索引的选择
从存储层读取数据是 SQL 计算过程中最为耗时的部分之一，TiDB 目前支持从不同的存储和不同的索引中读取数据，索引选择得是否合理将很大程度上决定一个查询的运行速度。
本章节将介绍 TiDB 如何选择索引去读入数据，以及相关的一些控制索引选择的方式。
读表
在介绍索引的选择之前，首先要了解 TiDB 有哪些读表的方式，这些方式的触发条件是什么，不同方式有什么区别，各有什么优劣。
读表算子
| 读表算子 | 触发条件 | 适用场景 | 说明 |
|---|---|---|---|
| PointGet/BatchPointGet | 读表的范围是一个或多个单点范围 | 任何场景 | 如果能被触发，通常被认为是最快的算子，因为其直接调用 kvget 的接口进行计算，不走 coprocessor |
| TableReader | 无 | 任何场景 | 该 TableReader 算子用于 TiKV。从 TiKV 端直接扫描表数据，一般被认为是效率最低的算子，除非在 _tidb_rowid 这一列上存在范围查询，或者无其他可以选择的读表算子时，才会选择这个算子 |
| TableReader | 表在 TiFlash 节点上存在副本 | 需要读取的列比较少，但是需要计算的行很多 | 该 TableReader 算子用于 TiFlash。TiFlash 是列式存储，如果需要对少量的列和大量的行进行计算，一般会选择这个算子 |
| IndexReader | 表有一个或多个索引，且计算所需的列被包含在索引里 | 存在较小的索引上的范围查询，或者对索引列有顺序需求的时候 | 当存在多个索引的时候，会根据估算代价选择合理的索引 |
| IndexLookupReader | 表有一个或多个索引，且计算所需的列不完全被包含在索引里 | 同 IndexReader | 因为计算列不完全被包含在索引里，所以读完索引后需要回表，这里会比 IndexReader 多一些开销 |
| IndexMerge | 表有多个索引或多值索引 | 使用多值索引或同时使用多个索引的时候 | 可以通过 optimizer hints 指定使用该算子，或让优化器根据代价估算自动选择该算子，参见用 EXPLAIN 查看索引合并的 SQL 执行计划 |
索引的选择
TiDB 基于规则或基于代价来选择索引。基于的规则包括前置规则和 Skyline-Pruning。在选择索引时，TiDB 会先尝试前置规则。如果存在索引满足某一条前置规则，则直接选择该索引。否则，TiDB 会采用 Skyline-Pruning 来排除不合适的索引，然后基于每个读表算子的代价估算，选择代价最小的索引。
基于规则选择
前置规则
TiDB 采用如下的启发式前置规则来选择索引：
规则 1：如果存在索引满足“唯一性索引全匹配 + 不需要回表（即该索引生成的计划是 IndexReader）”时，直接选择该索引。
规则 2：如果存在索引满足“唯一性索引全匹配 + 需要回表（即该索引生成的计划是 IndexLookupReader）”时，选择满足该条件且回表行数最小的索引作为候选索引。
规则 3：如果存在索引满足“普通索引不需要回表 + 读取行数小于一定阈值”时，选择满足该条件且读取行数最小的索引作为候选索引。
规则 4：如果规则 2 和 3 之中仅选出一条候选索引，则选择该候选索引。如果规则 2 和 3 均选出候选索引，则选择读取行数（读索引行数 + 回表行数）较小的索引。
上述规则中的“索引全匹配”指每个索引列上均存在等值条件。在执行 EXPLAIN FORMAT = 'verbose' ... 语句时，如果前置规则匹配了某一索引，TiDB 会输出一条 NOTE 级别的 warning 提示该索引匹配了前置规则。
在以下示例中，因为索引 idx_b 满足规则 2 中“唯一性索引全匹配 + 需要回表”的条件，TiDB 选择索引 idx_b 作为访问路径，SHOW WARNING 返回了索引 idx_b 命中前置规则的提示。
mysql> CREATE TABLE t(a INT PRIMARY KEY, b INT, c INT, UNIQUE INDEX idx_b(b));
Query OK, 0 rows affected (0.01 sec)
mysql> EXPLAIN FORMAT = 'verbose' SELECT b, c FROM t WHERE b = 3 OR b = 6;
+-------------------+---------+---------+------+-------------------------+------------------------------+
| id | estRows | estCost | task | access object | operator info |
+-------------------+---------+---------+------+-------------------------+------------------------------+
| Batch_Point_Get_5 | 2.00 | 8.80 | root | table:t, index:idx_b(b) | keep order:false, desc:false |
+-------------------+---------+---------+------+-------------------------+------------------------------+
1 row in set, 1 warning (0.00 sec)
mysql> SHOW WARNINGS;
+-------+------+-------------------------------------------------------------------------------------------+
| Level | Code | Message |
+-------+------+-------------------------------------------------------------------------------------------+
| Note | 1105 | unique index idx_b of t is selected since the path only has point ranges with double scan |
+-------+------+-------------------------------------------------------------------------------------------+
1 row in set (0.00 sec)
Skyline-Pruning
Skyline-Pruning 是一个针对索引的启发式过滤规则，能降低错误估算导致选错索引的概率。Skyline-Pruning 从以下三个维度衡量一个索引的优劣：
索引的列涵盖了多少访问条件。“访问条件”指的是可以转化为某列范围的
where条件，如果某个索引的列集合涵盖的访问条件越多，那么它在这个维度上更优。选择该索引读表时，是否需要回表（即该索引生成的计划是 IndexReader 还是 IndexLookupReader）。不用回表的索引在这个维度上优于需要回表的索引。如果均需要回表，则比较索引的列涵盖了多少过滤条件。过滤条件指的是可以根据索引判断的
where条件。如果某个索引的列集合涵盖的访问条件越多，则回表数量越少，那么它在这个维度上越优。
- 选择该索引是否能满足一定的顺序。因为索引的读取可以保证某些列集合的顺序，所以满足查询要求顺序的索引在这个维度上优于不满足的索引。
对于以上三种维度，如果索引 idx_a 在这三个维度上都不比 idx_b 差，且有一个维度比 idx_b 好，那么 TiDB 会优先选择 idx_a。在执行 EXPLAIN FORMAT = 'verbose' ... 语句时，如果 Skyline-Pruning 排除了某些索引，TiDB 会输出一条 NOTE 级别的 warning 提示哪些索引在 Skyline-Pruning 排除之后保留下来。
