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_bidx_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

  1. 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。

  2. 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。

  3. 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_INDEXUSE_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_ofjson_containsjson_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)

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