额外信息(Extra):Extra列提供了额外的执行信息,如Using where、Using index、Using temporary等,这些信息可以帮助理解查询的具体执行情况。 4. 注意优化常见的不走索引等慢sql: -- 索引的最左匹配原则 * : 指在使用复合索引(Composite Index)时,索引的多个列按顺序组成了索引键。当你查询时,可以利用索引中...
unique_subquery:用于where中的in形式子查询,子查询返回不重复值唯一值 index_subquery:用于in形式子查询使用到了辅助索引或者in常数列表,子查询可能返回重复值,可以使用索引将子查询去重。 range:只检索给定范围的行,使用一个索引来选着行。key列显示使用了哪个索引。一般在你的WHERE 语句中出现between 、< 、> 、i...
MySQL explain中key_len的计算 /group by这一部分被选中的索引列的。 例如,有个联合索引idx(c1,c2,c3),3列均是intnotnull,那么下面的SQL执行计划中,key_len的值是8而不是12: select...key_len表示索引使用的字节数,根据这个值可以判断索引的使用情况,特别是在组合索引的时候,判断该索引有多少部分被使用到非...
EXPLAIN 是MySQL 中的一个关键字,用于分析 SQL 查询语句的执行计划。通过 EXPLAIN 返回的信息,用户可以了解查询优化器是如何选择执行计划的,以及可能的性能瓶颈。 id 每个查询的唯一标识符。对于多表查询,id的值会增大。 select_type 查询的类型,主要有以下几种: SIMPLE: 简单查询,不包含子查询或UNION。 PRIMA...
Understand Complex SQL Queries Easily using AI Expert SQL Query Write down your SQL Query in the following box and select target datbase. Once done, click onExplain CodeButton to generate the explanation of your SQL query. SELECT U.userid, U.fullname, U.emailid, U.state, U.country, U....
NULL 조건 EXISTS 조건 IN 조건 SQL 명령 ABORT ALTER DATABASE ALTER DATASHARE ALTER DEFAULT PRIVILEGES ALTER EXTERNAL SCHEMA ALTER EXTERNAL VIEW ALTER FUNCTION ALTER GROUP ALTER IDENTITY PROVIDER 마스킹 정책 변경 ALTER MATERIALIZED VIEW DISTSTYLE 및 SORTKEY 예제 ...
There’s a lot of new data. For starters, we can see useful information in ‘used_key_parts’ section. Instead of guessing which parts of the composite index were used in the query, we have a nice list presented in the explain’s output. We have also precise information about what’s...
Under certain conditions the plan shown when using EXPLAIN PLAN can be different from the plan shown using V$SQL_PLAN. For example, when the SQL statement contains bind variables the plan shown from using EXPLAIN PLAN ignores the bind variable values while the plan shown in V$SQL_PLAN takes ...
By looking at the key_len column, we can see that the composite index for thevotestable is used in full - 8 bytes, which covers both the VoteTypeId and PostId columns. The last important change we see is the amount of rows MySQL estimates it needs to inspect in order to run evaluate...
oftheuseofcompositeindexes,Oraclehasthefollowing characteristics: 1,whenarulebasedoptimizer(RBO)isused,theindexisused onlywhentheleadingcolumnofthecompositeindexappearsin thewhereclauseoftheSQLstatement; 2,beforeusingOracle9ibasedonthecostoftheoptimizer ...