在以下示例中，索引 idx_b 和 idx_e 均劣于 idx_b_c，因而被 Skyline-Pruning 排除，SHOW WARNING 的返回结果显示了经过 Skyline-Pruning 后剩余的索引。
mysql> CREATE TABLE t(a INT PRIMARY KEY, b INT, c INT, d INT, e INT, INDEX idx_b(b), INDEX idx_b_c(b, c), INDEX idx_e(e));
Query OK, 0 rows affected (0.01 sec)
mysql> EXPLAIN FORMAT = 'verbose' SELECT * FROM t WHERE b = 2 AND c > 4;
+-------------------------------+---------+---------+-----------+------------------------------+----------------------------------------------------+
| id | estRows | estCost | task | access object | operator info |
+-------------------------------+---------+---------+-----------+------------------------------+----------------------------------------------------+
| IndexLookUp_10 | 33.33 | 738.29 | root | | |
| ├─IndexRangeScan_8(Build) | 33.33 | 2370.00 | cop[tikv] | table:t, index:idx_b_c(b, c) | range:(2 4,2 +inf], keep order:false, stats:pseudo |
| └─TableRowIDScan_9(Probe) | 33.33 | 2370.00 | cop[tikv] | table:t | keep order:false, stats:pseudo |
+-------------------------------+---------+---------+-----------+------------------------------+----------------------------------------------------+
3 rows in set, 1 warning (0.00 sec)
mysql> SHOW WARNINGS;
+-------+------+------------------------------------------------------------------------------------------+
| Level | Code | Message |
+-------+------+------------------------------------------------------------------------------------------+
| Note | 1105 | [t,idx_b_c] remain after pruning paths for t given Prop{SortItems: [], TaskTp: rootTask} |
+-------+------+------------------------------------------------------------------------------------------+
1 row in set (0.00 sec)
基于代价选择
在使用 Skyline-Pruning 规则排除了不合适的索引之后，索引的选择完全基于代价估算，读表的代价估算需要考虑以下几个方面：
- 索引的每行数据在存储层的平均长度。
- 索引生成的查询范围的行数量。
- 索引的回表代价。
- 索引查询时的范围数量。
根据这些因子和代价模型，优化器会选择一个代价最低的索引进行读表。
代价选择调优的常见问题
估算的行数量不准确？
一般是统计信息过期或者准确度不够造成的，可以重新执行
analyze table或者修改analyze table的参数。统计信息准确，为什么读 TiFlash 更快，而优化器选择了 TiKV？
目前区别 TiFlash 和 TiKV 的代价模型还比较粗糙，可以调小
tidb_opt_seek_factor的值，让优化器倾向于选择 TiFlash。统计信息准确，某个索引要回表，但是它比另一个不用回表的索引实际执行更快，为什么选择了不用回表的索引？
碰到这种情况，可能是代价估算时对于回表的代价计算得过大，可以调小
tidb_opt_network_factor，降低回表的代价。
控制索引的选择
通过 Optimizer Hints 可以实现单条查询对索引选择的控制。
USE_INDEX/IGNORE_INDEX可以强制优化器使用/不使用某些索引。FORCE_INDEX和USE_INDEX的作用相同。READ_FROM_STORAGE可以强制优化器对于某些表选择 TiKV/TiFlash 的存储引擎进行查询。
使用多值索引
多值索引和普通索引有所不同，TiDB 目前只会使用 IndexMerge 来访问多值索引。因此要想使用多值索引进行数据访问，请确保tidb_enable_index_merge 被设置为 ON。
多值索引的使用限制请参考 CREATE INDEX。
支持多值索引的场景
目前 TiDB 支持将 json_member_of、json_contains 和 json_overlaps 条件自动转换成 IndexMerge 来访问多值索引。既可以依赖优化器根据代价自动选择，也可通过 use_index_merge optimizer hint 或 use_index 指定选择多值索引，见下面例子：
mysql> CREATE TABLE t1 (j JSON, INDEX idx((CAST(j->'$.path' AS SIGNED ARRAY)))); -- 使用 '$.path' 作为路径创建多值索引
Query OK, 0 rows affected (0.04 sec)
mysql> EXPLAIN SELECT /*+ use_index_merge(t1, idx) */ * FROM t1 WHERE (1 MEMBER OF (j->'$.path'));
+---------------------------------+---------+-----------+-----------------------------------------------------------------------------+------------------------------------------------------------------------+
| id | estRows | task | access object | operator info |
+---------------------------------+---------+-----------+-----------------------------------------------------------------------------+------------------------------------------------------------------------+
