FirstMatch execution strategy (首次匹配) FirstMatch是一种最原始的半连接执行方式,简单来说就是说先取一条外层查询的中的记录,然后到子查询的表中寻找符合匹配条件的记录,如果能找到一条,则将该外层查询的记录放入最终的结果集并且停止查找更多匹配的记录,如果找不到则把该外层查询的记录丢弃掉;然后再开始取下一条...
EXPLAIN is used to obtain a query execution plan (that is, an explanation of how MySQL would execute a query). 简单翻译一下,就是explain用于获取查询执行计划(即MySQL是如何执行一个查询的)。 工作中,我们会遇到慢查询,这个时候我们就可以在select语句之前增加explain关键字,模拟MySQL优化器执行SQL语句,从而...
row *** id: 1 select_type: PRIMARY table: orders partitions: NULL type: range possible_keys: i_o_orderdate key: i_o_orderdate key_len: 3 ref: NULL rows: 568369 filtered: 100.00 Extra: Parallel scan (2 workers); Using index condition; Using where; Using temporary *** 3. row **...
unionM,N: The row refers to the union of the rows with id values of M and N. derivedN: The row refers to the derived table result for the row with an id value of N. A derived table may result, for example, from a subquery in the FROM clause. subqueryN: The row refers to the...
Using temporary:临时表用于保存中间结果,MySQL在对结果进行排序时使用临时表,常见于排序 order by 和分组查询 group by 。 Using index:表示对应的select操作使用Covering Index,避免访问表的数据行,效率较高。如果还出现using where,则表明该索引用于执行索引键值查找。如果没有同时使用where,则使用索引来读取数据而不...
* print out the execution plan for one Query * * "into" is NULL unless we are explaining the contents of a CreateTableAsStmt. */ static void ExplainOneQuery(Query *query, int cursorOptions, IntoClause *into, ExplainState *es, const char *queryString, ParamListInfo params, QueryEnvironment...
If the index bounds consists of several key ranges, the index scan execution process may examine out-of-bounds keys in order to skip from the end of one range to the beginning of the next. Consider the following example, where there is an index of field x and the collection contains 100...
ID A number assigned to each step in the execution plan. PARENT_ID The ID of the next execution step that operates on the output of the ID step. POSITION The order of processing for steps that all have the same PARENT_ID. OTHER Other information that is specific to the executio...
Partitioning, such as the set of accessed partitions Parallel execution, such as the distribution method of join inputs The EXPLAIN PLAN results let you determine whether the optimizer selects a particular execution plan, such as, nested loops join. The results also help you to understand the...
EXPLAIN: This statement returns the SQL execution plan that is estimated by the QO based on the characteristics of SQL statements, rather than the actual execution plan. This estimated execution plan provides reference for SQL statement execution. EXPLAIN ANALYZE: This statement returns the actual ...