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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 |
注意
TableReader 是基于 _tidb_rowid 的索引，TiFlash 是列存索引，所以索引的选择即是读表算子的选择。
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：
CREATE TABLE t2 (a INT, j JSON, b INT, k JSON, INDEX idx(a, (CAST(j->'$.path' AS SIGNED ARRAY)), b), INDEX idx2(b, (CAST(k->'$.path' AS SIGNED ARRAY))));EXPLAIN SELECT /*+ use_index_merge(t2, idx) */ * FROM t2 WHERE a=1 AND (1 MEMBER OF (j->'$.path')) AND b=2;EXPLAIN SELECT /*+ use_index_merge(t2, idx) */ * FROM t2 WHERE a=1 AND JSON_CONTAINS((j->'$.path'), '[1, 2, 3]');EXPLAIN SELECT /*+ use_index_merge(t2, idx) */ * FROM t2 WHERE a=1 AND JSON_OVERLAPS((j->'$.path'), '[1, 2, 3]');EXPLAIN SELECT /*+ use_index_merge(t2, idx, idx2) */ * FROM t2 WHERE (a=1 AND 1 member of (j->'$.path')) AND (b=1 AND 2 member of (k->'$.path'));
> EXPLAIN SELECT /*+ use_index_merge(t2, idx) */ * FROM t2 WHERE a=1 AND (1 MEMBER OF (j->'$.path')) AND b=2;+-------------------------------+---------+-----------+-----------------------------------------------------------------------------------+-----------------------------------------------------+| id | estRows | task | access object | operator info |+-------------------------------+---------+-----------+-----------------------------------------------------------------------------------+-----------------------------------------------------+| IndexMerge_7 | 0.00 | root | | type: union || ├─IndexRangeScan_5(Build) | 0.00 | cop[tikv] | table:t2, index:idx(a, cast(json_extract(`j`, _utf8'$.path') as signed array), b) | range:[1 1 2,1 1 2], keep order:false, stats:pseudo || └─TableRowIDScan_6(Probe) | 0.00 | cop[tikv] | table:t2 | keep order:false, stats:pseudo |+-------------------------------+---------+-----------+-----------------------------------------------------------------------------------+-----------------------------------------------------+> EXPLAIN SELECT /*+ use_index_merge(t2, idx) */ * FROM t2 WHERE a=1 AND JSON_CONTAINS((j->'$.path'), '[1, 2, 3]');+-------------------------------+---------+-----------+-----------------------------------------------------------------------------------+-------------------------------------------------+| id | estRows | task | access object | operator info |+-------------------------------+---------+-----------+-----------------------------------------------------------------------------------+-------------------------------------------------+| IndexMerge_9 | 0.00 | 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'$.path') as signed array), b) | range:[1 3,1 3], keep order:false, stats:pseudo || └─TableRowIDScan_8(Probe) | 0.00 | cop[tikv] | table:t2 | keep order:false, stats:pseudo |+-------------------------------+---------+-----------+-----------------------------------------------------------------------------------+-------------------------------------------------+> EXPLAIN SELECT /*+ use_index_merge(t2, idx) */ * FROM t2 WHERE a=1 AND JSON_OVERLAPS((j->'$.path'), '[1, 2, 3]');+---------------------------------+---------+-----------+-----------------------------------------------------------------------------------+----------------------------------------------------------------------------------+| id | estRows | task | access object | operator info |+---------------------------------+---------+-----------+-----------------------------------------------------------------------------------+----------------------------------------------------------------------------------+| Selection_5 | 0.24 | root | | json_overlaps(json_extract(test.t2.j, "$.path"), cast("[1, 2, 3]", json BINARY)) || └─IndexMerge_10 | 0.30 | root | | type: union || ├─IndexRangeScan_6(Build) | 0.10 | cop[tikv] | table:t2, index:idx(a, cast(json_extract(`j`, _utf8'$.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) | 0.30 | cop[tikv] | table:t2 | keep order:false, stats:pseudo |+---------------------------------+---------+-----------+-----------------------------------------------------------------------------------+----------------------------------------------------------------------------------+> EXPLAIN SELECT /*+ use_index_merge(t2, idx, idx2) */ * FROM t2 WHERE (a=1 AND 1 member of (j->'$.path')) AND (b=1 AND 2 member of (k->'$.path'));+-------------------------------+---------+-----------+-----------------------------------------------------------------------------------+-----------------------------------------------------+| id | estRows | task | access object | operator info |+-------------------------------+---------+-----------+-----------------------------------------------------------------------------------+-----------------------------------------------------+| IndexMerge_8 | 0.00 | root | | type: intersection || ├─IndexRangeScan_5(Build) | 0.00 | cop[tikv] | table:t2, index:idx(a, cast(json_extract(`j`, _utf8'$.path') as signed array), b) | range:[1 1 1,1 1 1], keep order:false, stats:pseudo || ├─IndexRangeScan_6(Build) | 0.10 | cop[tikv] | table:t2, index:idx2(b, cast(json_extract(`k`, _utf8'$.path') as signed array)) | range:[1 2,1 2], keep order:false, stats:pseudo || └─TableRowIDScan_7(Probe) | 0.00 | cop[tikv] | table:t2 | keep order:false, stats:pseudo |+-------------------------------+---------+-----------+-----------------------------------------------------------------------------------+-----------------------------------------------------+
