本篇内容主要讲解“MySQL字符集不同引起的join无法走索引的问题怎么解决”，感兴趣的朋友不妨来看看。本文介绍的方法操作简单快捷，实用性强。下面就让小编来带大家学习“MYSQL字符集不同引起的join无法走索引的问题怎么解决”吧!
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在对sql进行转换时，可以发现了以下将s表的deptid 转换成utf8mb4 的情况，即是说，两个表在left join时存在关联字段deptid 字符集不同的情况。
on((`SSS`.`d`.`Deptid` = convert(`ppp`.`s`.`Deptid` using utf8mb4))))
细查发现d表的deptid字段是utf8mb4，但是s表的deptid字段是utf8。
而根据经验，两边关联时字段字符集不一确实会导致无法走索引，因为这里是发生了隐式转换了。此时s表上的索引便无法生效。
此时我有个疑问是当以上将条件d.DEPTID = '00001111' 换成s.DEPTID = '00001111',其可以选择了索引，当此时字符集不同的情况仍然存在，发现以上选择d表的主键的原因是clustered_pk_chosen_by_heuristics。因为是主键而选择？这个便不是很理解了
"considered_access_paths": [
{
"access_type": "eq_ref",
"index": "PRIMARY",
"rows": 1,
"cost": 1.2,
"chosen": true,
"cause": "clustered_pk_chosen_by_heuristics"
},
在修改字符集统一为utf8mb4后就正常了：
+----+-------------+-------+------------+-------+----------------------------+-----------------+---------+-------+------+----------+-------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+-------+------------+-------+----------------------------+-----------------+---------+-------+------+----------+-------+
| 1 | SIMPLE | d | NULL | const | PRIMARY,INDEX_DEPARTMENT_5 | PRIMARY | 130 | const | 1 | 100.00 | NULL |
| 1 | SIMPLE | s | NULL | ref | IND_SHOP_DEPTID | IND_SHOP_DEPTID | 131 | const | 1 | 100.00 | NULL |
+----+-------------+-------+------------+-------+----------------------------+-----------------+---------+-------+------+----------+-------+
"ref_optimizer_key_uses": [
{
"table": "`sss`.`department` `d`",
"field": "Deptid",
"equals": "'00001111'",
"null_rejecting": false
},
{
"table": "`sss`.`department` `d`",
"field": "Deptid",
"equals": "'00001111'",
"null_rejecting": false
},
{
"table": "`ppp`.`shop` `s`",
"field": "Deptid",
"equals": "`sss`.`d`.`Deptid`",
"null_rejecting": false
}
] /* ref_optimizer_key_uses */
},
{
"rows_estimation": [
{
"table": "`sss`.`department` `d`",
"rows": 1,
"cost": 1,
"table_type": "const",
"empty": false
},
{
"table": "`ppp`.`shop` `s`",
"range_analysis": {
"table_scan": {
"rows": 998690,
"cost": 207849
} /* table_scan */,
"potential_range_indexes": [
{
"index": "PRIMARY",
"usable": false,
"cause": "not_applicable"
},
{
"index": "IND_SHOP_DEPTID",
"usable": true,
"key_parts": [
"Deptid",
"Shopid"
] /* key_parts */
},
{
"index": "IND_SHOP_DOMAIN",
"usable": false,
"cause": "not_applicable"
}
] /* potential_range_indexes */,
"setup_range_conditions": [
] /* setup_range_conditions */,
"group_index_range": {
"chosen": false,
"cause": "not_single_table"
} /* group_index_range */,
"analyzing_range_alternatives": {
"range_scan_alternatives": [
{
"index": "IND_SHOP_DEPTID",
"ranges": [
"00001111 <= Deptid <= 00001111"
] /* ranges */,
"index_dives_for_eq_ranges": true,
"rowid_ordered": true,
"using_mrr": false,
"index_only": false,
"rows": 1,
"cost": 2.21,
"chosen": true
}
] /* range_scan_alternatives */,
"analyzing_roworder_intersect": {
"usable": false,
"cause": "too_few_roworder_scans"
} /* analyzing_roworder_intersect */
} /* analyzing_range_alternatives */,
"chosen_range_access_summary": {
"range_access_plan": {
"type": "range_scan",
"index": "IND_SHOP_DEPTID",
"rows": 1,
"ranges": [
"00001111 <= Deptid <= 00001111"
] /* ranges */
} /* range_access_plan */,
"rows_for_plan": 1,
"cost_for_plan": 2.21,
"chosen": true
} /* chosen_range_access_summary */
} /* range_analysis */
}
] /* rows_estimation */
},
{
"considered_execution_plans": [
{
"plan_prefix": [
"`sss`.`department` `d`"
] /* plan_prefix */,
"table": "`ppp`.`shop` `s`",
"best_access_path": {
"considered_access_paths": [
{
"access_type": "ref",
"index": "IND_SHOP_DEPTID",
"rows": 1,
"cost": 1.2,
"chosen": true
},
{
"access_type": "range",
"range_details": {
"used_index": "IND_SHOP_DEPTID"
} /* range_details */,
"chosen": false,
"cause": "heuristic_index_cheaper"
}
] /* considered_access_paths */
} /* best_access_path */,
"condition_filtering_pct": 100,
"rows_for_plan": 1,
"cost_for_plan": 1.2,
"chosen": true
}
] /* considered_execution_plans */
},到此，相信大家对“MYSQL字符集不同引起的join无法走索引的问题怎么解决”有了更深的了解，不妨来实际操作一番吧！这里是创新互联网站，更多相关内容可以进入相关频道进行查询，关注我们，继续学习！
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