在postgresql15中,可以使用并发,使用多个cpu进程。这一特性涉及好几个参数,但是,我们只聚焦在参数max_parallel_workers_per_gather。 为了演示这个改进,我们创建三个表,没有索引,填充大约5000000条记录。注意,表的列数分别为1,5,10。 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 ...
PostgreSQL 是一种开源的关系型数据库管理系统,广泛应用于云计算领域。对于查询两列中任意一列的 DISTINCT(不重复)值,可以使用以下方法: 1. 使用 SELECT DISTINCT ...
1. DISTINCT is a reserved keyword in PostgreSQL, so we cannot specify it as an object name. postgres=#createtabledistinct(nint);ERROR: syntax error at or near "distinct" 2. In a SELECT query we cannot have more than one DISTINCT keyword: postgres=#selectdistinct1,distinct2;ERROR: syntax ...
A set of rows for which all the expressions are equal are considered duplicates, and only the first row of the set is kept in the output. Note that the “first row” of a set is unpredictable unless the query is sorted on enough columns to guarantee a unique ordering of the rows ...
PostgreSQL 的 count(distinct ...) 的实现方式是排序而不是使用 hash, 所以速度很慢. 应该要换成 hash 方式, 只是因为各种原因还没有实现. 规避途径一: 通过 COUNT 子查询 使用下面的方式, 查询时间能缩短一半以上 SELECT COUNT(col) FROM( SELECTDISTINCTfield_1AScolFROMtable_1 ...
postgresql中使用distinct去重 select语法 [ WITH [ RECURSIVE ] with_query [, ...] ] SELECT [ ALL | DISTINCT [ ON ( expression [, ...]...WINDOW window_name AS ( window_definition ) [, ...] ] [ { UNION | INTERSECT | EXCEPT } [ ALL | DISTINCT...只有code, name拼接的组合相同时,...
PostgreSQL has a really interesting and powerful construct called SELECT DISTINCT ON. No, this is not a typical DISTINCT. This is different. It is perfect when you have groups of data that are similar and want to pull a singl
Optimizer status: legacy query optimizer Total runtime:122.173ms (22rows) 执行计划解读: 非分布键 GROUP BY,首先会在本地节点group by,然后按GROUP BY字段进行数据重分布,然后再在本地节点GROUP BY,最后返回GROUP BY结果给master节点,返回给用户。
Summary: in this tutorial, you will learn how to use the PostgreSQL SELECT DISTINCT clause to remove duplicate rows from a result set returned by a query. Introduction to PostgreSQL SELECT DISTINCT clause The SELECT DISTINCT removes duplicate rows from a result set. The SELECT DISTINCT clause ret...
Using the DISTINCT Clause in PostgreSQL The DISTINCT clause in PostgreSQL is used to return unique values from a column or combination of columns within a query. This feature is helpful when you need to eliminate duplicate results and retrieve only unique data entries, improving query precision and...