同时，多值索引可以和普通索引共同组成 IndexMerge 使用。例如：
CREATE TABLE t3(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));EXPLAIN SELECT /*+ use_index_merge(t3, k1, k2, ka) */ * FROM t3 WHERE 1 member of (j1->'$.path') OR a = 3;EXPLAIN SELECT /*+ use_index_merge(t3, k1, k2, ka) */ * FROM t3 WHERE 1 member of (j1->'$.path') AND 2 member of (j2->'$.path') AND (a = 3);
> EXPLAIN SELECT /*+ use_index_merge(t3, k1, k2, ka) */ * FROM t3 WHERE 1 member of (j1->'$.path') OR a = 3;+-------------------------------+---------+-----------+-----------------------------------------------------------------------------+---------------------------------------------+| id | estRows | task | access object | operator info |+-------------------------------+---------+-----------+-----------------------------------------------------------------------------+---------------------------------------------+| IndexMerge_8 | 19.99 | root | | type: union || ├─IndexRangeScan_5(Build) | 10.00 | cop[tikv] | table:t3, index:k1(cast(json_extract(`j1`, _utf8'$.path') as signed array)) | range:[1,1], keep order:false, stats:pseudo || ├─IndexRangeScan_6(Build) | 10.00 | cop[tikv] | table:t3, index:ka(a) | range:[3,3], keep order:false, stats:pseudo || └─TableRowIDScan_7(Probe) | 19.99 | cop[tikv] | table:t3 | keep order:false, stats:pseudo |+-------------------------------+---------+-----------+-----------------------------------------------------------------------------+---------------------------------------------+> EXPLAIN SELECT /*+ use_index_merge(t3, k1, k2, ka) */ * FROM t3 WHERE 1 member of (j1->'$.path') AND 2 member of (j2->'$.path') AND (a = 3);+-------------------------------+---------+-----------+-----------------------------------------------------------------------------+---------------------------------------------+| id | estRows | task | access object | operator info |+-------------------------------+---------+-----------+-----------------------------------------------------------------------------+---------------------------------------------+| IndexMerge_9 | 0.00 | root | | type: intersection || ├─IndexRangeScan_5(Build) | 10.00 | cop[tikv] | table:t3, index:ka(a) | range:[3,3], keep order:false, stats:pseudo || ├─IndexRangeScan_6(Build) | 10.00 | cop[tikv] | table:t3, index:k1(cast(json_extract(`j1`, _utf8'$.path') as signed array)) | range:[1,1], keep order:false, stats:pseudo || ├─IndexRangeScan_7(Build) | 10.00 | cop[tikv] | table:t3, index:k2(cast(json_extract(`j2`, _utf8'$.path') as signed array)) | range:[2,2], keep order:false, stats:pseudo || └─TableRowIDScan_8(Probe) | 0.00 | cop[tikv] | table:t3 | keep order:false, stats:pseudo |+-------------------------------+---------+-----------+-----------------------------------------------------------------------------+---------------------------------------------+
对于由 OR 或 AND 连接而成的多个 json_member_of、json_contains 或 json_overlaps 条件，需要满足以下条件才能使用 IndexMerge 访问：
CREATE TABLE t4(a INT, j JSON, INDEX mvi1((CAST(j->'$.a' AS UNSIGNED ARRAY))), INDEX mvi2((CAST(j->'$.b' AS UNSIGNED ARRAY))));
对于由
OR连接而成的条件，其中每个子条件都需要分别能够使用 IndexMerge 访问。例如：EXPLAIN SELECT /*+ use_index_merge(t4, mvi1) */ * FROM t4 WHERE json_overlaps(j->'$.a', '[1, 2]') OR json_overlaps(j->'$.a', '[3, 4]');EXPLAIN SELECT /*+ use_index_merge(t4, mvi1) */ * FROM t4 WHERE json_overlaps(j->'$.a', '[1, 2]') OR json_length(j->'$.a') = 3;SHOW WARNINGS;
> EXPLAIN SELECT /*+ use_index_merge(t4, mvi1) */ * FROM t4 WHERE json_overlaps(j->'$.a', '[1, 2]') OR json_overlaps(j->'$.a', '[3, 4]');+----------------------------------+---------+-----------+-----------------------------------------------------------------------------+------------------------------------------------------------------------------------------------------------------------------------------------------------+| id | estRows | task | access object | operator info |+----------------------------------+---------+-----------+-----------------------------------------------------------------------------+------------------------------------------------------------------------------------------------------------------------------------------------------------+| Selection_5 | 31.95 | root | | or(json_overlaps(json_extract(test.t4.j, "$.a"), cast("[1, 2]", json BINARY)), json_overlaps(json_extract(test.t4.j, "$.a"), cast("[3, 4]", json BINARY))) || └─IndexMerge_11 | 39.94 | root | | type: union || ├─IndexRangeScan_6(Build) | 10.00 | cop[tikv] | table:t4, index:mvi1(cast(json_extract(`j`, _utf8'$.a') as unsigned array)) | range:[1,1], keep order:false, stats:pseudo || ├─IndexRangeScan_7(Build) | 10.00 | cop[tikv] | table:t4, index:mvi1(cast(json_extract(`j`, _utf8'$.a') as unsigned array)) | range:[2,2], keep order:false, stats:pseudo || ├─IndexRangeScan_8(Build) | 10.00 | cop[tikv] | table:t4, index:mvi1(cast(json_extract(`j`, _utf8'$.a') as unsigned array)) | range:[3,3], keep order:false, stats:pseudo || ├─IndexRangeScan_9(Build) | 10.00 | cop[tikv] | table:t4, index:mvi1(cast(json_extract(`j`, _utf8'$.a') as unsigned array)) | range:[4,4], keep order:false, stats:pseudo || └─TableRowIDScan_10(Probe) | 39.94 | cop[tikv] | table:t4 | keep order:false, stats:pseudo |+----------------------------------+---------+-----------+-----------------------------------------------------------------------------+------------------------------------------------------------------------------------------------------------------------------------------------------------+-- json_length(j->'$.a') = 3 无法使用 IndexMerge 访问，因此该 SQL 整体无法使用 IndexMerge 访问> EXPLAIN SELECT /*+ use_index_merge(t4, mvi1) */ * FROM t4 WHERE json_overlaps(j->'$.a', '[1, 2]') OR json_length(j->'$.a') = 3;+-------------------------+----------+-----------+---------------+------------------------------------------------------------------------------------------------------------------------------------+| id | estRows | task | access object | operator info |+-------------------------+----------+-----------+---------------+------------------------------------------------------------------------------------------------------------------------------------+| Selection_5 | 8000.00 | root | | or(json_overlaps(json_extract(test.t4.j, "$.a"), cast("[1, 2]", json BINARY)), eq(json_length(json_extract(test.t4.j, "$.a")), 3)) || └─TableReader_7 | 10000.00 | root | | data:TableFullScan_6 || └─TableFullScan_6 | 10000.00 | cop[tikv] | table:t4 | keep order:false, stats:pseudo |+-------------------------+----------+-----------+---------------+------------------------------------------------------------------------------------------------------------------------------------+> SHOW WARNINGS;+---------+------+----------------------------+| Level | Code | Message |+---------+------+----------------------------+| Warning | 1105 | IndexMerge is inapplicable |+---------+------+----------------------------+
对于由
AND连接而成的条件，其中一部分子条件需要分别能够使用 IndexMerge 访问。最终使用 IndexMerge 访问时，也只能利用这一部分条件。例如：EXPLAIN SELECT /*+ use_index_merge(t4, mvi1) */ * FROM t4 WHERE json_contains(j->'$.a', '[1, 2]') AND json_contains(j->'$.a', '[3, 4]');EXPLAIN SELECT /*+ use_index_merge(t4, mvi1) */ * FROM t4 WHERE json_contains(j->'$.a', '[1, 2]') AND json_contains(j->'$.a', '[3, 4]') AND json_length(j->'$.a') = 2;
> EXPLAIN SELECT /*+ use_index_merge(t4, mvi1) */ * FROM t4 WHERE json_contains(j->'$.a', '[1, 2]') AND json_contains(j->'$.a', '[3, 4]');+-------------------------------+---------+-----------+-----------------------------------------------------------------------------+---------------------------------------------+| id | estRows | task | access object | operator info |+-------------------------------+---------+-----------+-----------------------------------------------------------------------------+---------------------------------------------+| IndexMerge_10 | 0.00 | root | | type: intersection || ├─IndexRangeScan_5(Build) | 10.00 | cop[tikv] | table:t4, index:mvi1(cast(json_extract(`j`, _utf8'$.a') as unsigned array)) | range:[1,1], keep order:false, stats:pseudo || ├─IndexRangeScan_6(Build) | 10.00 | cop[tikv] | table:t4, index:mvi1(cast(json_extract(`j`, _utf8'$.a') as unsigned array)) | range:[2,2], keep order:false, stats:pseudo || ├─IndexRangeScan_7(Build) | 10.00 | cop[tikv] | table:t4, index:mvi1(cast(json_extract(`j`, _utf8'$.a') as unsigned array)) | range:[3,3], keep order:false, stats:pseudo || ├─IndexRangeScan_8(Build) | 10.00 | cop[tikv] | table:t4, index:mvi1(cast(json_extract(`j`, _utf8'$.a') as unsigned array)) | range:[4,4], keep order:false, stats:pseudo || └─TableRowIDScan_9(Probe) | 0.00 | cop[tikv] | table:t4 | keep order:false, stats:pseudo |+-------------------------------+---------+-----------+-----------------------------------------------------------------------------+---------------------------------------------+-- json_length(j->'$.a') = 3 无法使用 IndexMerge 访问，因此只有另外两个 json_contains 表达式可以使用 IndexMerge 访问，而 json_length(j->'$.a') = 3 成为一个单独的 Selection 算子> EXPLAIN SELECT /*+ use_index_merge(t4, mvi1) */ * FROM t4 WHERE json_contains(j->'$.a', '[1, 2]') AND json_contains(j->'$.a', '[3, 4]') AND json_length(j->'$.a') = 2;+-------------------------------+---------+-----------+-----------------------------------------------------------------------------+----------------------------------------------------+| id | estRows | task | access object | operator info |+-------------------------------+---------+-----------+-----------------------------------------------------------------------------+----------------------------------------------------+| IndexMerge_11 | 0.00 | root | | type: intersection || ├─IndexRangeScan_5(Build) | 10.00 | cop[tikv] | table:t4, index:mvi1(cast(json_extract(`j`, _utf8'$.a') as unsigned array)) | range:[1,1], keep order:false, stats:pseudo || ├─IndexRangeScan_6(Build) | 10.00 | cop[tikv] | table:t4, index:mvi1(cast(json_extract(`j`, _utf8'$.a') as unsigned array)) | range:[2,2], keep order:false, stats:pseudo || ├─IndexRangeScan_7(Build) | 10.00 | cop[tikv] | table:t4, index:mvi1(cast(json_extract(`j`, _utf8'$.a') as unsigned array)) | range:[3,3], keep order:false, stats:pseudo || ├─IndexRangeScan_8(Build) | 10.00 | cop[tikv] | table:t4, index:mvi1(cast(json_extract(`j`, _utf8'$.a') as unsigned array)) | range:[4,4], keep order:false, stats:pseudo || └─Selection_10(Probe) | 0.00 | cop[tikv] | | eq(json_length(json_extract(test.t4.j, "$.a")), 2) || └─TableRowIDScan_9 | 0.00 | cop[tikv] | table:t4 | keep order:false, stats:pseudo |+-------------------------------+---------+-----------+-----------------------------------------------------------------------------+----------------------------------------------------+
