GreatSQL函数索引失效分析：排序规则匹配机制
GreatSQL函数索引失效分析：排序规则匹配机制某项目中，客户使用SQL查询时，索引未生效，经排查发现查询使用的排序规则与函数索引的排序规则不一致，导致无法使用该函数索引。
一、排序规则不匹配的测试案例
'测试表结构如下'
greatsql> SHOW CREATE TABLE test_findex;
+--------------+-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| Table | Create Table |
+--------------+-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| test_findex | CREATE TABLE `test_findex` (
`id` int NOT NULL AUTO_INCREMENT,
`c` char(120) CHARACTER SET utf8mb4 COLLATE utf8mb4_0900_ai_ci NOT NULL DEFAULT '',
`pad` char(60) CHARACTER SET utf8mb4 COLLATE utf8mb4_bin NOT NULL DEFAULT '',
PRIMARY KEY (`id`),
KEY `bbb` ((substr(`pad`,1,10))),
KEY `ccc` ((concat(`c`,`pad`)))
) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_bin |
+--------------+-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
'按照排序规则 utf8mb4_bin 进行查询，排序规则匹配可以使用函数索引'
greatsql> EXPLAIN SELECT * FROM test_findex WHERE concat(`c`,`pad`)='aaa' COLLATE utf8mb4_bin;
+----+-------------+--------------+------------+------+---------------+------+---------+-------+------+----------+-------+
| id | select_type | table | partitions | type | possible_keys | key| key_len | ref | rows | filtered | Extra |
+----+-------------+--------------+------------+------+---------------+------+---------+-------+------+----------+-------+
|1 | SIMPLE | test_findex | NULL | ref| ccc | ccc| 723 | const | 1 | 100.00 | NULL|
+----+-------------+--------------+------------+------+---------------+------+---------+-------+------+----------+-------+
1 row in set, 1 warning (0.01 sec)
greatsql> SHOW WARNINGS;
+-------+------+-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| Level | Code | Message |
+-------+------+-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| Note| 1003 | /* select#1 */ select `test`.`test_findex`.`id` AS `id`,`test`.`test_findex`.`c` AS `c`,`test`.`test_findex`.`pad` AS `pad` from `test`.`test_findex` where (concat(`c`,`pad`) = ('aaa' collate utf8mb4_bin)) |
+-------+------+-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
1 row in set (0.00 sec)
'按照排序规则 utf8mb4_0900_ai_ci 进行查询，排序规则与索引不一致，同时warnings中给出了不能使用函数索引的原因'
greatsql> EXPLAIN SELECT * FROM test_findex WHERE concat(`c`,`pad`)='aaa' COLLATE utf8mb4_0900_ai_ci;
+----+-------------+--------------+------------+------+---------------+------+---------+------+------+----------+-------------+
| id | select_type | table | partitions | type | possible_keys | key| key_len | ref| rows | filtered | Extra |
+----+-------------+--------------+------------+------+---------------+------+---------+------+------+----------+-------------+
|1 | SIMPLE | test_findex | NULL | ALL| ccc | NULL | NULL | NULL | 1 | 100.00 | Using where |
+----+-------------+--------------+------------+------+---------------+------+---------+------+------+----------+-------------+
1 row in set, 3 warnings (0.00 sec)
greatsql> SHOW WARNINGS;
+---------+------+-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| Level | Code | Message |
+---------+------+-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| Warning | 3909 | Cannot use functional index 'ccc' due to type or collation conversion. |
| Warning | 1739 | Cannot use range access on index 'ccc' due to type or collation conversion on field '!hidden!ccc!0!0' |
| Note | 1003 | /* select#1 */ select `test`.`test_findex`.`id` AS `id`,`test`.`test_findex`.`c` AS `c`,`test`.`test_findex`.`pad` AS `pad` from `test`.`test_findex` where (concat(`c`,`pad`) = <cache>(('aaa' collate utf8mb4_0900_ai_ci))) |
+---------+------+-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+测试现象：在使用索引时，如果查询条件的排序规则和索引的排序规则不匹配（不相同或不兼容），则无法使用这个函数索引。
二、函数索引的底层存储机制
通过提取SDI信息分析发现，每个函数索引对应一个隐藏列，其collation_id决定排序规则：
测试表结构如下：
greatsql> SHOW CREATE TABLE test_findex2;
+-------------+-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| Table | Create Table |
+-------------+-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| test_findex2 | CREATE TABLE `test_findex2` (
`id` int NOT NULL AUTO_INCREMENT,
`pad` char(60) CHARACTER SET utf8mb4 COLLATE utf8mb4_bin NOT NULL DEFAULT '',
PRIMARY KEY (`id`),
KEY `ddd` (((substr(`pad`,1,10) collate utf8mb4_0900_ai_ci))),
KEY `fff` (((substr(`pad`,1,10) collate utf8mb4_bin)))
) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_bin |
+-------------+-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
$ ./bin/ibd2sdi/usr/local/db/dbdata/test/test_findex2.ibd
...
{
"name": "!hidden!ddd!0!0",
....
