本篇内容介绍了“分析PostgreSQL中的distinct和group by”的有关知识，在实际案例的操作过程中，不少人都会遇到这样的困境，接下来就让小编带领大家学习一下如何处理这些情况吧！希望大家仔细阅读，能够学有所成！
通常来说,获取唯一值,既可以用distinct也可以用group by,但在存在主键时,group by会做相应的优化,把多个分组键规约为主键.
没有主键的情况
[pg12@localhost ~]$ psql
Expanded display is used automatically.
psql (12.2)
Type "help" for help.
[local:/data/run/pg12]:5120 pg12@testdb=# create table tbl1 (id int,c1 text,c2 int,c3 varchar);
CREATE TABLE
[local:/data/run/pg12]:5120 pg12@testdb=# insert into tbl1 (id,c1,c2,c3) select x,x||'c1',x,x||'c3' from generate_series(1,100000) as x;
INSERT 0 100000
[local:/data/run/pg12]:5120 pg12@testdb=# explain select distinct id,c1,c2,c3 from tbl1;
QUERY PLAN
------------------------------------------------------------------
HashAggregate (cost=1668.94..1720.54 rows=5160 width=72)
Group Key: id, c1, c2, c3
-> Seq Scan on tbl1 (cost=0.00..1152.97 rows=51597 width=72)
(3 rows)
[local:/data/run/pg12]:5120 pg12@testdb=# explain select id,c1,c2,c3 from tbl1 group by id,c1,c2,c3;
QUERY PLAN
------------------------------------------------------------------
HashAggregate (cost=1668.94..1720.54 rows=5160 width=72)
Group Key: id, c1, c2, c3
-> Seq Scan on tbl1 (cost=0.00..1152.97 rows=51597 width=72)
(3 rows)存在主键的情况
[local:/data/run/pg12]:5120 pg12@testdb=# alter table tbl1 add primary key(id);
'ALTER TABLE
[local:/data/run/pg12]:5120 pg12@testdb=# explain select distinct id,c1,c2,c3 from tbl1;
QUERY PLAN
-------------------------------------------------------------------------
Unique (cost=14043.82..15293.82 rows=100000 width=72)
-> Sort (cost=14043.82..14293.82 rows=100000 width=72)
Sort Key: id, c1, c2, c3
-> Seq Scan on tbl1 (cost=0.00..1637.00 rows=100000 width=72)
(4 rows)
[local:/data/run/pg12]:5120 pg12@testdb=# explain select id,c1,c2,c3 from tbl1 group by id,c1,c2,c3;
QUERY PLAN
-------------------------------------------------------------------------------------
Group (cost=0.29..5402.29 rows=100000 width=72)
Group Key: id
-> Index Scan using tbl1_pkey on tbl1 (cost=0.29..5152.29 rows=100000 width=72)
(3 rows)
[local:/data/run/pg12]:5120 pg12@testdb=#在存在主键的情况下,使用group by时,分组键只需要主键即可.
“分析PostgreSQL中的distinct和group by”的内容就介绍到这里了，感谢大家的阅读。如果想了解更多行业相关的知识可以关注亿速云网站，小编将为大家输出更多高质量的实用文章！
