本节简单介绍了PostgreSQL中和distinct和group by. 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.