| Selection_5 | 8000.00 | root | | json_memberof(cast(1, json BINARY), json_extract(test.t1.j, "$.path")) |
| └─IndexMerge_8 | 10.00 | root | | type: union |
| ├─IndexRangeScan_6(Build) | 10.00 | cop[tikv] | table:t1, index:idx(cast(json_extract(`j`, _utf8'$.path') as signed array)) | range:[1,1], keep order:false, stats:pseudo |
| └─TableRowIDScan_7(Probe) | 10.00 | cop[tikv] | table:t1 | keep order:false, stats:pseudo |
+---------------------------------+---------+-----------+-----------------------------------------------------------------------------+------------------------------------------------------------------------+
4 rows in set, 1 warning (0.00 sec)
mysql> EXPLAIN SELECT /*+ use_index_merge(t1, idx) */ * FROM t1 WHERE JSON_CONTAINS((j->'$.path'), '[1, 2, 3]');
+-------------------------------+---------+-----------+-----------------------------------------------------------------------------+---------------------------------------------+
| id | estRows | task | access object | operator info |
+-------------------------------+---------+-----------+-----------------------------------------------------------------------------+---------------------------------------------+
| IndexMerge_9 | 10.00 | root | | type: intersection |
| ├─IndexRangeScan_5(Build) | 10.00 | cop[tikv] | table:t1, index:idx(cast(json_extract(`j`, _utf8'$.path') as signed array)) | range:[1,1], keep order:false, stats:pseudo |
| ├─IndexRangeScan_6(Build) | 10.00 | cop[tikv] | table:t1, index:idx(cast(json_extract(`j`, _utf8'$.path') as signed array)) | range:[2,2], keep order:false, stats:pseudo |
| ├─IndexRangeScan_7(Build) | 10.00 | cop[tikv] | table:t1, index:idx(cast(json_extract(`j`, _utf8'$.path') as signed array)) | range:[3,3], keep order:false, stats:pseudo |
| └─TableRowIDScan_8(Probe) | 10.00 | cop[tikv] | table:t1 | keep order:false, stats:pseudo |
+-------------------------------+---------+-----------+-----------------------------------------------------------------------------+---------------------------------------------+
5 rows in set (0.00 sec)
mysql> EXPLAIN SELECT /*+ use_index_merge(t1, idx) */ * FROM t1 WHERE JSON_OVERLAPS((j->'$.path'), '[1, 2, 3]');
+---------------------------------+---------+-----------+-----------------------------------------------------------------------------+----------------------------------------------------------------------------------+
| id | estRows | task | access object | operator info |
+---------------------------------+---------+-----------+-----------------------------------------------------------------------------+----------------------------------------------------------------------------------+
| Selection_5 | 8000.00 | root | | json_overlaps(json_extract(test.t1.j, "$.path"), cast("[1, 2, 3]", json BINARY)) |
| └─IndexMerge_10 | 10.00 | root | | type: union |
| ├─IndexRangeScan_6(Build) | 10.00 | cop[tikv] | table:t1, index:idx(cast(json_extract(`j`, _utf8'$.path') as signed array)) | range:[1,1], keep order:false, stats:pseudo |
| ├─IndexRangeScan_7(Build) | 10.00 | cop[tikv] | table:t1, index:idx(cast(json_extract(`j`, _utf8'$.path') as signed array)) | range:[2,2], keep order:false, stats:pseudo |
| ├─IndexRangeScan_8(Build) | 10.00 | cop[tikv] | table:t1, index:idx(cast(json_extract(`j`, _utf8'$.path') as signed array)) | range:[3,3], keep order:false, stats:pseudo |
| └─TableRowIDScan_9(Probe) | 10.00 | cop[tikv] | table:t1 | keep order:false, stats:pseudo |
