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:

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 ORAND 连接而成的多个 json_member_ofjson_containsjson_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 ORAND 的语义。具体如下:

    • json_containsAND 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_overlapsOR 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_ofORAND 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_containsOR 连接,或包含多个值的 json_overlapsAND 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 | +-------------------------------+---------+-----------+-----------------------------------------------------------------------------+---------------------------------------------+
    • ORAND 同时出现(本质上是 ORAND 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 ORAND,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));

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 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)

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()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)

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