通过EXPLAIN 分析低效 SQL的执行计划 1,先看例子 每个列的简单解释如下: select_type:表示 SELECT 的类型,常见的取值有 SIMPLE(简单表,即不使用表连接 或者子查询)、PRIMARY(主查询,即外层的查询)、UNION(UNION 中的第二个或 者后面的查询语句)、SUBQUERY(子查询中的第一个SELECT)等。 table:输出...
I have a script where it's displaying user info on a leader board. It's grabbing each user's display info through the 'registrations' table as shown in the top sql, however their back-end info (userna... IOBluetoothHandsFreeDevice audio static ...
(1)order by clauses (2)group by (3)sort merge join –-这三个会产生排序运算 2.filter --过滤,如not in、min函数等容易产生 Has a number of different meanings, used to indicate partition elimination, may also indicate an actual filter step where one row source is filtering, another, functio...
注意:由第二个执行计划可以分析得出,key_len只表示sql通过索引过滤结果集时所选择使用的索引的长度,并不包括order by的字段(类似的还有无需回表的快速索引扫描,select last_name from employees where first_name = ‘Tomofumi’),所以在第二个执行计划中,虽然order by也能使用到索引(可以通过Extra中是否有sort判断...
ERROR: column "course.language_id" must appear in the GROUP BY clause or be used in an aggregate function复制代码 此时我们可以考虑使用 partition by 来解决这个问题。 除此之外,我们使用使用 order by 时,如果列字段不存在于 group by 子语句中,也会提示上述报错。
SQL - Drop Views SQL - Rename Views SQL Operators and Clauses SQL - Where Clause SQL - Top Clause SQL - Distinct Clause SQL - Order By Clause SQL - Group By Clause SQL - Having Clause SQL - AND & OR SQL - BOOLEAN (BIT) Operator SQL - LIKE Operator SQL - IN Operator SQL - ANY...
Depending on thedetails of your tables, columns, indexes, and the conditions in your WHEREclause, the MySQL optimizer considers many techniques to efficiently performthe lookups involved in an SQL query. A query on a huge table can be performedwithout reading all the rows; a join involving seve...
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* This is exported because it's called back from prepare.c in the * EXPLAIN EXECUTE case. */voidExplainOneUtility(Node *utilityStmt, IntoClause *into, ExplainState *es, const char *queryString, ParamListInfo params, QueryEnvironment *queryEnv){ if (utilityStmt == NULL) return; if (IsA(...
When 'Column Extra' saysUsingIndexCondition, all columns in where condition are using index. If there are any columns out of index, then Column Extra sayUsingWhere;UsingIndex(in this case, Mysql need look for in data row to apply where clause). It's betterUsingIndexCondition. ...