用于构成整体 IndexMerge 计划的每个子条件，需要分别符合用于连接的
OR或AND的语义。具体如下：json_contains由AND连接，符合语义。例如：EXPLAIN SELECT /*+ use_index_merge(t4, mvi1, mvi2) */ * FROM t4 WHERE json_contains(j->'$.a', '[1]') AND json_contains(j->'$.b', '[2, 3]');EXPLAIN SELECT /*+ use_index_merge(t4, mvi1, mvi2) */ * FROM t4 WHERE json_contains(j->'$.a', '[1]') OR json_contains(j->'$.b', '[2, 3]');
> EXPLAIN SELECT /*+ use_index_merge(t4, mvi1, mvi2) */ * FROM t4 WHERE json_contains(j->'$.a', '[1]') AND json_contains(j->'$.b', '[2, 3]');+-------------------------------+---------+-----------+-----------------------------------------------------------------------------+---------------------------------------------+| id | estRows | task | access object | operator info |+-------------------------------+---------+-----------+-----------------------------------------------------------------------------+---------------------------------------------+| IndexMerge_9 | 0.00 | root | | type: intersection || ├─IndexRangeScan_5(Build) | 10.00 | cop[tikv] | table:t4, index:mvi1(cast(json_extract(`j`, _utf8'$.a') as unsigned array)) | range:[1,1], keep order:false, stats:pseudo || ├─IndexRangeScan_6(Build) | 10.00 | cop[tikv] | table:t4, index:mvi2(cast(json_extract(`j`, _utf8'$.b') as unsigned array)) | range:[2,2], keep order:false, stats:pseudo || ├─IndexRangeScan_7(Build) | 10.00 | cop[tikv] | table:t4, index:mvi2(cast(json_extract(`j`, _utf8'$.b') as unsigned array)) | range:[3,3], keep order:false, stats:pseudo || └─TableRowIDScan_8(Probe) | 0.00 | cop[tikv] | table:t4 | keep order:false, stats:pseudo |+-------------------------------+---------+-----------+-----------------------------------------------------------------------------+---------------------------------------------+-- 不符合语义，如前文所述，该 SQL 整体无法使用 IndexMerge 访问> EXPLAIN SELECT /*+ use_index_merge(t4, mvi1, mvi2) */ * FROM t4 WHERE json_contains(j->'$.a', '[1]') OR json_contains(j->'$.b', '[2, 3]');+-------------------------+----------+-----------+---------------+---------------------------------------------------------------------------------------------------------------------------------------------------------+| id | estRows | task | access object | operator info |+-------------------------+----------+-----------+---------------+---------------------------------------------------------------------------------------------------------------------------------------------------------+| TableReader_7 | 10.01 | root | | data:Selection_6 || └─Selection_6 | 10.01 | cop[tikv] | | or(json_contains(json_extract(test.t4.j, "$.a"), cast("[1]", json BINARY)), json_contains(json_extract(test.t4.j, "$.b"), cast("[2, 3]", json BINARY))) || └─TableFullScan_5 | 10000.00 | cop[tikv] | table:t4 | keep order:false, stats:pseudo |+-------------------------+----------+-----------+---------------+---------------------------------------------------------------------------------------------------------------------------------------------------------+
json_overlaps由OR连接，符合语义。例如：EXPLAIN SELECT /*+ use_index_merge(t4, mvi1, mvi2) */ * FROM t4 WHERE json_overlaps(j->'$.a', '[1]') OR json_overlaps(j->'$.b', '[2, 3]');EXPLAIN SELECT /*+ use_index_merge(t4, mvi1, mvi2) */ * FROM t4 WHERE json_overlaps(j->'$.a', '[1]') AND json_overlaps(j->'$.b', '[2, 3]');
> EXPLAIN SELECT /*+ use_index_merge(t4, mvi1, mvi2) */ * FROM t4 WHERE json_overlaps(j->'$.a', '[1]') OR json_overlaps(j->'$.b', '[2, 3]');+---------------------------------+---------+-----------+-----------------------------------------------------------------------------+---------------------------------------------------------------------------------------------------------------------------------------------------------+| id | estRows | task | access object | operator info |+---------------------------------+---------+-----------+-----------------------------------------------------------------------------+---------------------------------------------------------------------------------------------------------------------------------------------------------+| Selection_5 | 23.98 | root | | or(json_overlaps(json_extract(test.t4.j, "$.a"), cast("[1]", json BINARY)), json_overlaps(json_extract(test.t4.j, "$.b"), cast("[2, 3]", json BINARY))) || └─IndexMerge_10 | 29.97 | root | | type: union || ├─IndexRangeScan_6(Build) | 10.00 | cop[tikv] | table:t4, index:mvi1(cast(json_extract(`j`, _utf8'$.a') as unsigned array)) | range:[1,1], keep order:false, stats:pseudo || ├─IndexRangeScan_7(Build) | 10.00 | cop[tikv] | table:t4, index:mvi2(cast(json_extract(`j`, _utf8'$.b') as unsigned array)) | range:[2,2], keep order:false, stats:pseudo || ├─IndexRangeScan_8(Build) | 10.00 | cop[tikv] | table:t4, index:mvi2(cast(json_extract(`j`, _utf8'$.b') as unsigned array)) | range:[3,3], keep order:false, stats:pseudo || └─TableRowIDScan_9(Probe) | 29.97 | cop[tikv] | table:t4 | keep order:false, stats:pseudo |+---------------------------------+---------+-----------+-----------------------------------------------------------------------------+---------------------------------------------------------------------------------------------------------------------------------------------------------+-- 