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MySQL查询该怎么用 mysqljoin查询( 三 )

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我们加上hint给相同的查询，再次看看查询计划 。
这个时候用到了index_merge,union了三个列 。扫描的行数为1103 ， cost为441.09，明显比之前的快了好几倍 。
mysql explainformat=json select /*+ index_merge(t1) */ * from t1where rank1 =1 or rank2 = 2 or rank3 = 2\G*************************** 1. row ***************************EXPLAIN: {"query_block": {"select_id": 1,"cost_info": {"query_cost": "441.09"},"table": {"table_name": "t1","access_type": "index_merge","possible_keys": ["idx_rank1","idx_rank2","idx_rank3"],"key": "union(idx_rank1,idx_rank2,idx_rank3)","key_length": "5,5,5","rows_examined_per_scan": 1103,"rows_produced_per_join": 1103,"filtered": "100.00","cost_info": {"read_cost": "330.79","eval_cost": "110.30","prefix_cost": "441.09","data_read_per_join": "473K"},"used_columns": ["id","rank1","rank2","log_time","prefix_uid","desc1","rank3"],"attached_condition": "((`ytt`.`t1`.`rank1` = 1) or (`ytt`.`t1`.`rank2` = 2) or (`ytt`.`t1`.`rank3` = 2))"}}}1 row in set, 1 warning (0.00 sec)
我们再看下SQL D的计划：
不加HINT，
mysql explain format=json select * from t1 where rank1 =100 and rank2 =100 and rank3 =100\G*************************** 1. row ***************************EXPLAIN: {"query_block": {"select_id": 1,"cost_info": {"query_cost": "534.34"},"table": {"table_name": "t1","access_type": "ref","possible_keys": ["idx_rank1","idx_rank2","idx_rank3"],"key": "idx_rank1","used_key_parts": ["rank1"],"key_length": "5","ref": ["const"],"rows_examined_per_scan": 555,"rows_produced_per_join": 0,"filtered": "0.07","cost_info": {"read_cost": "478.84","eval_cost": "0.04","prefix_cost": "534.34","data_read_per_join": "176"},"used_columns": ["id","rank1","rank2","log_time","prefix_uid","desc1","rank3"],"attached_condition": "((`ytt`.`t1`.`rank3` = 100) and (`ytt`.`t1`.`rank2` = 100))"}}}1 row in set, 1 warning (0.00 sec)
加了HINT，
mysql explain format=json select /*+ index_merge(t1)*/ * from t1 where rank1 =100 and rank2 =100 and rank3 =100\G*************************** 1. row ***************************EXPLAIN: {"query_block": {"select_id": 1,"cost_info": {"query_cost": "5.23"},"table": {"table_name": "t1","access_type": "index_merge","possible_keys": ["idx_rank1","idx_rank2","idx_rank3"],"key": "intersect(idx_rank1,idx_rank2,idx_rank3)","key_length": "5,5,5","rows_examined_per_scan": 1,"rows_produced_per_join": 1,"filtered": "100.00","cost_info": {"read_cost": "5.13","eval_cost": "0.10","prefix_cost": "5.23","data_read_per_join": "440"},"used_columns": ["id","rank1","rank2","log_time","prefix_uid","desc1","rank3"],"attached_condition": "((`ytt`.`t1`.`rank3` = 100) and (`ytt`.`t1`.`rank2` = 100) and (`ytt`.`t1`.`rank1` = 100))"}}}1 row in set, 1 warning (0.00 sec)
对比下以上两个 ， 加了HINT的比不加HINT的cost小了100倍 。
总结下，就是说表的cardinality值影响这张的查询计划，如果这个值没有正常更新的话，就需要手工加HINT了 。相信MySQL未来的版本会带来更多的HINT 。
mysql 如何使用sql查询某一个表的表头工具/材料：Management Studio 。
1、首先在桌面上，点击“Management Studio”图标 。
2、之后在该界面中 ， 点击左上角“新建查询”按钮 。
3、接着在该界面中 ， 输入查询某一个表的表头sql语句“select column_name from Information_schema.columns where table_Name = 'test2'” 。
4、然后在该界面中 ， 点击左上方“执行”按钮 。
5、最后在该界面中，显示某一个表的表头内容 。
如何使用mysql查询前十条数据记录MySql查询前10条数据sql语句为MySQL查询该怎么用：select * from table_name limit 0,10。
通常0是可以省略MySQL查询该怎么用的MySQL查询该怎么用 ， 直接写成limit 10 。0代表从第0条记录后面开始MySQL查询该怎么用，也就是从第一条开始 。
扩展资料MySQL查询该怎么用：

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