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...
2,执行计划有时预测数据量,不是真实运行,可能不准确 group by的MR hive语句最好写子查询嵌套,这样分阶段的导入数据,可以逐步减少数据量。但可能会浪费时间。所以需要设计好。groupby本身也是一种数据筛选,可以大量减少数据,尤其用于去重等方面,功效显著。但group by产生MR有时不可控,不知道在哪个阶段更好。尤其,map...
<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....
(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...
explain select id, sum(moneys) from sales2 group by id order by null \G 1. 2. 你可以通过比较发现第一条语句会比第二句在Extra:里面多了Using filesort.而恰恰filesort是最耗时的。 优化ORDER BY语句 在某些情况中,MySQL可以使用一个索引来满足ORDER BY子句,而不需要额外的排序。WHERE 条件和 ORDER ...
In this case the Optimizer assumes a uniform distribution and calculates the cardinality for the query by dividing the total number of rows in the table by the number of distinct values in the column used in the where clause predicate. Figure 9 shows a query running against the employees ...
The HAVING clause is always false and cannot select any rows.(HAVING子句总是为false,不能选择任何行) Impossible WHERE The WHERE clause is always false and cannot select any rows.(WHERE子句始终为false,不能选择任何行) Impossible WHERE noticed after reading const tables ...
T.SalesTerritoryGroup IN ('North America', 'Pacific') 14 AND Gender = 'F' 15 GROUP BY G.StateProvinceName, T.SalesTerritoryGroup 16 ORDER BY AVG(YearlyIncome) DESC</sql>17<dsql_operationstotal_cost="0.926237696"total_number_operations="9">18<dsql_operationoperation_type="RND_ID">19<...
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. ...
InExample 9-3,bad1is specified as the statement identifier: Example 9-3 Using EXPLAIN PLAN with the STATEMENT ID Clause EXPLAIN PLANSET STATEMENT_ID = 'bad1' FORSELECT last_name FROM employees; Specifying Different Tables for EXPLAIN PLAN ...