https://dev.mysql.com/doc/workbench/en/wb-performance-explain.html 标准框--红色:全表扫描(花费极高),全索引扫描(花费高) 圆脚框--group/sort,分组或排序 钻石框--joins Framed boxes(有外框)--子查询。 Join type连接类型: https://dev.mysql.com/doc/refman/8.0/en/explain-output.html#explain-joi...
EXPLAIN returnsa row of information for each table used in the SELECT statement. It lists thetables in the output in the order that MySQL would read them while processingthe statement. MySQL resolves all joins using a nested-loop join method. Thismeans that MySQL reads a row from the first ...
如果MySQL在t2中找到一个匹配的行,它会知道t2.id永远不会为NULL,并且不会扫描t2中具有相同id值的其余行。也就是说,对于t1中的每一行,MySQL只需要在t2中只执行一次查找,而不考虑在t2中实际匹配的行数。 在MySQL 8.0.17及更高版本中,如果出现此提示,还可表示形如 NOT IN (subquery) 或 NOT EXISTS (subquer...
这个算法与 MySQL 主要使用的 Join 算法类似。在下表的示例中,TableReader_29(Build) 算子首先读取表 t1,然后根据在 t1 中匹配到的每行数据,依次探查表 t2 中的数据: 注意 在执行计划返回结果中,自 v6.4.0 版本起,特定算子(即 IndexJoin 和Apply 算子的 Probe 端所有子节点)的 estRows 字段意义与 v6.4....
mysql root@localhost:(none)>selectversion();+---+|version()|+---+|5.7.24-log|+---+1rowinsetTime:0.066s 回到顶部 主要用法 { EXPLAIN | DESCRIBE } [EXTENDED | PARTITIONS | FORMAT=[TRADITIONAL | JSON]]SQL_STATEMENT; EXPLAIN和DESCRIBE(可以简写成DESC)都可以用来...
merges two record sets by looping through every record in the first set and trying to find a match in the second set. All matching records are returned. 遍历模式 索引模式 5、聚合 - Aggregate group by 操作 groups records together based on a GROUP BY or aggregate function (like sum()). ...
the order that MySQL would read them while processingthe statement. MySQL resolves all joins using a -loop join method. Thismeans that MySQL reads a row the first table, and then finds a matchingrow in the second table, the third table, and so on. When all tables areprocessed, MySQL...
for such a query if table is InnoDB. Also HASH joins are visible only in TREE format (and EXPLAIN ANALYZE of course). Currently some queries, which don’t use the iterator executor, won’t be explained andran by EXPLAIN ANALYZE at all and you’ll get only “not executable by ...
joins the tables in an optimal order. To give a hint to the optimizer to use a join order corresponding to the order in which the tables are namedin a SELECT statement, begin the statement with SELECT STRAIGHT_JOIN rather than just SELECT. (See Section 15.2.13, “SELECT Statement”.) ...
This tableis referenced as the child of table in a join that can be pushed down to the NDBkernel. Applies only in MySQL Cluster, when pushed-down joins are enabled. Seethe description of the ndb_join_pushdown serversystem variable for more information and examples. ...