PostgreSQL在使用Limit但没有显示指定order by的情况下，结果可能会不一样，原因是不同的执行计划会导致返回的结果不一样。 **测试脚本** ``` -- 脚本 drop table t; create table t(id int ,c1 int,c2 int); insert into t select x,mod(x,100),x from generate_series(1,10000) as x; insert into t select x,mod(x,100),x from generate_series(1,10000) as x; select c1,count(*) from t group by c1 limit 2; -- 执行结果 testdb=# insert into t select x,mod(x,100),x from generate_series(1,10000) as x; INSERT 0 10000 testdb=# insert into t select x,mod(x,100),x from generate_series(1,10000) as x; INSERT 0 10000 testdb=# testdb=# select c1,count(*) from t group by c1 limit 2; c1 | count ----+------- 87 | 200 71 | 200 (2 rows) testdb=# explain analyze select c1,count(*) from t group by c1 limit 2; QUERY PLAN ----------------------------------------------------------------------------------------------------------------- Limit (cost=442.54..442.56 rows=2 width=12) (actual time=10.778..10.780 rows=2 loops=1) -> HashAggregate (cost=442.54..444.54 rows=200 width=12) (actual time=10.776..10.777 rows=2 loops=1) Group Key: c1 Batches: 1 Memory Usage: 48kB -> Seq Scan on t (cost=0.00..331.36 rows=22236 width=4) (actual time=0.010..2.804 rows=20000 loops=1) Planning Time: 0.067 ms Execution Time: 10.824 ms (7 rows) -- 禁用hashagg，用groupagg，结果不同 testdb=# set enable_hashagg = off; SET testdb=# select c1,count(*) from t group by c1 limit 2; c1 | count ----+------- 0 | 200 1 | 200 (2 rows) testdb=# -- 从结果上来也符合预期，group agg是先排序再聚集，因此前2行是0和1 ``` 一个细节问题，平时留意即可。