Question

何时评估“分组依据”表达式？当Postgres决定，这两个表达式实际上是一样的吗？

示例：

db=> \timing
Timing is on.

db=> select pg_sleep(1) group by pg_sleep(1)::varchar;
 pg_sleep 
----------

(1 row)

Time: 2002.416 ms

db=> select pg_sleep(1)::varchar group by pg_sleep(1)::varchar;
 pg_sleep 
----------

(1 row)

Time: 1001.367 ms

由于可能的开销，“group by”中的别名是否比表达式执行得更好？例如，它会比使用表达式“group by”的版本更好：

db=> select pg_sleep(1)::varchar as e group by e;
 e 
---

(1 row)

Time: 1001.688 ms

即使AST不同，也不会重新评估表达式：

db=> select pg_sleep(1 + 0.5)::varchar group by pg_sleep(0.5 + 1)::varchar;
 pg_sleep 
----------

(1 row)

Time: 1500.971 ms

Answer 1

我不确定我是否理解这个问题，所以也许回答错误的问题。然而 - 在这里。它获得后将结果分组。我很惊讶pg_sleep没有被同一个演员重新执行。聪明的策划者很聪明。聚合方法似乎没有改变它。关于别名 - 如果规划者理解表达式是相同的并且不需要再次执行函数它没有别名的帮助就做了，所以可能别名不会改变任何东西。

so=# explain analyze select pg_sleep(1) group by pg_sleep(1)::varchar;
                                           QUERY PLAN
-------------------------------------------------------------------------------------------------
 HashAggregate  (cost=0.02..0.04 rows=1 width=0) (actual time=2002.201..2002.203 rows=1 loops=1)
   ->  Result  (cost=0.00..0.02 rows=1 width=0) (actual time=1001.093..1001.093 rows=1 loops=1)
 Total runtime: 2002.236 ms
(3 rows)

Time: 2002.779 ms

so=# set enable_hashagg to off;
SET

so=# explain analyze select pg_sleep(1)::text group by pg_sleep(1)::varchar;
                                              QUERY PLAN
------------------------------------------------------------------------------------------------------
 Group  (cost=0.03..0.05 rows=1 width=0) (actual time=2002.219..2002.220 rows=1 loops=1)
   ->  Sort  (cost=0.03..0.03 rows=1 width=0) (actual time=1001.111..1001.112 rows=1 loops=1)
         Sort Key: ((pg_sleep(1::double precision))::character varying)
         Sort Method: quicksort  Memory: 25kB
         ->  Result  (cost=0.00..0.02 rows=1 width=0) (actual time=1001.100..1001.100 rows=1 loops=1)
 Total runtime: 2002.245 ms
(6 rows)

Time: 2002.564 ms
so=# explain analyze select pg_sleep(1)::varchar as e group by e;
                                              QUERY PLAN
------------------------------------------------------------------------------------------------------
 Group  (cost=0.03..0.04 rows=1 width=0) (actual time=1001.109..1001.110 rows=1 loops=1)
   ->  Sort  (cost=0.03..0.03 rows=1 width=0) (actual time=1001.107..1001.108 rows=1 loops=1)
         Sort Key: ((pg_sleep(1::double precision))::character varying)
         Sort Method: quicksort  Memory: 25kB
         ->  Result  (cost=0.00..0.02 rows=1 width=0) (actual time=1001.098..1001.099 rows=1 loops=1)
 Total runtime: 1001.132 ms
(6 rows)

Time: 1001.470 ms

最后，我不确定这种行为是否直观：

so=# explain analyze select pg_sleep(random())::text group by pg_sleep(random())::varchar;
                                            QUERY PLAN
--------------------------------------------------------------------------------------------------
 Group  (cost=0.03..0.06 rows=1 width=0) (actual time=742.928..742.930 rows=1 loops=1)
   ->  Sort  (cost=0.03..0.04 rows=1 width=0) (actual time=53.152..53.153 rows=1 loops=1)
         Sort Key: ((pg_sleep(random()))::character varying)
         Sort Method: quicksort  Memory: 25kB
         ->  Result  (cost=0.00..0.02 rows=1 width=0) (actual time=53.143..53.143 rows=1 loops=1)
 Total runtime: 742.958 ms
(6 rows)

Time: 743.271 ms
so=# explain analyze select pg_sleep(random())::text group by pg_sleep(random())::text;
                                             QUERY PLAN
----------------------------------------------------------------------------------------------------
 Group  (cost=0.03..0.05 rows=1 width=0) (actual time=963.075..963.076 rows=1 loops=1)
   ->  Sort  (cost=0.03..0.04 rows=1 width=0) (actual time=963.073..963.073 rows=1 loops=1)
         Sort Key: ((pg_sleep(random()))::text)
         Sort Method: quicksort  Memory: 25kB
         ->  Result  (cost=0.00..0.02 rows=1 width=0) (actual time=963.063..963.065 rows=1 loops=1)
 Total runtime: 963.099 ms
(6 rows)

Time: 963.419 ms

我希望explain analyze select pg_sleep(random())::text group by pg_sleep(random())::text;能够pg_sleep重新执行group by

Postgres小组通过表达式评估

1 个答案: