PolarDB-X 1.0中的排序算子主要包括 MemSort、TopN,以及 MergeSort。 MemSort PolarDB-X 1.0中的通用的排序实现为MemSort算子,即内存中运行快速排序(Quick Sort)算法。 下面是一个用到MemSort算子的例子: > explain select t1.namefromt1 join t2 on t1.id = t2.id order by t1.name,t2.name; Project(n...
数据量比较大的情况下,“HASH GROUP BY”要更快,当然不能得出“HASH GROUP BY”就一定快的结论。 实际上是因为避免了排序操作所以“HASH GROUP BY”会比”SORT GROUP BY“更快。 无法使用”HASH GROUP BY“的两种情况 情况1:GROUP BY后有对字段进行ORDER BY。 比如: 17:35:32ZKM@dev-app73/pdb(9)>sele...
distinct效率高于group by。原因是distinct 和 group by都会进行分组操作,但group by可能会进行排序,触发filesort,导致sql执行效率低下。 基于这个结论,你可能会问: 为什么在语义相同,有索引的情况下,group by和distinct效率相同? 且在什么情况下,group by会进行排序操作? 带着这两个问题找答案。接下来,我们先来看...
由于a.company_code字段没有用到索引,所以出现了Using filesort的问题,效率比上面的差了不少 执行SQL:DESC SELECT * FROM `expense_application` a LEFT JOIN expense_application_detail b ON a.order_no = b.order_no where a.reply_num like 'QC00%' and b.customer_code = '200120' ORDER BY b.cos...
example, in theDetailsgroup, you control the order of the detail rows. For a child group, you control the order of group instances for the child group within the parent group. By default, when you create a group, the sort expression is set to the group expression and to asce...
Use the Group and Sort dialog box to group and sort items in your current display. Show Group Totals: Mark to display totals in the grouping bands. Clear the checkbox to hide all totals in the grouping bands. Show Grand Totals: Mark to show a grand total row at the top of the layout...
1.使用松散(Loose)索引扫描实现 GROUP BY 何谓松散索引扫描实现 GROUP BY 呢?实际上就是当 MySQL 完全利用索引扫描来实现 GROUP BY 的时候,并不需要扫描所有满足条件的索引键即可完成操作得出结果。 下面我们通过一个示例来描述松散索引扫描实现 GROUP BY,在示例之前我们需要首先调整一下 group_message 表的索引,将...
The default sorting is ascending and is specified with theASCkeyword, and you don't need to explicitly add it, but if you want to sort by descending order, you need to use theDESCkeyword. If we use the query above and just addDESCat the end as follows: ...
packagecom.esri.samples.statistical_query_group_and_sort;importjavafx.beans.property.SimpleBooleanProperty;importjavafx.beans.property.SimpleStringProperty;/*** Convenience bean class for representing a group-by field. The grouping property can be bound to a CheckBoxListCell* to choose whether the field...
方案一: 为了去掉filesort我们可以把索引建成(age,NAME) CREATE INDEX idx_age_name ON student(age,NAME);EXPLAIN SELECT SQL_NO_CACHE * FROM studentWHERE age = 30 AND stuno <101000 ORDER BY NAME ;[SQL]SELECT SQL_NO_CACHE * FROM student WHERE age = 30 AND stuno <101000 ORDER BY NAME ;...