"generation_expression": "(substr(`pad`,1,10) collate utf8mb4_0900_ai_ci)",
"generation_expression_utf8": "(substr(`pad`,1,10) collate utf8mb4_0900_ai_ci)",
....
"collation_id": 255, // utf8mb4_0900_ai_ci 通过information_schema.COLLATIONS表查询
"is_explicit_collation": false
},
{
"name": "!hidden!fff!0!0",
...
"generation_expression": "(substr(`pad`,1,10) collate utf8mb4_bin)",
"generation_expression_utf8": "(substr(`pad`,1,10) collate utf8mb4_bin)",
...
"collation_id": 46, // utf8mb4_bin
"is_explicit_collation": false
},
...排序规则ID映射关系
greatsql> SELECT * FROM information_schema.COLLATIONS WHERE ID IN(46,255);
+--------------------+--------------------+-----+------------+-------------+---------+---------------+
| COLLATION_NAME | CHARACTER_SET_NAME | ID| IS_DEFAULT | IS_COMPILED | SORTLEN | PAD_ATTRIBUTE |
+--------------------+--------------------+-----+------------+-------------+---------+---------------+
| utf8mb4_bin | utf8mb4 |46 | | Yes | 1 | PAD SPACE |
| utf8mb4_0900_ai_ci | utf8mb4 | 255 | Yes | Yes | 0 | NO PAD |
+--------------------+--------------------+-----+------------+-------------+---------+---------------+三、排序规则匹配场景测试
1. 查询时指定了排序规则
查询时指定了排序规则,按照指定的排序规则选择索引。
'索引 fff 和 ddd 都指定了排序规则，执行计划均选择了对应排序规则的索引'
greatsql> EXPLAIN SELECT * FROM test_findex2 WHERE substr(`pad`,1,10) COLLATE utf8mb4_bin ='1111111111';
+----+-------------+--------------+------------+------+---------------+------+---------+-------+------+----------+-------+
| id | select_type | table | partitions | type | possible_keys | key| key_len | ref | rows | filtered | Extra |
+----+-------------+--------------+------------+------+---------------+------+---------+-------+------+----------+-------+
|1 | SIMPLE | test_findex2 | NULL | ref| fff | fff| 43 | const | 1 | 100.00 | NULL|
+----+-------------+--------------+------------+------+---------------+------+---------+-------+------+----------+-------+
greatsql> EXPLAIN SELECT * FROM test_findex2 WHERE substr(`pad`,1,10) COLLATE utf8mb4_0900_ai_ci ='1111111111';
+----+-------------+--------------+------------+------+---------------+------+---------+-------+------+----------+-------+
| id | select_type | table | partitions | type | possible_keys | key| key_len | ref | rows | filtered | Extra |
+----+-------------+--------------+------------+------+---------------+------+---------+-------+------+----------+-------+
|1 | SIMPLE | test_findex2 | NULL | ref| ddd | ddd| 43 | const | 1 | 100.00 | NULL|
+----+-------------+--------------+------------+------+---------------+------+---------+-------+------+----------+-------+2. 查询时未指定排序规则
查询时未指定排序规则,使用索引列排序规则对应的索引。
'查询时未指定排序规则，选择了索引 fff（其排序规则是 utf8mb4_bin ）与 pad 列的排序规则相同'
greatsql> EXPLAIN SELECT * FROM test_findex2 WHERE substr(`pad`,1,10)='1111111111';
+----+-------------+--------------+------------+------+---------------+------+---------+-------+------+----------+-------+
| id | select_type | table | partitions | type | possible_keys | key| key_len | ref | rows | filtered | Extra |
+----+-------------+--------------+------------+------+---------------+------+---------+-------+------+----------+-------+
|1 | SIMPLE | test_findex2 | NULL | ref| fff | fff| 43 | const | 1 | 100.00 | NULL|
+----+-------------+--------------+------------+------+---------------+------+---------+-------+------+----------+-------+
1 row in set, 1 warning (0.01 sec)
greatsql> SHOW WARNINGS;
+-------+------+------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| Level | Code | Message |
+-------+------+------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| Note| 1003 | /* select#1 */ select `test`.`test_findex2`.`id` AS `id`,`test`.`test_findex2`.`pad` AS `pad` from `test`.`test_findex2` where ((substr(`pad`,1,10) collate utf8mb4_bin) = '1111111111') |
+-------+------+------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
1 row in set (0.00 sec)
'将表列 pad 的排序规则改为 utf8mb4_0900_ai_ci ，
查询时未指定排序规则，选择了索引 ddd 其排序规则是 utf8mb4_0900_ai_ci ）与 pad 列的排序规则相同'
greatsql>ALTER TABLE test_findex2 MODIFY pad char(60) COLLATE utf8mb4_0900_ai_ci;
Query OK, 0 rows affected (0.12 sec)
Records: 0Duplicates: 0Warnings: 0