不符合语义，如前文所述，只能利用一部分条件使用 IndexMerge 访问> EXPLAIN SELECT /*+ use_index_merge(t4, mvi1, mvi2) */ * FROM t4 WHERE json_overlaps(j->'$.a', '[1]') AND json_overlaps(j->'$.b', '[2, 3]');+---------------------------------+---------+-----------+-----------------------------------------------------------------------------+-----------------------------------------------------------------------------------------------------------------------------------------------------+| id | estRows | task | access object | operator info |+---------------------------------+---------+-----------+-----------------------------------------------------------------------------+-----------------------------------------------------------------------------------------------------------------------------------------------------+| Selection_5 | 15.99 | root | | json_overlaps(json_extract(test.t4.j, "$.a"), cast("[1]", json BINARY)), json_overlaps(json_extract(test.t4.j, "$.b"), cast("[2, 3]", json BINARY)) || └─IndexMerge_8 | 10.00 | root | | type: union || ├─IndexRangeScan_6(Build) | 10.00 | cop[tikv] | table:t4, index:mvi1(cast(json_extract(`j`, _utf8'$.a') as unsigned array)) | range:[1,1], keep order:false, stats:pseudo || └─TableRowIDScan_7(Probe) | 10.00 | cop[tikv] | table:t4 | keep order:false, stats:pseudo |+---------------------------------+---------+-----------+-----------------------------------------------------------------------------+-----------------------------------------------------------------------------------------------------------------------------------------------------+
json_member_of由OR或AND连接，均符合语义。例如：EXPLAIN SELECT /*+ use_index_merge(t4, mvi1, mvi2) */ * FROM t4 WHERE 1 member of (j->'$.a') AND 2 member of (j->'$.b') AND 3 member of (j->'$.a');EXPLAIN SELECT /*+ use_index_merge(t4, mvi1, mvi2) */ * FROM t4 WHERE 1 member of (j->'$.a') OR 2 member of (j->'$.b') OR 3 member of (j->'$.a');
> EXPLAIN SELECT /*+ use_index_merge(t4, mvi1, mvi2) */ * FROM t4 WHERE 1 member of (j->'$.a') AND 2 member of (j->'$.b') AND 3 member of (j->'$.a');+-------------------------------+---------+-----------+-----------------------------------------------------------------------------+---------------------------------------------+| id | estRows | task | access object | operator info |+-------------------------------+---------+-----------+-----------------------------------------------------------------------------+---------------------------------------------+| IndexMerge_9 | 0.00 | root | | type: intersection || ├─IndexRangeScan_5(Build) | 10.00 | cop[tikv] | table:t4, index:mvi1(cast(json_extract(`j`, _utf8'$.a') as unsigned array)) | range:[1,1], keep order:false, stats:pseudo || ├─IndexRangeScan_6(Build) | 10.00 | cop[tikv] | table:t4, index:mvi1(cast(json_extract(`j`, _utf8'$.a') as unsigned array)) | range:[3,3], keep order:false, stats:pseudo || ├─IndexRangeScan_7(Build) | 10.00 | cop[tikv] | table:t4, index:mvi2(cast(json_extract(`j`, _utf8'$.b') as unsigned array)) | range:[2,2], keep order:false, stats:pseudo || └─TableRowIDScan_8(Probe) | 0.00 | cop[tikv] | table:t4 | keep order:false, stats:pseudo |+-------------------------------+---------+-----------+-----------------------------------------------------------------------------+---------------------------------------------+> EXPLAIN SELECT /*+ use_index_merge(t4, mvi1, mvi2) */ * FROM t4 WHERE 1 member of (j->'$.a') OR 2 member of (j->'$.b') OR 3 member of (j->'$.a');+-------------------------------+---------+-----------+-----------------------------------------------------------------------------+---------------------------------------------+| id | estRows | task | access object | operator info |+-------------------------------+---------+-----------+-----------------------------------------------------------------------------+---------------------------------------------+| IndexMerge_9 | 29.97 | root | | type: union || ├─IndexRangeScan_5(Build) | 10.00 | cop[tikv] | table:t4, index:mvi1(cast(json_extract(`j`, _utf8'$.a') as unsigned array)) | range:[1,1], keep order:false, stats:pseudo || ├─IndexRangeScan_6(Build) | 10.00 | cop[tikv] | table:t4, index:mvi2(cast(json_extract(`j`, _utf8'$.b') as unsigned array)) | range:[2,2], keep order:false, stats:pseudo || ├─IndexRangeScan_7(Build) | 10.00 | cop[tikv] | table:t4, index:mvi1(cast(json_extract(`j`, _utf8'$.a') as unsigned array)) | range:[3,3], keep order:false, stats:pseudo || └─TableRowIDScan_8(Probe) | 29.97 | cop[tikv] | table:t4 | keep order:false, stats:pseudo |+-------------------------------+---------+-----------+-----------------------------------------------------------------------------+---------------------------------------------+
包含多个值的
json_contains由OR连接，或包含多个值的json_overlaps由AND连接，不符合语义，而如果它们只包含一个值则符合语义。例如：-- 不符合语义的例子可参考前文，此处只举符合语义的例子EXPLAIN SELECT /*+ use_index_merge(t4, mvi1, mvi2) */ * FROM t4 WHERE json_overlaps(j->'$.a', '[1]') AND json_overlaps(j->'$.b', '[2]');EXPLAIN SELECT /*+ use_index_merge(t4, mvi1, mvi2) */ * FROM t4 WHERE json_contains(j->'$.a', '[1]') OR json_contains(j->'$.b', '[2]');