+---------------------------------+---------+-----------+-----------------------------------------------------------------------------+----------------------------------------------------------------------------------+
6 rows in set, 1 warning (0.00 sec)
复合的多值索引，也一样可以使用 IndexMerge 进行访问：
mysql> CREATE TABLE t2 (a INT, j JSON, b INT, INDEX idx(a, (CAST(j->'$.path' AS SIGNED ARRAY)), b));
Query OK, 0 rows affected (0.04 sec)
mysql> EXPLAIN SELECT /*+ use_index_merge(t2, idx) */ * FROM t2 WHERE a=1 AND (1 MEMBER OF (j->'$.path')) AND b=2;
+---------------------------------+---------+-----------+-----------------------------------------------------------------------------------+------------------------------------------------------------------------+
| id | estRows | task | access object | operator info |
+---------------------------------+---------+-----------+-----------------------------------------------------------------------------------+------------------------------------------------------------------------+
| Selection_5 | 0.01 | root | | json_memberof(cast(1, json BINARY), json_extract(test.t2.j, "$.path")) |
| └─IndexMerge_8 | 0.00 | root | | type: union |
| ├─IndexRangeScan_6(Build) | 0.00 | cop[tikv] | table:t2, index:idx(a, cast(json_extract(`j`, _utf8'$.path') as signed array), b) | range:[1 1 2,1 1 2], keep order:false, stats:pseudo |
| └─TableRowIDScan_7(Probe) | 0.00 | cop[tikv] | table:t2 | keep order:false, stats:pseudo |
+---------------------------------+---------+-----------+-----------------------------------------------------------------------------------+------------------------------------------------------------------------+
4 rows in set, 1 warning (0.00 sec)
mysql> EXPLAIN SELECT /*+ use_index_merge(t2, idx) */ * FROM t2 WHERE a=1 AND JSON_CONTAINS((j->'$.path'), '[1, 2, 3]');
+-------------------------------+---------+-----------+-----------------------------------------------------------------------------------+-------------------------------------------------+
| id | estRows | task | access object | operator info |
+-------------------------------+---------+-----------+-----------------------------------------------------------------------------------+-------------------------------------------------+
| IndexMerge_9 | 0.10 | root | | type: intersection |
| ├─IndexRangeScan_5(Build) | 0.10 | cop[tikv] | table:t2, index:idx(a, cast(json_extract(`j`, _utf8'$.path') as signed array), b) | range:[1 1,1 1], keep order:false, stats:pseudo |
| ├─IndexRangeScan_6(Build) | 0.10 | cop[tikv] | table:t2, index:idx(a, cast(json_extract(`j`, _utf8'$.path') as signed array), b) | range:[1 2,1 2], keep order:false, stats:pseudo |
| ├─IndexRangeScan_7(Build) | 0.10 | cop[tikv] | table:t2, index:idx(a, cast(json_extract(`j`, _utf8'$.path') as signed array), b) | range:[1 3,1 3], keep order:false, stats:pseudo |
| └─TableRowIDScan_8(Probe) | 0.10 | cop[tikv] | table:t2 | keep order:false, stats:pseudo |
+-------------------------------+---------+-----------+-----------------------------------------------------------------------------------+-------------------------------------------------+
5 rows in set (0.00 sec)
mysql> EXPLAIN SELECT /*+ use_index_merge(t2, idx) */ * FROM t2 WHERE a=1 AND JSON_OVERLAPS((j->'$.path'), '[1, 2, 3]');
+---------------------------------+---------+-----------+-----------------------------------------------------------------------------------+----------------------------------------------------------------------------------+