greatsql> EXPLAIN SELECT * FROM test_findex2 WHERE substr(`pad`,1,10)='1111111111';
+----+-------------+--------------+------------+------+---------------+------+---------+-------+------+----------+-------+
| id | select_type | table | partitions | type | possible_keys | key| key_len | ref | rows | filtered | Extra |
+----+-------------+--------------+------------+------+---------------+------+---------+-------+------+----------+-------+
|1 | SIMPLE | test_findex2 | NULL | ref| ddd | ddd| 43 | const | 1 | 100.00 | NULL|
+----+-------------+--------------+------------+------+---------------+------+---------+-------+------+----------+-------+
1 row in set, 1 warning (0.00 sec)
greatsql> SHOW WARNINGS;
+-------+------+-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| Level | Code | Message |
+-------+------+-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| Note| 1003 | /* select#1 */ select `test`.`test_findex2`.`id` AS `id`,`test`.`test_findex2`.`pad` AS `pad` from `test`.`test_findex2` where ((substr(`pad`,1,10) collate utf8mb4_0900_ai_ci) = '1111111111') |
+-------+------+-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
1 row in set (0.00 sec)当SQL 中 没有显式写出排序规则时，优化器会自动继承查询条件中字段的排序规则，并补上排序规则。
3. 建表时未指定排序规则
系统按照database的字符集和排序规则创建表。数据列按照表的排序规则创建，遵循第2条：使用索引列排序规则对应的索引。
'创建表时不指定排序规则（继承数据库默认）'
CREATE TABLE `test_findex3` (
`id` int NOT NULL AUTO_INCREMENT,
`pad` char(60) NOT NULL DEFAULT '',
PRIMARY KEY (`id`),
KEY `ddd` ((substr(`pad`,1,10) COLLATE utf8mb4_0900_ai_ci)),
KEY `fff` ((substr(`pad`,1,10) COLLATE utf8mb4_bin))
) ENGINE=InnoDB;
'数据库默认字符集 utf8mb4 ，默认排序规则 utf8mb4_0900_ai_ci '
greatsql> SHOW CREATE TABLE test_findex3;
+--------------+----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| Table | Create Table |
+--------------+----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| test_findex3 | CREATE TABLE `test_findex3` (
`id` int NOT NULL AUTO_INCREMENT,
`pad` char(60) NOT NULL DEFAULT '',
PRIMARY KEY (`id`),
KEY `ddd` (((substr(`pad`,1,10) collate utf8mb4_0900_ai_ci))),
KEY `fff` (((substr(`pad`,1,10) collate utf8mb4_bin)))
) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_0900_ai_ci |
+--------------+----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
'查询时未指定排序规则且列未指定排序规则，继承建表的排序规则，同第二条'
greatsql> EXPLAIN SELECT * FROM test_findex3 WHERE substr(`pad`,1,10)='1111111111';
+----+-------------+--------------+------------+------+---------------+------+---------+-------+------+----------+-------+
| id | select_type | table | partitions | type | possible_keys | key| key_len | ref | rows | filtered | Extra |
+----+-------------+--------------+------------+------+---------------+------+---------+-------+------+----------+-------+
|1 | SIMPLE | test_findex3 | NULL | ref| ddd | ddd| 43 | const | 1 | 100.00 | NULL|
+----+-------------+--------------+------------+------+---------------+------+---------+-------+------+----------+-------+
1 row in set, 1 warning (0.00 sec)
greatsql> SHOW WARNINGS;
+-------+------+-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| Level | Code | Message |
+-------+------+-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| Note| 1003 | /* select#1 */ select `test`.`test_findex3`.`id` AS `id`,`test`.`test_findex3`.`pad` AS `pad` from `test`.`test_findex3` where ((substr(`pad`,1,10) collate utf8mb4_0900_ai_ci) = '1111111111') |
+-------+------+-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
1 row in set (0.00 sec)4. 测试结果
[*]命中索引：
[*]查询时明确指定与函数索引相同的排序规则（排序规则匹配，优化器可用该索引）
[*]查询未指定排序规则，但列的排序规则与索引匹配（优化器自动继承列排序规则并选择匹配索引）
[*]无法命中索引：
[*]查询时指定不同排序规则（排序规则不兼容，触发 Collation Conversion）
[*]查询未指定排序规则，列和索引排序规则不一致（优化器使用列排序规则，但与索引不匹配）
四、避坑建议
[*]显式指定排序规则 在创建函数索引和编写查询时，显式声明排序规则，避免隐式转换：
greatsql> CREATE INDEX idx ON test_table ((substr(col,1,10) COLLATE utf8mb4_bin));
greatsql> SELECT * FROM test_table WHERE substr(col,1,10) COLLATE utf8mb4_bin = 'value';
[*]保持排序规则一致性
[*]表列、函数索引、查询条件三者的排序规则尽量一致，避免排序规则混用；
[*]修改列排序规则后，需重建索引（因索引依赖列定义），否则索引可能失效。
[*]监控索引失效警告 定期检查慢查询日志，关注 SHOW WARNINGS 中collation转换提示。
[*]根据业务模型选择兼容性强的排序规则 例如 utf8mb4_0900_ai_ci，支持更广泛的Unicode字符。
Enjoy GreatSQL
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