> EXPLAIN SELECT /*+ use_index_merge(t4, mvi1, mvi2) */ * FROM t4 WHERE json_overlaps(j->'$.a', '[1]') AND json_overlaps(j->'$.b', '[2]');+---------------------------------+---------+-----------+-----------------------------------------------------------------------------+--------------------------------------------------------------------------------------------------------------------------------------------------+| id | estRows | task | access object | operator info |+---------------------------------+---------+-----------+-----------------------------------------------------------------------------+--------------------------------------------------------------------------------------------------------------------------------------------------+| Selection_5 | 8.00 | root | | json_overlaps(json_extract(test.t4.j, "$.a"), cast("[1]", json BINARY)), json_overlaps(json_extract(test.t4.j, "$.b"), cast("[2]", json BINARY)) || └─IndexMerge_9 | 0.01 | root | | type: intersection || ├─IndexRangeScan_6(Build) | 10.00 | cop[tikv] | table:t4, index:mvi1(cast(json_extract(`j`, _utf8'$.a') as unsigned array)) | range:[1,1], keep order:false, stats:pseudo || ├─IndexRangeScan_7(Build) | 10.00 | cop[tikv] | table:t4, index:mvi2(cast(json_extract(`j`, _utf8'$.b') as unsigned array)) | range:[2,2], keep order:false, stats:pseudo || └─TableRowIDScan_8(Probe) | 0.01 | cop[tikv] | table:t4 | keep order:false, stats:pseudo |+---------------------------------+---------+-----------+-----------------------------------------------------------------------------+--------------------------------------------------------------------------------------------------------------------------------------------------+> EXPLAIN SELECT /*+ use_index_merge(t4, mvi1, mvi2) */ * FROM t4 WHERE json_contains(j->'$.a', '[1]') OR json_contains(j->'$.b', '[2]');+-------------------------------+---------+-----------+-----------------------------------------------------------------------------+---------------------------------------------+| id | estRows | task | access object | operator info |+-------------------------------+---------+-----------+-----------------------------------------------------------------------------+---------------------------------------------+| IndexMerge_8 | 19.99 | root | | type: union || ├─IndexRangeScan_5(Build) | 10.00 | cop[tikv] | table:t4, index:mvi1(cast(json_extract(`j`, _utf8'$.a') as unsigned array)) | range:[1,1], keep order:false, stats:pseudo || ├─IndexRangeScan_6(Build) | 10.00 | cop[tikv] | table:t4, index:mvi2(cast(json_extract(`j`, _utf8'$.b') as unsigned array)) | range:[2,2], keep order:false, stats:pseudo || └─TableRowIDScan_7(Probe) | 19.99 | cop[tikv] | table:t4 | keep order:false, stats:pseudo |+-------------------------------+---------+-----------+-----------------------------------------------------------------------------+---------------------------------------------+
当
OR和AND同时出现（本质上是OR和AND嵌套）时，构成 IndexMerge 的条件只能全部符合OR的语义或者全部符合AND的语义，不能部分符合OR的语义而另一部分符合AND的语义。例如：EXPLAIN SELECT /*+ use_index_merge(t4, mvi1, mvi2) */ * FROM t4 WHERE 1 member of (j->'$.a') AND (2 member of (j->'$.b') OR 3 member of (j->'$.a'));EXPLAIN SELECT /*+ use_index_merge(t4, mvi1, mvi2) */ * FROM t4 WHERE 1 member of (j->'$.a') OR (2 member of (j->'$.b') AND 3 member of (j->'$.a'));
-- 只有符合 OR 语义的 2 member of (j->'$.b') 和 3 member of (j->'$.a') 构成了 IndexMerge，而符合 AND 语义的 1 member of (j->'$.a') 没有构成 IndexMerge> EXPLAIN SELECT /*+ use_index_merge(t4, mvi1, mvi2) */ * FROM t4 WHERE 1 member of (j->'$.a') AND (2 member of (j->'$.b') OR 3 member of (j->'$.a'));+-------------------------------+---------+-----------+-----------------------------------------------------------------------------+-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+| id | estRows | task | access object | operator info |+-------------------------------+---------+-----------+-----------------------------------------------------------------------------+-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+| IndexMerge_9 | 0.00 | root | | type: union || ├─IndexRangeScan_5(Build) | 10.00 | cop[tikv] | table:t4, index:mvi2(cast(json_extract(`j`, _utf8'$.b') as unsigned array)) | range:[2,2], keep order:false, stats:pseudo || ├─IndexRangeScan_6(Build) | 10.00 | cop[tikv] | table:t4, index:mvi1(cast(json_extract(`j`, _utf8'$.a') as unsigned array)) | range:[3,3], keep order:false, stats:pseudo || └─Selection_8(Probe) | 0.00 | cop[tikv] | | json_memberof(cast(1, json BINARY), json_extract(test.t4.j, "$.a")), or(json_memberof(cast(2, json BINARY), json_extract(test.t4.j, "$.b")), json_memberof(cast(3, json BINARY), json_extract(test.t4.j, "$.a"))) || └─TableRowIDScan_7 | 19.99 | cop[tikv] | table:t4 | keep order:false, stats:pseudo |+-------------------------------+---------+-----------+-----------------------------------------------------------------------------+-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+-- 只有符合 OR 语义的 1 member of (j->'$.a') 和 3 member of (j->'$.a') 构成了 IndexMerge，而嵌套的符合 AND 语义的 2 member of (j->'$.b') 没有构成 IndexMerge> EXPLAIN SELECT /*+ use_index_merge(t4, mvi1, mvi2) */ * FROM t4 WHERE 1 member of (j->'$.a') OR (2 member of (j->'$.b') AND 3 member of (j->'$.a'));+-------------------------------+---------+-----------+-----------------------------------------------------------------------------+------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+| id | estRows | task | access