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本文档中的示例都基于以下数据：
TiDB 从二级索引检索数据时会使用 IndexLookup 算子。例如，以下所有查询均会在 intkey 列的索引上使用 IndexLookup 算子：
EXPLAIN SELECT * FROM t1 WHERE intkey = 123;EXPLAIN SELECT * FROM t1 WHERE intkey < 10;EXPLAIN SELECT * FROM t1 WHERE intkey BETWEEN 300 AND 310;EXPLAIN SELECT * FROM t1 WHERE intkey BETWEEN 300 AND 310;EXPLAIN SELECT * FROM t1 WHERE intkey IN (123,29,98);EXPLAIN SELECT * FROM t1 WHERE intkey >= 99 AND intkey <= 103;
+-------------------------------+---------+-----------+--------------------------------+-----------------------------------+| id | estRows | task | access object | operator info |+-------------------------------+---------+-----------+--------------------------------+-----------------------------------+| IndexLookUp_10 | 1.00 | root | | || ├─IndexRangeScan_8(Build) | 1.00 | cop[tikv] | table:t1, index:intkey(intkey) | range:[123,123], keep order:false || └─TableRowIDScan_9(Probe) | 1.00 | cop[tikv] | table:t1 | keep order:false |+-------------------------------+---------+-----------+--------------------------------+-----------------------------------+3 rows in set (0.00 sec)+-------------------------------+---------+-----------+--------------------------------+-----------------------------------+| id | estRows | task | access object | operator info |+-------------------------------+---------+-----------+--------------------------------+-----------------------------------+| IndexLookUp_10 | 3.60 | root | | || ├─IndexRangeScan_8(Build) | 3.60 | cop[tikv] | table:t1, index:intkey(intkey) | range:[-inf,10), keep order:false || └─TableRowIDScan_9(Probe) | 3.60 | cop[tikv] | table:t1 | keep order:false |+-------------------------------+---------+-----------+--------------------------------+-----------------------------------+3 rows in set (0.00 sec)+-------------------------------+---------+-----------+--------------------------------+-----------------------------------+| id | estRows | task | access object | operator info |+-------------------------------+---------+-----------+--------------------------------+-----------------------------------+| IndexLookUp_10 | 5.67 | root | | || ├─IndexRangeScan_8(Build) | 5.67 | cop[tikv] | table:t1, index:intkey(intkey) | range:[300,310], keep order:false || └─TableRowIDScan_9(Probe) | 5.67 | cop[tikv] | table:t1 | keep order:false |+-------------------------------+---------+-----------+--------------------------------+-----------------------------------+3 rows in set (0.00 sec)+-------------------------------+---------+-----------+--------------------------------+-----------------------------------+| id | estRows | task | access object | operator info |+-------------------------------+---------+-----------+--------------------------------+-----------------------------------+| IndexLookUp_10 | 5.67 | root | | || ├─IndexRangeScan_8(Build) | 5.67 | cop[tikv] | table:t1, index:intkey(intkey) | range:[300,310], keep order:false || └─TableRowIDScan_9(Probe) | 5.67 | cop[tikv] | table:t1 | keep order:false |+-------------------------------+---------+-----------+--------------------------------+-----------------------------------+3 rows in set (0.00 sec)+-------------------------------+---------+-----------+--------------------------------+-----------------------------------------------------+| id | estRows | task | access object | operator info |+-------------------------------+---------+-----------+--------------------------------+-----------------------------------------------------+| IndexLookUp_10 | 4.00 | root | | || ├─IndexRangeScan_8(Build) | 4.00 | cop[tikv] | table:t1, index:intkey(intkey) | range:[29,29], [98,98], [123,123], keep order:false || └─TableRowIDScan_9(Probe) | 4.00 | cop[tikv] | table:t1 | keep order:false |+-------------------------------+---------+-----------+--------------------------------+-----------------------------------------------------+3 rows in set (0.00 sec)+-------------------------------+---------+-----------+--------------------------------+----------------------------------+| id | estRows | task | access object | operator info |+-------------------------------+---------+-----------+--------------------------------+----------------------------------+| IndexLookUp_10 | 6.00 | root | | || ├─IndexRangeScan_8(Build) | 6.00 | cop[tikv] | table:t1, index:intkey(intkey) | range:[99,103], keep order:false || └─TableRowIDScan_9(Probe) | 6.00 | cop[tikv] | table:t1 | keep order:false |+-------------------------------+---------+-----------+--------------------------------+----------------------------------+3 rows in set (0.00 sec)
