├─IndexReader_24(Build) | 180000.00 | root | | index:IndexFullScan_23 |
| │ └─IndexFullScan_23 | 180000.00 | cop[tikv] | table:t2, index:t1_id(t1_id) | keep order:true |
| └─TableReader_22(Probe) | 56808.00 | root | | data:Selection_21 |
| └─Selection_21 | 56808.00 | cop[tikv] | | ne(test.t1.id, test.t1.int_col) |
| └─TableFullScan_20 | 71010.00 | cop[tikv] | table:t1 | keep order:true |
+-----------------------------+-----------+-----------+------------------------------+--------------------------------------------------------------------------------------------------------+
6 rows in set (0.00 sec)
```

From the result above, you can see that TiDB uses a `Semi Join` algorithm. Semi-join differs from inner join: semi-join only permits the first value on the right key (`t2.t1_id`), which means that the duplicates are eliminated as a part of the join operator task. The join algorithm is also Merge Join, which is like an efficient zipper-merge as the operator reads data from both the left and the right side in sorted order.
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