object | operator info |+-------------------------------+---------+-----------+-----------------------------------------------------------------------------+------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+| IndexMerge_9 | 0.02 | root | | type: union || ├─IndexRangeScan_5(Build) | 10.00 | cop[tikv] | table:t4, index:mvi1(cast(json_extract(`j`, _utf8'$.a') as unsigned array)) | range:[1,1], keep order:false, stats:pseudo || ├─IndexRangeScan_6(Build) | 10.00 | cop[tikv] | table:t4, index:mvi1(cast(json_extract(`j`, _utf8'$.a') as unsigned array)) | range:[3,3], keep order:false, stats:pseudo || └─Selection_8(Probe) | 0.02 | cop[tikv] | | or(json_memberof(cast(1, json BINARY), json_extract(test.t4.j, "$.a")), and(json_memberof(cast(2, json BINARY), json_extract(test.t4.j, "$.b")), json_memberof(cast(3, json BINARY), json_extract(test.t4.j, "$.a")))) || └─TableRowIDScan_7 | 19.99 | cop[tikv] | table:t4 | keep order:false, stats:pseudo |+-------------------------------+---------+-----------+-----------------------------------------------------------------------------+------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
如果表达式包含嵌套的 OR 或 AND，或者表达式需要进行展开等变形之后才能直接一一对应各索引列，TiDB 可能无法使用 IndexMerge 或不能充分利用所有过滤条件。建议针对具体场景预先进行测试验证。以下是部分场景示例：
CREATE TABLE t5 (a INT, j JSON, b INT, k JSON, INDEX idx(a, (CAST(j AS SIGNED ARRAY))), INDEX idx2(b, (CAST(k as SIGNED ARRAY))));CREATE TABLE t6 (a INT, j JSON, b INT, k JSON, INDEX idx(a, (CAST(j AS SIGNED ARRAY)), b), INDEX idx2(a, (CAST(k as SIGNED ARRAY)), b));
当 AND 嵌套在由 OR 连接的条件中，且嵌套的 AND 所连接的子条件与多列索引的各个列能够一一对应时，TiDB 通常能够充分利用过滤条件。例如：
EXPLAIN SELECT /*+ use_index_merge(t5, idx, idx2) */ * FROM t5 WHERE (a=1 AND 1 member of (j)) OR (b=2 AND 2 member of (k));
> EXPLAIN SELECT /*+ use_index_merge(t5, idx, idx2) */ * FROM t5 WHERE (a=1 AND 1 member of (j)) OR (b=2 AND 2 member of (k));+-------------------------------+---------+-----------+----------------------------------------------------+-------------------------------------------------+| id | estRows | task | access object | operator info |+-------------------------------+---------+-----------+----------------------------------------------------+-------------------------------------------------+| IndexMerge_8 | 0.20 | root | | type: union || ├─IndexRangeScan_5(Build) | 0.10 | cop[tikv] | table:t5, 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:t5, index:idx2(b, cast(`k` as signed array)) | range:[2 2,2 2], keep order:false, stats:pseudo || └─TableRowIDScan_7(Probe) | 0.20 | cop[tikv] | table:t5 | keep order:false, stats:pseudo |+-------------------------------+---------+-----------+----------------------------------------------------+-------------------------------------------------+
当单个 OR 嵌套在由 AND 连接的条件中，且 OR 所连接的子条件通过展开表达式可以直接对应索引列时，TiDB 通常能够充分利用过滤条件。例如：
EXPLAIN SELECT /*+ use_index_merge(t6, idx, idx2) */ * FROM t6 WHERE a=1 AND (1 member of (j) OR 2 member of (k));
+-------------------------------+---------+-----------+-------------------------------------------------------+-------------------------------------------------------------------------------------------------------------------------+| id | estRows | task | access object | operator info |+-------------------------------+---------+-----------+-------------------------------------------------------+-------------------------------------------------------------------------------------------------------------------------+| IndexMerge_9 | 0.20 | root | | type: union || ├─IndexRangeScan_5(Build) | 0.10 | cop[tikv] | table:t6, index:idx(a, cast(`j` as signed array), b) | range:[1 1,1 1], keep order:false, stats:pseudo || ├─IndexRangeScan_6(Build) | 0.10 | cop[tikv] | table:t6, index:idx2(a, cast(`k` as signed array), b) | range:[1 2,1 2], keep order:false, stats:pseudo || └─Selection_8(Probe) | 0.20 | cop[tikv] | | eq(test2.t6.a, 1), or(json_memberof(cast(1, json BINARY), test2.t6.j), json_memberof(cast(2, json BINARY), test2.t6.k)) || └─TableRowIDScan_7 | 0.20 | cop[tikv] | table:t6 | keep order:false, stats:pseudo |+-------------------------------+---------+-----------+-------------------------------------------------------+-------------------------------------------------------------------------------------------------------------------------+
当多个 OR 嵌套在由 AND 连接的条件中，且 OR 所连接的子条件需要展开表达式才能直接对应索引列的时候，TiDB 可能无法充分利用过滤条件。例如：
EXPLAIN SELECT /*+ use_index_merge(t6, idx, idx2) */ * FROM t6 WHERE a=1 AND (1 member of (j) OR 2 member of (k)) and (b = 1 OR b = 2);EXPLAIN SELECT /*+ use_index_merge(t6, idx, idx2) */ * FROM t6 WHERE a=1 AND ((1 member of (j) AND b = 1) OR (1 member of (j) AND b = 2) OR (2 member of (k) AND b = 1) OR (2 member of (k) AND b = 2));