| id | estRows | task | access object | operator info |
+---------------------------------+---------+-----------+-----------------------------------------------------------------------------------+----------------------------------------------------------------------------------+
| Selection_5 | 8.00 | root | | json_overlaps(json_extract(test.t2.j, "$.path"), cast("[1, 2, 3]", json BINARY)) |
| └─IndexMerge_10 | 0.10 | root | | type: union |
| ├─IndexRangeScan_6(Build) | 0.10 | cop[tikv] | table:t2, index:idx(a, cast(json_extract(`j`, _utf8'$.path') as signed array), b) | range:[1 1,1 1], keep order:false, stats:pseudo |
| ├─IndexRangeScan_7(Build) | 0.10 | cop[tikv] | table:t2, index:idx(a, cast(json_extract(`j`, _utf8'$.path') as signed array), b) | range:[1 2,1 2], keep order:false, stats:pseudo |
| ├─IndexRangeScan_8(Build) | 0.10 | cop[tikv] | table:t2, index:idx(a, cast(json_extract(`j`, _utf8'$.path') as signed array), b) | range:[1 3,1 3], keep order:false, stats:pseudo |
| └─TableRowIDScan_9(Probe) | 0.10 | cop[tikv] | table:t2 | keep order:false, stats:pseudo |
+---------------------------------+---------+-----------+-----------------------------------------------------------------------------------+----------------------------------------------------------------------------------+
6 rows in set, 1 warning (0.00 sec)
对于由多个 member of 组成的 OR/AND 条件，且这些条件可以被作用在同一个或者多个多值索引上，由于 member of 的单路性质，也可以使用 IndexMerge UNION 或者 INTERSECTION 进行访问：
mysql> CREATE TABLE t3 (a INT, j JSON, b INT, k JSON, INDEX idx(a, (CAST(j AS SIGNED ARRAY))), INDEX idx2(b,(CAST(k as SIGNED ARRAY))));
Query OK, 0 rows affected (0.04 sec)
使用 UNION 进行访问：
mysql> EXPLAIN SELECT /*+ use_index_merge(t3, idx) */ * FROM t3 WHERE ((a=1 AND (1 member of (j)))) OR ((a=2 AND (2 member of (j))));
+---------------------------------+---------+-----------+---------------------------------------------------+--------------------------------------------------------------------------------------------------------------------------------------------------+
| id | estRows | task | access object | operator info |
+---------------------------------+---------+-----------+---------------------------------------------------+--------------------------------------------------------------------------------------------------------------------------------------------------+
| Selection_5 | 0.08 | root | | or(and(eq(test.t3.a, 1), json_memberof(cast(1, json BINARY), test.t3.j)), and(eq(test.t3.a, 2), json_memberof(cast(2, json BINARY), test.t3.j))) |
| └─IndexMerge_9 | 0.10 | root | | type: union |
| ├─IndexRangeScan_6(Build) | 0.10 | cop[tikv] | table:t3, index:idx(a, cast(`j` as signed array)) | range:[1 1,1 1], keep order:false, stats:pseudo |
| ├─IndexRangeScan_7(Build) | 0.10 | cop[tikv] | table:t3, index:idx(a, cast(`j` as signed array)) | range:[2 2,2 2], keep order:false, stats:pseudo |
| └─TableRowIDScan_8(Probe) | 0.10 | cop[tikv] | table:t3 | keep order:false, stats:pseudo |
+---------------------------------+---------+-----------+---------------------------------------------------+--------------------------------------------------------------------------------------------------------------------------------------------------+
使用 INTERSECTION 进行访问：
tidb> EXPLAIN SELECT /*+ use_index_merge(t3, idx, idx2) */ * FROM t3 WHERE ((a=1 AND (1 member of (j)))) AND ((b=1 AND (2 member of (k))));
+-------------------------------+---------+-----------+----------------------------------------------------+-------------------------------------------------+
