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sql tuning: add EXPLAIN views (pingcap#4624)
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Co-authored-by: Zhang Jian <zjsariel@gmail.com>

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Co-authored-by: TomShawn <41534398+TomShawn@users.noreply.github.com>
Co-authored-by: Zhang Jian <zjsariel@gmail.com>
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---
title: 用 EXPLAIN 查看带视图的 SQL 执行计划
summary: 了解 TiDB 中视图相关语句的执行计划。
---

# 用 EXPLAIN 查看带视图的 SQL 执行计划

`EXPLAIN` 语句返回的结果会显示[视图](/views.md)引用的表和索引，而不是视图本身的名称。这是因为视图是一张虚拟表，本身并不存储任何数据。视图的定义会和查询语句的其余部分在 SQL 优化过程中进行合并。

参考 [bikeshare 数据库示例（英文）](https://docs.pingcap.com/tidb/stable/import-example-data)，以下两个示例查询的执行方式类似：

{{< copyable "sql" >}}

```sql
ALTER TABLE trips ADD INDEX (duration);
CREATE OR REPLACE VIEW long_trips AS SELECT * FROM trips WHERE duration > 3600;
EXPLAIN SELECT * FROM long_trips;
EXPLAIN SELECT * FROM trips WHERE duration > 3600;
```

```sql
Query OK, 0 rows affected (2 min 10.11 sec)

Query OK, 0 rows affected (0.13 sec)

+--------------------------------+------------+-----------+---------------------------------------+-------------------------------------+
| id | estRows | task | access object | operator info |
+--------------------------------+------------+-----------+---------------------------------------+-------------------------------------+
| IndexLookUp_12 | 6372547.67 | root | | |
| ├─IndexRangeScan_10(Build) | 6372547.67 | cop[tikv] | table:trips, index:duration(duration) | range:(3600,+inf], keep order:false |
| └─TableRowIDScan_11(Probe) | 6372547.67 | cop[tikv] | table:trips | keep order:false |
+--------------------------------+------------+-----------+---------------------------------------+-------------------------------------+
3 rows in set (0.00 sec)

+-------------------------------+-----------+-----------+---------------------------------------+-------------------------------------+
| id | estRows | task | access object | operator info |
+-------------------------------+-----------+-----------+---------------------------------------+-------------------------------------+
| IndexLookUp_10 | 833219.37 | root | | |
| ├─IndexRangeScan_8(Build) | 833219.37 | cop[tikv] | table:trips, index:duration(duration) | range:(3600,+inf], keep order:false |
| └─TableRowIDScan_9(Probe) | 833219.37 | cop[tikv] | table:trips | keep order:false |
+-------------------------------+-----------+-----------+---------------------------------------+-------------------------------------+
3 rows in set (0.00 sec)
```

同样，该视图中的谓词被下推至基表：

{{< copyable "sql" >}}

```sql
EXPLAIN SELECT * FROM long_trips WHERE bike_number = 'W00950';
EXPLAIN SELECT * FROM trips WHERE bike_number = 'W00950';
```

```sql
+--------------------------------+---------+-----------+---------------------------------------+---------------------------------------------------+
| id | estRows | task | access object | operator info |
+--------------------------------+---------+-----------+---------------------------------------+---------------------------------------------------+
| IndexLookUp_14 | 3.33 | root | | |
| ├─IndexRangeScan_11(Build) | 3333.33 | cop[tikv] | table:trips, index:duration(duration) | range:(3600,+inf], keep order:false, stats:pseudo |
| └─Selection_13(Probe) | 3.33 | cop[tikv] | | eq(bikeshare.trips.bike_number, "W00950") |
| └─TableRowIDScan_12 | 3333.33 | cop[tikv] | table:trips | keep order:false, stats:pseudo |
+--------------------------------+---------+-----------+---------------------------------------+---------------------------------------------------+
4 rows in set (0.00 sec)

+-------------------------+-------------+-----------+---------------+-------------------------------------------+
| id | estRows | task | access object | operator info |
+-------------------------+-------------+-----------+---------------+-------------------------------------------+
| TableReader_7 | 43.00 | root | | data:Selection_6 |
| └─Selection_6 | 43.00 | cop[tikv] | | eq(bikeshare.trips.bike_number, "W00950") |
| └─TableFullScan_5 | 19117643.00 | cop[tikv] | table:trips | keep order:false |
+-------------------------+-------------+-----------+---------------+-------------------------------------------+
3 rows in set (0.00 sec)
```

执行以上第一条语句时使用了索引，满足视图定义，接着在 TiDB 读取行时应用了 `bike_number = 'W00950'` 条件。执行以上第二条语句时，不存在满足该语句的索引，因此使用了 `TableFullScan`。

TiDB 使用的索引可以同时满足视图定义和语句本身，如以下组合索引所示：

{{< copyable "sql" >}}

```sql
ALTER TABLE trips ADD INDEX (bike_number, duration);
EXPLAIN SELECT * FROM long_trips WHERE bike_number = 'W00950';
EXPLAIN SELECT * FROM trips WHERE bike_number = 'W00950';
```

```sql
Query OK, 0 rows affected (2 min 31.20 sec)

+--------------------------------+----------+-----------+-------------------------------------------------------+-------------------------------------------------------+
| id | estRows | task | access object | operator info |
+--------------------------------+----------+-----------+-------------------------------------------------------+-------------------------------------------------------+
| IndexLookUp_13 | 63725.48 | root | | |
| ├─IndexRangeScan_11(Build) | 63725.48 | cop[tikv] | table:trips, index:bike_number(bike_number, duration) | range:("W00950" 3600,"W00950" +inf], keep order:false |
| └─TableRowIDScan_12(Probe) | 63725.48 | cop[tikv] | table:trips | keep order:false |
+--------------------------------+----------+-----------+-------------------------------------------------------+-------------------------------------------------------+
3 rows in set (0.00 sec)

+-------------------------------+----------+-----------+-------------------------------------------------------+---------------------------------------------+
| id | estRows | task | access object | operator info |
+-------------------------------+----------+-----------+-------------------------------------------------------+---------------------------------------------+
| IndexLookUp_10 | 19117.64 | root | | |
| ├─IndexRangeScan_8(Build) | 19117.64 | cop[tikv] | table:trips, index:bike_number(bike_number, duration) | range:["W00950","W00950"], keep order:false |
| └─TableRowIDScan_9(Probe) | 19117.64 | cop[tikv] | table:trips | keep order:false |
+-------------------------------+----------+-----------+-------------------------------------------------------+---------------------------------------------+
3 rows in set (0.00 sec)
```

在第一条语句中，TiDB 能够使用组合索引的两个部分 `(bike_number, duration)`。在第二条语句，TiDB 仅使用了索引 `(bike_number, duration)` 的第一部分 `bike_number`。

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