-- 由于目前实现上的限制，(b = 1 or b = 2) 没有参与构成 IndexMerge，而是成为了一个单独的 Selection 算子> EXPLAIN SELECT /*+ use_index_merge(t6, idx, idx2) */ * FROM t6 WHERE a=1 AND (1 member of (j) OR 2 member of (k)) AND (b = 1 OR b = 2);+-------------------------------+---------+-----------+-------------------------------------------------------+--------------------------------------------------------------------------------------------------------------------------------------------------------------+| id | estRows | task | access object | operator info |+-------------------------------+---------+-----------+-------------------------------------------------------+--------------------------------------------------------------------------------------------------------------------------------------------------------------+| IndexMerge_9 | 0.20 | root | | type: union || ├─IndexRangeScan_5(Build) | 0.10 | cop[tikv] | table:t6, index:idx(a, cast(`j` as signed array), b) | range:[1 1,1 1], keep order:false, stats:pseudo || ├─IndexRangeScan_6(Build) | 0.10 | cop[tikv] | table:t6, index:idx2(a, cast(`k` as signed array), b) | range:[1 2,1 2], keep order:false, stats:pseudo || └─Selection_8(Probe) | 0.20 | cop[tikv] | | eq(test.t6.a, 1), or(eq(test.t6.b, 1), eq(test.t6.b, 2)), or(json_memberof(cast(1, json BINARY), test.t6.j), json_memberof(cast(2, json BINARY), test.t6.k)) || └─TableRowIDScan_7 | 0.20 | cop[tikv] | table:t6 | keep order:false, stats:pseudo |+-------------------------------+---------+-----------+-------------------------------------------------------+--------------------------------------------------------------------------------------------------------------------------------------------------------------+-- 将 SQL 中由 AND 连接的两个 OR 表达式手动展开之后，TiDB 能够充分利用这些条件> EXPLAIN SELECT /*+ use_index_merge(t6, idx, idx2) */ * FROM t6 WHERE a=1 AND ((1 member of (j) AND b = 1) OR (1 member of (j) AND b = 2) OR (2 member of (k) AND b = 1) OR (2 member of (k) AND b = 2));+-------------------------------+---------+-----------+-------------------------------------------------------+--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+| id | estRows | task | access object | operator info |+-------------------------------+---------+-----------+-------------------------------------------------------+--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+| IndexMerge_11 | 0.00 | root | | type: union || ├─IndexRangeScan_5(Build) | 0.00 | cop[tikv] | table:t6, index:idx(a, cast(`j` as signed array), b) | range:[1 1 1,1 1 1], keep order:false, stats:pseudo || ├─IndexRangeScan_6(Build) | 0.00 | cop[tikv] | table:t6, index:idx(a, cast(`j` as signed array), b) | range:[1 1 2,1 1 2], keep order:false, stats:pseudo || ├─IndexRangeScan_7(Build) | 0.00 | cop[tikv] | table:t6, index:idx2(a, cast(`k` as signed array), b) | range:[1 2 1,1 2 1], keep order:false, stats:pseudo || ├─IndexRangeScan_8(Build) | 0.00 | cop[tikv] | table:t6, index:idx2(a, cast(`k` as signed array), b) | range:[1 2 2,1 2 2], keep order:false, stats:pseudo || └─Selection_10(Probe) | 0.00 | cop[tikv] | | eq(test.t6.a, 1), or(or(and(json_memberof(cast(1, json BINARY), test.t6.j), eq(test.t6.b, 1)), and(json_memberof(cast(1, json BINARY), test.t6.j), eq(test.t6.b, 2))), or(and(json_memberof(cast(2, json BINARY), test.t6.k), eq(test.t6.b, 1)), and(json_memberof(cast(2, json BINARY), test.t6.k), eq(test.t6.b, 2)))) || └─TableRowIDScan_9 | 0.00 | cop[tikv] | table:t6 | keep order:false, stats:pseudo |+-------------------------------+---------+-----------+-------------------------------------------------------+--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
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 querymysql> 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)
多值索引与执行计划缓存
通过 member of 条件选择的多值索引的执行计划可以被缓存。如果执行计划是通过 JSON_CONTAINS() 或 JSON_OVERLAPS() 函数选择的多值索引，则该计划暂时无法被缓存。
下面是可以缓存的例子：
mysql> CREATE TABLE t5 (j1 JSON, j2 JSON, INDEX idx1((CAST(j1 AS SIGNED ARRAY))));Query OK, 0 rows affected (0.04 sec)mysql> PREPARE st FROM 'SELECT /*+ use_index(t5, idx1) */ * FROM t5 WHERE (? member of (j1))';Query OK, 0 rows affected (0.00 sec)mysql> SET @a=1;Query OK, 0 rows affected (0.00 sec)mysql> EXECUTE st USING @a;Empty set (0.01 sec)mysql> EXECUTE st USING @a;Empty set (0.00 sec)mysql> SELECT @@last_plan_from_cache;+------------------------+| @@last_plan_from_cache |+------------------------+| 1 |+------------------------+1 row in set (0.00 sec)mysql> PREPARE st FROM 'SELECT /*+ use_index(t5, idx1) */ * FROM t5 WHERE (? member of (j1)) AND JSON_CONTAINS(j2, ?)';Query OK, 0 rows affected (0.00 sec)mysql> SET @a=1, @b='[1,2]';Query OK, 0 rows affected (0.00 sec)mysql> EXECUTE st USING @a, @b;Empty set (0.00 sec)mysql> EXECUTE st USING @a, @b;Empty set (0.00 sec)mysql> SELECT @@LAST_PLAN_FROM_CACHE; -- can hit plan cache if the JSON_CONTAINS doesn't impact index selection+------------------------+| @@LAST_PLAN_FROM_CACHE |+------------------------+| 1 |+------------------------+1 row in set (0.00 sec)
下面是无法缓存的例子：
mysql> PREPARE st2 FROM 'SELECT /*+ use_index(t5, idx1) */ * FROM t5 WHERE JSON_CONTAINS(j1, ?)';Query OK, 0 rows affected (0.00 sec)mysql> SET @a='[1,2]';Query OK, 0 rows affected (0.01 sec)mysql> EXECUTE st2 USING @a;Empty set, 1 warning (0.00 sec)mysql> SHOW WARNINGS; -- cannot hit plan cache since the JSON_CONTAINS predicate might affect index selection+---------+------+-------------------------------------------------------------------------------------------------------+| Level | Code | Message |+---------+------+-------------------------------------------------------------------------------------------------------+| Warning | 1105 | skip prepared plan-cache: json_contains function with immutable parameters can affect index selection |+---------+------+-------------------------------------------------------------------------------------------------------+1 row in set (0.01 sec)