| id | estRows | task | access object | operator info |
+-------------------------------+---------+-----------+----------------------------------------------------+-------------------------------------------------+
| IndexMerge_8 | 0.10 | root | | type: intersection |
| ├─IndexRangeScan_5(Build) | 0.10 | cop[tikv] | table:t3, index:idx(a, cast(`j` as signed array)) | range:[1 1,1 1], keep order:false, stats:pseudo |
| ├─IndexRangeScan_6(Build) | 0.10 | cop[tikv] | table:t3, index:idx2(b, cast(`k` as signed array)) | range:[1 2,1 2], keep order:false, stats:pseudo |
| └─TableRowIDScan_7(Probe) | 0.10 | cop[tikv] | table:t3 | keep order:false, stats:pseudo |
+-------------------------------+---------+-----------+----------------------------------------------------+-------------------------------------------------+
4 rows in set (0.01 sec)
部分支持多值索引的场景
如果多个条件通过 AND/OR 进行组合，每个 item 条件对应多个不同的索引，则有以下情况：
- 如果单个 item 的路径也是 index merge，则只要其和外部的
AND/OR的逻辑一致，就可以和外部 index merge 融合。 - 如果单个 item 的路径也是 index merge，且其 index partial path 只有一条，那么无论其自身 index merge 的逻辑是
AND/OR，都可以和外部 index merge 融合。
mysql> CREATE TABLE t(j1 JSON, j2 JSON, a INT, INDEX k1((CAST(j1->'$.path' AS SIGNED ARRAY))), INDEX k2((CAST(j2->'$.path' AS SIGNED ARRAY))), INDEX ka(a));
Query OK, 0 rows affected (0.02 sec)
tidb> EXPLAIN SELECT /*+ use_index_merge(t, k1, k2, ka) */ * FROM t WHERE (1 member of (j1->'$.path')) AND (2 member of (j2->'$.path')) AND (a = 3);
+-------------------------------+---------+-----------+----------------------------------------------------------------------------+---------------------------------------------+
| id | estRows | task | access object | operator info |
+-------------------------------+---------+-----------+----------------------------------------------------------------------------+---------------------------------------------+
| IndexMerge_9 | 10.00 | root | | type: intersection |
| ├─IndexRangeScan_5(Build) | 10.00 | cop[tikv] | table:t, index:ka(a) | range:[3,3], keep order:false, stats:pseudo |
| ├─IndexRangeScan_6(Build) | 10.00 | cop[tikv] | table:t, index:k1(cast(json_extract(`j1`, _utf8'$.path') as signed array)) | range:[1,1], keep order:false, stats:pseudo |
| ├─IndexRangeScan_7(Build) | 10.00 | cop[tikv] | table:t, index:k2(cast(json_extract(`j2`, _utf8'$.path') as signed array)) | range:[2,2], keep order:false, stats:pseudo |
| └─TableRowIDScan_8(Probe) | 10.00 | cop[tikv] | table:t | keep order:false, stats:pseudo |
+-------------------------------+---------+-----------+----------------------------------------------------------------------------+---------------------------------------------+
5 rows in set (0.00 sec)
目前只能使用 1 个索引进行访问，而无法产生如下面这样同时使用多个索引的计划：
Selection
└─IndexMerge
├─IndexRangeScan(k1)
├─IndexRangeScan(k2)
├─IndexRangeScan(ka)
└─Selection
└─TableRowIDScan
不支持多值索引的场景
如果多个条件通过 OR 进行组合，且这些条件对应多个不同的索引，则无法使用多值索引，如：
mysql> create table t(j1 json, j2 json, a int, INDEX k1((CAST(j1->'$.path' AS SIGNED ARRAY))), INDEX k2((CAST(j2->'$.path' AS SIGNED ARRAY))), INDEX ka(a));
Query OK, 0 rows affected (0.03 sec)
mysql> explain select /*+ use_index_merge(t, k1, k2, ka) */ * from t where (1 member of (j1->'$.path')) or (2 member of (j2->'$.path'));
+-------------------------+----------+-----------+---------------+----------------------------------------------------------------------------------------------------------------------------------------------------+
