为什么MySQL不使用我的索引?优化mysql选择查询
我正在尝试优化MySQL select请求:为什么MySQL不使用我的索引?优化mysql选择查询,mysql,optimization,indexing,query-optimization,database-schema,Mysql,Optimization,Indexing,Query Optimization,Database Schema,我正在尝试优化MySQL select请求: SELECT * FROM `sales` WHERE ((sales.private = false AND (sales.buyer_id IS NULL OR NOT sales.buyer_id=142) AND (sales.merchand_id IS NULL OR NOT sales.merchand_id=142) AND (sales.private_item = false) ) AND ((sales.buyer
SELECT * FROM `sales`
WHERE ((sales.private = false AND (sales.buyer_id IS NULL OR NOT sales.buyer_id=142)
AND (sales.merchand_id IS NULL OR NOT sales.merchand_id=142)
AND (sales.private_item = false) )
AND ((sales.buyer_id=32 OR sales.merchand_id=32)
AND (sales.admin=0 AND NOT sales.type IN ('book'))))
ORDER BY sales.created_at DESC, sales.id DESC LIMIT 0, 10;
表的架构是
mysql> SHOW columns from sales;
+------------------------+--------------+------+-----+---------+----------------+
| Field | Type | Null | Key | Default | Extra |
+------------------------+--------------+------+-----+---------+----------------+
| id | int(11) | NO | PRI | NULL | auto_increment |
| type | varchar(255) | YES | MUL | NULL | |
| buyer_id | int(11) | YES | MUL | NULL | |
| merchand_id | int(11) | YES | MUL | NULL | |
| private | tinyint(1) | YES | | 0 | |
| admin | tinyint(1) | YES | | 0 | |
| created_at | datetime | YES | | NULL | |
| updated_at | datetime | YES | | NULL | |
| country_id | int(11) | YES | MUL | 0 | |
| private_item | tinyint(1) | YES | | 0 | |
+------------------------+--------------+------+-----+---------+----------------+
这些指数是:
mysql> show indexes from sales;
+-----------------+------------+--------------------------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+
| Table | Non_unique | Key_name | Seq_in_index | Column_name | Collation | Cardinality | Sub_part | Packed | Null | Index_type | Comment |
+-----------------+------------+--------------------------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+
| sales | 0 | PRIMARY | 1 | id | A | 286509 | NULL | NULL | | BTREE | |
| sales | 1 | index_sales_on_type | 1 | type | A | 123 | NULL | NULL | YES | BTREE | |
| sales | 1 | index_sales_on_buyer_id | 1 | buyer_id | A | 40929 | NULL | NULL | YES | BTREE | |
| sales | 1 | index_sales_on_merchand_id | 1 | merchand_id | A | 40929 | NULL | NULL | YES | BTREE | |
| sales | 1 | index_sales_on_country_id | 1 | country_id | A | 6 | NULL | NULL | YES | BTREE | |
| sales | 1 | index_sales_on_type_and_country_id | 1 | type | A | 151 | NULL | NULL | YES | BTREE | |
| sales | 1 | index_sales_on_type_and_country_id | 2 | country_id | A | 428 | NULL | NULL | YES | BTREE | |
| sales | 1 | index_sales_viewed | 1 | buyer_id | A | 35813 | NULL | NULL | YES | BTREE | |
| sales | 1 | index_sales_viewed | 2 | merchand_id | A | 286509 | NULL | NULL | YES | BTREE | |
| sales | 1 | index_sales_viewed | 3 | private_item| A | 285009 | NULL | NULL | YES | BTREE | |
| sales | 1 | index_sales_viewed | 4 | admin | A | 285009 | NULL | NULL | YES | BTREE | |
| sales | 1 | index_sales_viewed | 5 | type | A | 285009 | NULL | NULL | YES | BTREE | |
| sales | 1 | index_sales_viewed | 6 | private | A | 285009 | NULL | NULL | YES | BTREE | |
| sales | 1 | index_sales_viewed | 7 | created_at | A | 285009 | NULL | NULL | YES | BTREE | |
+-------+------------+------------------------------------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+
执行查询时,即使查询中没有国家/地区id,它也会在类型和国家/地区id上使用索引\u sales\u\u
使用此索引进行查询需要2.5秒
但是,当我使用使用索引(查看了索引的销售额)时,它会下降到0.2秒
以下是对查询的解释:
+----+-------------+-----------------+------+----------------------------------------------+------+---------+------+--------+---------------------
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+-----------------+------+----------------------------------------------+------+---------+------+--------+---------------------
| 1 | SIMPLE | sales | range | see bellow | index_sales_on_type_and_country_id | 258 | NULL | 208725 | Using where; Using filesort |
+----+-------------+-----------------+------+----------------------------------------------+------+---------+------+--------+---------------------
可能的关键是:
index_sales_on_type,
index_sales_on_buyer_id,
index_sales_on_merchand_id,
index_sales_on_type_and_country_id,
index_sales_public_recent_activity
为什么MySQL在默认情况下不使用索引\u查看的销售额?有没有更好的指数
谢谢大家! 这是对NULL的错误使用,请将索引中使用的所有列更改为非NULL 参考此 如果此列为空,则没有相关索引。在这种情况下,您可以通过检查WHERE子句来检查它是否引用了一些适合索引的列,从而提高查询的性能。如果是这样,请创建一个适当的索引并再次使用EXPLAIN检查查询
Mysql在类型和国家id上选择了索引,因为您不能与空值进行比较发布查询的“解释”,这样我们就可以看到数据库的想法。