C:使用not in()形式子查询或not exists运算符的连接查询,这种叫做反连接。即,一般连接查询是先查询内表,再查询外表,反连接就是先查询外表,再查询内表。 D:using filesort:排序时无法使用到索引时,就会出现这个。常见于order by和group by语句中。 E:using index:查询时不需要回表查询,直接通过索引就可以获取查...
Scans all the block in the index, Rows are not returned in sorted order, Introduced in 7.3 and requires V733_PLANS_ENABLED=TRUE and CBO, may be hinted using INDEX_FFS hint, uses multiblock i/o, can be executed in parallel, can be used to access second column of concatenated indexes. T...
explain结果 explain select sid from stu where grade_id in(2,3) and class_id=1 order by grade_id desc ;回到顶部 多表链接(https://www.cnblogs.com/uncleyong/p/14758383.html)中第28题,自己先写sql,然后做优化 最新原文:https://www.cnblogs.com/uncleyong/p/14805395.html...
没错,在sql前面加上explain关键字,就能够看到它的执行计划,通过执行计划,我们可以清楚的看到表和索引执行的情况,索引有没有执行、索引执行顺序和索引的类型等。索引优化的步骤是: 使用explain查看sql执行计划 判断哪些索引使用不当 优化sql,sql可能需要多次优化才能达到索引使用的最优值 既然索引优化的第一步是使用expl...
<derivedN>:ID值n的行的派生表结果,派生表可能是从FROM子句中的一个子查询生成的(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>:ID值n的行的物化子查询结果。 4....
SortsortSort performs the sorting specified by the ORDER BY clause as well as other operations such as UNIONs and joins. Can operate from disk. MergemergeProduces final sorted results of a query based on intermediate sorted results derived from operations performed in parallel. ...
IN ('North America', 'Pacific') 14 AND Gender = 'F' 15 GROUP BY G.StateProvinceName, T.SalesTerritoryGroup 16 ORDER BY AVG(YearlyIncome) DESC</sql> 17 <dsql_operations total_cost="0.926237696" total_number_operations="9"> 18 <dsql_operation operation_type="RND_ID"> 19 <identi...
After reviewing the query again, consider that a multiple-column index can meet the conditions of the WHERE clause that is based on both the o_orderdate and o_clerk columns as the next statement shows. CREATE INDEX io_clerk_date ON orders(o_clerk, o_orderdate)...
The query has no FROM clause, or has a FROM DUAL clause.(没有FROM子句或者使用DUAL虚拟表) .注:DUAL虚拟表纯粹是为了方便那些要求所有SELECT语句应该有FROM和可能的其他子句的人。 MySQL可能会忽略这些条款。 如果没有引用表,MySQL不需要FROM DUAL(https://dev.mysql.com/doc/refman/5.7/en/select.html) ...
SQL言語リファレンス 目的 EXPLAINPLAN文を使用すると、指定したSQL文を実行するためにOracle Databaseが使用する実行計画を決定できます。この文によって、実行計画の各ステップを記述している行が、指定した表に挿入されます。SQLトレース機能の一部としてEXPLAINPLAN文を発行することもできます。