| id | estRows | task | access object | operator info |
+-------------------------+----------+-----------+---------------+----------------------------------------------------------------------------------------------------------------------------------------------------+
| Selection_5 | 8000.00 | root | | or(json_memberof(cast(1, json BINARY), json_extract(test.t.j1, "$.path")), json_memberof(cast(2, json BINARY), json_extract(test.t.j2, "$.path"))) |
| └─TableReader_7 | 10000.00 | root | | data:TableFullScan_6 |
| └─TableFullScan_6 | 10000.00 | cop[tikv] | table:t | keep order:false, stats:pseudo |
+-------------------------+----------+-----------+---------------+----------------------------------------------------------------------------------------------------------------------------------------------------+
3 rows in set, 3 warnings (0.00 sec)
mysql> explain select /*+ use_index_merge(t, k1, k2, ka) */ * from t where (1 member of (j1->'$.path')) or (a = 3);
+-------------------------+----------+-----------+---------------+---------------------------------------------------------------------------------------------+
| id | estRows | task | access object | operator info |
+-------------------------+----------+-----------+---------------+---------------------------------------------------------------------------------------------+
| Selection_5 | 8000.00 | root | | or(json_memberof(cast(1, json BINARY), json_extract(test.t.j1, "$.path")), eq(test.t.a, 3)) |
| └─TableReader_7 | 10000.00 | root | | data:TableFullScan_6 |
| └─TableFullScan_6 | 10000.00 | cop[tikv] | table:t | keep order:false, stats:pseudo |
+-------------------------+----------+-----------+---------------+---------------------------------------------------------------------------------------------+
3 rows in set, 3 warnings (0.00 sec)
对于上述场景，你可以使用 Union All 改写查询以使用多值索引。
下面是一些更加复杂且暂时无法使用 IndexMerge 来访问多值索引的场景：
mysql> CREATE TABLE t4 (j JSON, INDEX idx((CAST(j AS SIGNED ARRAY))));
Query OK, 0 rows affected (0.04 sec)
-- 如果查询包含多个 json_contains 通过 OR 组成的条件，则无法通过 IndexMerge 访问索引
mysql> EXPLAIN SELECT /*+ use_index_merge(t3, idx) */ * FROM t3 WHERE (json_contains(j, '[1, 2]')) OR (json_contains(j, '[3, 4]'));
+-------------------------+----------+-----------+---------------+------------------------------------------------------------------------------------------------------------------+
| id | estRows | task | access object | operator info |
+-------------------------+----------+-----------+---------------+------------------------------------------------------------------------------------------------------------------+
| TableReader_7 | 9600.00 | root | | data:Selection_6 |
| └─Selection_6 | 9600.00 | cop[tikv] | | or(json_contains(test.t3.j, cast("[1, 2]", json BINARY)), json_contains(test.t3.j, cast("[3, 4]", json BINARY))) |
| └─TableFullScan_5 | 10000.00 | cop[tikv] | table:t3 | keep order:false, stats:pseudo |
+-------------------------+----------+-----------+---------------+------------------------------------------------------------------------------------------------------------------+
3 rows in set, 1 warning (0.00 sec)
mysql> SHOW WARNINGS;
+---------+------+----------------------------+
| Level | Code | Message |
+---------+------+----------------------------+
| Warning | 1105 | IndexMerge is inapplicable |
+---------+------+----------------------------+
1 row in set (0.00 sec)