IndexLookup 算子有以下两个子节点：
├─IndexRangeScan_8(Build)算子节点对intkey列的索引执行范围扫描，并检索内部的RowID值（对此表而言，即为主键）。└─TableRowIDScan_9(Probe)算子节点随后从表数据中检索整行。
IndexLookup 任务分以上两步执行。如果满足条件的行较多，SQL 优化器可能会根据选择使用 算子。在以下示例中，很多行都满足 intkey > 100 这一条件，因此优化器选择了 TableFullScan：
EXPLAIN SELECT * FROM t1 WHERE intkey > 100;
+-------------------------+---------+-----------+---------------+-------------------------+| id | estRows | task | access object | operator info |+-------------------------+---------+-----------+---------------+-------------------------+| TableReader_7 | 898.50 | root | | data:Selection_6 || └─Selection_6 | 898.50 | cop[tikv] | | gt(test.t1.intkey, 100) || └─TableFullScan_5 | 1010.00 | cop[tikv] | table:t1 | keep order:false |+-------------------------+---------+-----------+---------------+-------------------------+3 rows in set (0.00 sec)
EXPLAIN SELECT * FROM t1 ORDER BY intkey DESC LIMIT 10;
以上示例中，TiDB 从 intkey 索引读取最后 20 行，然后从表数据中检索这些行的 RowID 值。
TiDB 支持覆盖索引优化 (covering index optimization)。如果 TiDB 能从索引中检索出所有行，就会跳过 IndexLookup 任务中通常所需的第二步（即从表数据中检索整行）。示例如下：
EXPLAIN SELECT * FROM t1 WHERE intkey = 123;EXPLAIN SELECT id FROM t1 WHERE intkey = 123;
+-------------------------------+---------+-----------+--------------------------------+-----------------------------------+| id | estRows | task | access object | operator info |+-------------------------------+---------+-----------+--------------------------------+-----------------------------------+| IndexLookUp_10 | 1.00 | root | | || ├─IndexRangeScan_8(Build) | 1.00 | cop[tikv] | table:t1, index:intkey(intkey) | range:[123,123], keep order:false || └─TableRowIDScan_9(Probe) | 1.00 | cop[tikv] | table:t1 | keep order:false |+-------------------------------+---------+-----------+--------------------------------+-----------------------------------+3 rows in set (0.00 sec)+--------------------------+---------+-----------+--------------------------------+-----------------------------------+| id | estRows | task | access object | operator info |+--------------------------+---------+-----------+--------------------------------+-----------------------------------+| Projection_4 | 1.00 | root | | test.t1.id || └─IndexReader_6 | 1.00 | root | | index:IndexRangeScan_5 || └─IndexRangeScan_5 | 1.00 | cop[tikv] | table:t1, index:intkey(intkey) | range:[123,123], keep order:false |+--------------------------+---------+-----------+--------------------------------+-----------------------------------+3 rows in set (0.00 sec)
以上结果中，id 也是内部的 RowID 值，因此 id 也存储在 intkey 索引中。部分 └─IndexRangeScan_5 任务使用 intkey 索引后，可直接返回 RowID 值。
TiDB 直接从主键或唯一键检索数据时会使用 Point_Get 或 Batch_Point_Get 算子。这两个算子比 IndexLookup 更有效率。示例如下：
EXPLAIN SELECT * FROM t1 WHERE id = 1234;EXPLAIN SELECT * FROM t1 WHERE id IN (1234,123);ALTER TABLE t1 ADD unique_key INT;UPDATE t1 SET unique_key = id;ALTER TABLE t1 ADD UNIQUE KEY (unique_key);EXPLAIN SELECT * FROM t1 WHERE unique_key = 1234;EXPLAIN SELECT * FROM t1 WHERE unique_key IN (1234, 123);
+-------------+---------+------+---------------+---------------+| id | estRows | task | access object | operator info |+-------------+---------+------+---------------+---------------+| Point_Get_1 | 1.00 | root | table:t1 | handle:1234 |+-------------+---------+------+---------------+---------------+1 row in set (0.00 sec)+-------------------+---------+------+---------------+-------------------------------------------------+| id | estRows | task | access object | operator info |+-------------------+---------+------+---------------+-------------------------------------------------++-------------------+---------+------+---------------+-------------------------------------------------+1 row in set (0.00 sec)Query OK, 