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移除书签用 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），以下两个示例查询的执行方式类似：
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;
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：
EXPLAIN SELECT * FROM long_trips WHERE bike_number = 'W00950';EXPLAIN SELECT * FROM trips WHERE bike_number = 'W00950';
+--------------------------------+---------+-----------+---------------------------------------+---------------------------------------------------+| 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：
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';
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。