mysql> EXPLAIN SELECT /*+ use_index_merge(t3, idx) */ * FROM t3 WHERE (json_contains(j, '[1, 2]')) OR (json_contains(j, '[3, 4]'));
+-------------------------+----------+-----------+---------------+------------------------------------------------------------------------------------------------------------------+
| id | estRows | task | access object | operator info |
+-------------------------+----------+-----------+---------------+------------------------------------------------------------------------------------------------------------------+
| TableReader_7 | 9600.00 | root | | data:Selection_6 |
| └─Selection_6 | 9600.00 | cop[tikv] | | or(json_contains(test.t3.j, cast("[1, 2]", json BINARY)), json_contains(test.t3.j, cast("[3, 4]", json BINARY))) |
| └─TableFullScan_5 | 10000.00 | cop[tikv] | table:t3 | keep order:false, stats:pseudo |
+-------------------------+----------+-----------+---------------+------------------------------------------------------------------------------------------------------------------+
3 rows in set, 1 warning (0.01 sec)
-- 如果查询中包含较为复杂的多层 OR / AND 嵌套形成的表达式，则无法通过 IndexMerge 访问索引
mysql> EXPLAIN SELECT /*+ use_index_merge(t3, idx) */ * FROM t3 WHERE ((1 member of (j)) AND (2 member of (j))) OR ((3 member of (j)) AND (4 member of (j)));
+-------------------------+----------+-----------+---------------+--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| id | estRows | task | access object | operator info |
+-------------------------+----------+-----------+---------------+--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| Selection_5 | 8000.00 | root | | or(and(json_memberof(cast(1, json BINARY), test.t3.j), json_memberof(cast(2, json BINARY), test.t3.j)), and(json_memberof(cast(3, json BINARY), test.t3.j), json_memberof(cast(4, json BINARY), test.t3.j))) |
| └─TableReader_7 | 10000.00 | root | | data:TableFullScan_6 |
| └─TableFullScan_6 | 10000.00 | cop[tikv] | table:t3 | keep order:false, stats:pseudo |
+-------------------------+----------+-----------+---------------+--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
3 rows in set, 2 warnings (0.00 sec)
受限于多值索引的特性，当多值索引无法生效时，使用 use_index 可能会返回 Can't find a proper physical plan for this query 的错误，而使用 use_index_merge 不会，因此建议使用 use_index_merge：
mysql> EXPLAIN SELECT /*+ use_index(t3, idx) */ * FROM t3 WHERE ((1 member of (j)) AND (2 member of (j))) OR ((3 member of (j)) AND (4 member of (j)));
ERROR 1815 (HY000): Internal : Cant find a proper physical plan for this query
mysql> EXPLAIN SELECT /*+ use_index_merge(t3, idx) */ * FROM t3 WHERE ((1 member of (j)) AND (2 member of (j))) OR ((3 member of (j)) AND (4 member of (j)));
+-------------------------+----------+-----------+---------------+--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| id | estRows | task | access object | operator info |
+-------------------------+----------+-----------+---------------+--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| Selection_5 | 8000.00 | root | | or(and(json_memberof(cast(1, json BINARY), test.t3.j), json_memberof(cast(2, json BINARY), test.t3.j)), and(json_memberof(cast(3, json BINARY), test.t3.j), json_memberof(cast(4, json BINARY), test.t3.j))) |
| └─TableReader_7 | 10000.00 | root | | data:TableFullScan_6 |
| └─TableFullScan_6 | 10000.00 | cop[tikv] | table:t3 | keep order:false, stats:pseudo |
+-------------------------+----------+-----------+---------------+--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
3 rows in set, 2 warnings (0.00 sec)