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sql: update explain results (#2548) #2865

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Apr 23, 2020
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sql: update explain results (#2548) #2865

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c737a75
sql: update `explain` results (#2548)
LittleFall Apr 22, 2020
187e7e4
Merge branch 'release-4.0' into release-4.0-417355712e55
ran-huang Apr 23, 2020
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46 changes: 23 additions & 23 deletions reference/performance/index-merge.md
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Original file line number Diff line number Diff line change
Expand Up @@ -29,13 +29,13 @@ explain select * from t where a = 1 or b = 1;
由于查询的过滤条件是一个通过 `OR` 连接的表达式，我们在只能对每张表使用一个索引的限制下，无法将 `a = 1` 下推到索引 `a` 上，或将 `b = 1` 下推到索引 `b` 上，因此为了保证结果正确性，对这个查询只能生成 `TableScan` 的执行计划：

```
+---------------------+----------+-----------+------------------------------------------------------------+
| id | count | task | operator info |
+---------------------+----------+-----------+------------------------------------------------------------+
| TableReader_7 | 8000.00 | root | data:Selection_6 |
| └─Selection_6 | 8000.00 | cop[tikv] | or(eq(test.t.a, 1), eq(test.t.b, 1)) |
| └─TableScan_5 | 10000.00 | cop[tikv] | table:t, range:[-inf,+inf], keep order:false, stats:pseudo |
+---------------------+----------+-----------+------------------------------------------------------------+
+-------------------------+----------+-----------+---------------+--------------------------------------+
| id | estRows | task | access object | operator info |
+-------------------------+----------+-----------+---------------+--------------------------------------+
| TableReader_7 | 8000.00 | root | | data:Selection_6 |
| └─Selection_6 | 8000.00 | cop[tikv] | | or(eq(test.t.a, 1), eq(test.t.b, 1)) |
| └─TableFullScan_5 | 10000.00 | cop[tikv] | table:t | keep order:false, stats:pseudo |
+-------------------------+----------+-----------+---------------+--------------------------------------+
```

当 `t` 的数据量很大时，全表扫描的效率会很低，但这条查询最多却只会返回两行记录。针对这类场景，TiDB 引入了对表的新访问方式 `IndexMerge`。
Expand All @@ -45,14 +45,14 @@ explain select * from t where a = 1 or b = 1;
在 `IndexMerge` 访问方式下，优化器可以选择对一张表使用多个索引，并将每个索引的返回结果进行集合并操作。以上面查询为例，生成的执行计划将会变为：

```
+--------------------+-------+-----------+---------------------------------------------------------------+
| id | count | task | operator info |
+--------------------+-------+-----------+---------------------------------------------------------------+
| IndexMerge_11 | 2.00 | root | |
| ├─IndexScan_8 | 1.00 | cop[tikv] | table:t, index:a, range:[1,1], keep order:false, stats:pseudo |
| ├─IndexScan_9 | 1.00 | cop[tikv] | table:t, index:b, range:[1,1], keep order:false, stats:pseudo |
| └─TableScan_10 | 2.00 | cop[tikv] | table:t, keep order:false, stats:pseudo |
+--------------------+-------+-----------+---------------------------------------------------------------+
+--------------------------------+---------+-----------+---------------------+---------------------------------------------+
| id | estRows | task | access object | operator info |
+--------------------------------+---------+-----------+---------------------+---------------------------------------------+
| IndexMerge_11 | 2.00 | root | | |
| ├─IndexRangeScan_8(Build) | 1.00 | cop[tikv] | table:t, index:a(a) | range:[1,1], keep order:false, stats:pseudo |
| ├─IndexRangeScan_9(Build) | 1.00 | cop[tikv] | table:t, index:b(b) | range:[1,1], keep order:false, stats:pseudo |
| └─TableRowIDScan_10(Probe) | 2.00 | cop[tikv] | table:t | keep order:false, stats:pseudo |
+--------------------------------+---------+-----------+---------------------+---------------------------------------------+
```

`IndexMerge` 执行计划的结构和 `IndexLookUp` 很接近，都可以分为索引扫描和全表扫描两部分，只是 `IndexMerge` 的索引扫描部分可以包含多个 `IndexScan`，当表的主键索引是整数类型时，索引扫描部分甚至可能包含一个 `TableScan`，比如：
Expand All @@ -74,14 +74,14 @@ explain select * from t where a = 1 or b = 1;
```

```
+--------------------+-------+-----------+---------------------------------------------------------------+
| id | count | task | operator info |
+--------------------+-------+-----------+---------------------------------------------------------------+
| IndexMerge_11 | 2.00 | root | |
| ├─TableScan_8 | 1.00 | cop[tikv] | table:t, range:[1,1], keep order:false, stats:pseudo |
| ├─IndexScan_9 | 1.00 | cop[tikv] | table:t, index:b, range:[1,1], keep order:false, stats:pseudo |
| └─TableScan_10 | 2.00 | cop[tikv] | table:t, keep order:false, stats:pseudo |
+--------------------+-------+-----------+---------------------------------------------------------------+
+--------------------------------+---------+-----------+---------------------+---------------------------------------------+
| id | estRows | task | access object | operator info |
+--------------------------------+---------+-----------+---------------------+---------------------------------------------+
| IndexMerge_11 | 2.00 | root | | |
| ├─TableRangeScan_8(Build) | 1.00 | cop[tikv] | table:t | range:[1,1], keep order:false, stats:pseudo |
| ├─IndexRangeScan_9(Build) | 1.00 | cop[tikv] | table:t, index:b(b) | range:[1,1], keep order:false, stats:pseudo |
| └─TableRowIDScan_10(Probe) | 2.00 | cop[tikv] | table:t | keep order:false, stats:pseudo |
+--------------------------------+---------+-----------+---------------------+---------------------------------------------+
4 rows in set (0.01 sec)
```

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