This join type is like ref, but with the addition that MySQL does an extra search for rows that contain NULL values. This join type optimization is used most often in resolving subqueries. In the following examples, MySQL can use a ref_or_null join to process ref_table: 这个查询类型和ref...
-- 使用EXPLAIN查看SQL的执行计划EXPLAINSELECT*FROMt1;-- 获取上一条SQL的预估执行代价SHOWSTATUSLIKE'Last_query_cost'; 如果使用集群地址连接数据库,建议您在SHOW STATUS LIKE 'Last_query_cost'前添加HINT语法/*ROUTE_TO_LAST_USED*/,以确保能够在正确的节点上查询到上一条语句的预估执行代价。例如,/*ROUTE_...
MySQL explains how it would process the statement, including information about how tables are joined and in which order. For information about using EXPLAIN to obtain execution plan information, see Section 8.8.2, “EXPLAIN Output Format”. 当...
The refcolumnshows which columnsorconstants are comparedtotheindexnamedinthekeycolumntoselectrowsfromthetable.Ifthe valueisfunc, the value usedisthe resultofsomefunction.Tosee whichfunction,useSHOW WARNINGS following EXPLAINto see the extended EXPLAIN output. Thefunctionmight actually be an operator such...
在WHERE/HAVING/ON 条件中,TiDB 优化器会分析主键或索引键的查询返回。如数字、日期类型的比较符,如大于、小于、等于以及大于等于、小于等于,字符类型的 LIKE 符号等。 值得注意的是,TiDB 目前只支持比较符一端是列,另一端是常量,或可以计算成某一常量的情况,类似year(birth_day) < 1992的查询条件是不能利用索...
如上面描述当使用 explain format=tree可以展示sql执行的树状结构 MySQL执行计划是火山模型,执行计划树自顶向下调用Read()接口,数据则自底向上被拉取处理。因此FORMAT=TREE展示的树形计划,数据获取顺序是最底层的先执行获取。对于并列的operator,上面的operator是树的左节点,优先执行。 mysql> explain format=tree select...
range can be used when a key column is compared to a constant using any of the =, <>, >, >=, <, <=, IS NULL, <=>, BETWEEN, LIKE, or IN() operators: 通过索引范围查找多行数据,可以使用=, <>, >, >=, <, <=, IS NULL, <=>, BETWEEN, LIKE, 或 IN() 操作符。
索引范围扫描,常见于使用>,<,is null,between ,in ,like等运算符的查询中。 5.7 index 索引全表扫描,把索引从头到尾扫一遍,常见于使用索引列就能够处理不须要读取数据文件的查询、能够使用索引排序或者分组的查询。 5.8 ALL 这个就是全表扫描数据文件。然后再在server层进行过滤返回符合要求的记录。
This join type is likeref, but with the addition that MySQL does an extra search for rows that containNULLvalues. This join type optimization is used most often in resolving subqueries. In the following examples, MySQL can use aref_or_nulljoin to processref_table: ...
The structure of the return value XML looks like this:XML Kopiera \<?xml version="1.0" encoding="utf-8"?> <dsql_query> <sql>. . .</sql> <params /> <dsql_operations> <dsql_operation> . . . </dsql_operation> [ . . . n ] <dsql_operations> </dsql_query> ...