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...
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_...
SQL - Having Clause SQL - AND & OR SQL - BOOLEAN (BIT) Operator SQL - LIKE Operator SQL - IN Operator SQL - ANY, ALL Operators SQL - EXISTS Operator SQL - CASE SQL - NOT Operator SQL - NOT EQUAL SQL - IS NULL SQL - IS NOT NULL SQL - NOT NULL SQL - BETWEEN Operator SQL -...
ref can be usedforindexed columns that are compared using the = or <=> operator. In the following examples, MySQL can use a ref jointo process ref_table: 注意这里索引使用了最左侧前缀,因此值是ref。表结构如下: | dept_manager |CREATE TABLE `dept_manager` ( ...
查看SQL执行的频次 show GLOBAL status like ‘Com___’ 查看innoDB搜索引擎的执行sql频次 show GLOBAL status like ‘innoDB_row_%’ 定位低效sql语句show PROCESSLIST执行计划explain在sql前加上explainexplain 第五章:索引的使用-查询分析器explain 直接学习...
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> ...
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。
(void) get_equality_op_for_ordering_op(sortOperator, &reverse); } /* Add NULLS FIRST/LAST only if it wouldn't be default */ if (nullsFirst && !reverse) { appendStringInfoString(buf, " NULLS FIRST"); } else if (!nullsFirst && reverse) { appendStringInfoString(buf, " NULLS LAST"...
When theEXTENDED keyword is used, EXPLAIN produces extra information that can be viewedby issuing a SHOW WARNINGS statement following the EXPLAIN statement. Thisinformation displays how the optimizer qualifies table and column names in theSELECT statement, what the SELECT looks like after the applicatio...