0 rows affected (0.27 sec)Query OK, 1010 rows affected (0.06 sec)Query OK, 0 rows affected (0.37 sec)+-------------+---------+------+----------------------------------------+---------------+| id | estRows | task | access object | operator info |+-------------+---------+------+----------------------------------------+---------------+| Point_Get_1 | 1.00 | root | table:t1, index:unique_key(unique_key) | |+-------------+---------+------+----------------------------------------+---------------+1 row in set (0.00 sec)+-------------------+---------+------+----------------------------------------+------------------------------+| id | estRows | task | access object | operator info |+-------------------+---------+------+----------------------------------------+------------------------------+| Batch_Point_Get_1 | 2.00 | root | table:t1, index:unique_key(unique_key) | keep order:false, desc:false |+-------------------+---------+------+----------------------------------------+------------------------------+1 row in set (0.00 sec)
EXPLAIN SELECT MIN(intkey) FROM t1;EXPLAIN SELECT MAX(intkey) FROM t1;
以上语句的执行过程中，TiDB 在每一个 TiKV Region 上执行 IndexFullScan 操作。虽然算子名为 FullScan 即全扫描，TiDB 只读取第一行 (└─Limit_28)。每个 TiKV Region 返回各自的 MIN 或 MAX 值给 TiDB，TiDB 再执行流聚合运算来过滤出一行数据。即使表为空，带 MAX 或 MIN 函数的流聚合运算也能保证返回 NULL 值。
相反，在没有索引的值上执行 MIN 函数会在每一个 TiKV Region 上执行 TableFullScan 操作。该查询会要求在 TiKV 中扫描所有行，但 TopN 计算可保证每个 TiKV Region 只返回一行数据给 TiDB。尽管 TopN 能减少 TiDB 和 TiKV 之间的多余数据传输，但该查询的效率仍远不及以上示例（MIN 能够使用索引）。
EXPLAIN SELECT MIN(pad1) FROM t1;
+--------------------------------+---------+-----------+---------------+-----------------------------------+| id | estRows | task | access object | operator info |+--------------------------------+---------+-----------+---------------+-----------------------------------+| StreamAgg_13 | 1.00 | root | | funcs:min(test.t1.pad1)->Column#4 || └─TopN_14 | 1.00 | root | | test.t1.pad1, offset:0, count:1 || └─TableReader_23 | 1.00 | root | | data:TopN_22 || └─TopN_22 | 1.00 | cop[tikv] | | test.t1.pad1, offset:0, count:1 || └─Selection_21 | 1008.99 | cop[tikv] | | not(isnull(test.t1.pad1)) || └─TableFullScan_20 | 1010.00 | cop[tikv] | table:t1 | keep order:false |+--------------------------------+---------+-----------+---------------+-----------------------------------+6 rows in set (0.00 sec)
执行以下语句时，TiDB 将使用 IndexFullScan 算子扫描索引中的每一行：
EXPLAIN SELECT SUM(intkey) FROM t1;EXPLAIN SELECT AVG(intkey) FROM t1;
+----------------------------+---------+-----------+--------------------------------+-------------------------------------+| id | estRows | task | access object | operator info |+----------------------------+---------+-----------+--------------------------------+-------------------------------------+| StreamAgg_20 | 1.00 | root | | funcs:sum(Column#6)->Column#4 || └─IndexReader_21 | 1.00 | root | | index:StreamAgg_8 || └─StreamAgg_8 | 1.00 | cop[tikv] | | funcs:sum(test.t1.intkey)->Column#6 || └─IndexFullScan_19 | 1010.00 | cop[tikv] | table:t1, index:intkey(intkey) | keep order:false |+----------------------------+---------+-----------+--------------------------------+-------------------------------------+4 rows in set (0.00 sec)+----------------------------+---------+-----------+--------------------------------+----------------------------------------------------------------------------+| id | estRows | task | access object | operator info |+----------------------------+---------+-----------+--------------------------------+----------------------------------------------------------------------------+| StreamAgg_20 | 1.00 | root | | funcs:avg(Column#7, Column#8)->Column#4 || └─IndexReader_21 | 1.00 | root | | index:StreamAgg_8 || └─StreamAgg_8 | 1.00 | cop[tikv] | | funcs:count(test.t1.intkey)->Column#7, funcs:sum(test.t1.intkey)->Column#8 || └─IndexFullScan_19 | 1010.00 | cop[tikv] | table:t1, index:intkey(intkey) | keep order:false |+----------------------------+---------+-----------+--------------------------------+----------------------------------------------------------------------------+4 rows in set (0.00 sec)
以上示例中，IndexFullScan 比 TableFullScan 更有效率，因为 (intkey + RowID) 索引中值的长度小于整行的长度。
