如何阅读 Postgres 解释语句中的成本？

Question

这是来自 postgres 的示例解释：

 Aggregate  (cost=55881.29..55881.30 rows=1 width=64)
   ->  Nested Loop  (cost=1509.25..55881.28 rows=1 width=32)
         ->  Nested Loop  (cost=1508.82..55880.82 rows=1 width=23)
               ->  Nested Loop  (cost=1508.53..55880.48 rows=1 width=19)
                     Join Filter: (n.id = ci.person_id)
                     ->  Nested Loop  (cost=1508.09..55874.34 rows=1 width=27)
                           ->  Nested Loop  (cost=1507.67..55873.73 rows=1 width=23)
                                 ->  Nested Loop  (cost=1507.24..55865.27 rows=1 width=4)
                                       ->  Seq Scan on info_type it  (cost=0.00..2.41 rows=1 width=4)
                                             Filter: ((info)::text = 'mini biography'::text)
                                       ->  Bitmap Heap Scan on person_info pi  (cost=1507.24..55862.85 rows=1 width=8)
                                             Recheck Cond: (info_type_id = it.id)
                                             Filter: ((note)::text = 'Volker Boehm'::text)
                                             ->  Bitmap Index Scan on info_type_id_person_info  (cost=0.00..1507.24 rows=137974 width=0)
                                                   Index Cond: (info_type_id = it.id)
                                 ->  Index Scan using name_pkey on name n  (cost=0.43..8.46 rows=1 width=19)
                                       Index Cond: (id = pi.person_id)
                                       Filter: (((name_pcode_cf)::text >= 'A'::text) AND ((name_pcode_cf)::text <= 'F'::text) AND (((gender)::text = 'm'::text) OR (((gender)::text = 'f'::text) AND ((name)::text ~~ 'B%'::text))))
                           ->  Index Scan using person_id_aka_name on aka_name an  (cost=0.42..0.60 rows=2 width=4)
                                 Index Cond: (person_id = n.id)
                                 Filter: ((name)::text ~~ '%a%'::text)
                     ->  Index Scan using person_id_cast_info on cast_info ci  (cost=0.44..4.40 rows=139 width=8)
                           Index Cond: (person_id = an.person_id)
               ->  Index Only Scan using linked_movie_id_movie_link on movie_link ml  (cost=0.29..0.32 rows=2 width=4)
                     Index Cond: (linked_movie_id = ci.movie_id)
         ->  Index Scan using title_pkey on title t  (cost=0.43..0.46 rows=1 width=21)
               Index Cond: (id = ci.movie_id)
               Filter: ((production_year >= 1980) AND (production_year <= 1995))

我理解成本=a...b 表示 a 是启动成本，b 是总成本。但总成本是多少？其中发生的一切的总成本，还是仅仅是行动？例如，嵌套循环的成本，是嵌套循环本身的成本还是该成本加上其中发生的一切（更多连接、table 扫描等）？谢谢

Answer 1

来自文档：https://www.postgresql.org/docs/11/using-explain.html

It's important to understand that the cost of an upper-level node includes the cost of all its child nodes. It's also important to realize that the cost only reflects things that the planner cares about. In particular, the cost does not consider the time spent transmitting result rows to the client, which could be an important factor in the real elapsed time; but the planner ignores it because it cannot change it by altering the plan. (Every correct plan will output the same row set, we trust.)

如何阅读 Postgres 解释语句中的成本？

How to read costs in Postgres explain statements?

postgresql

explain

sql-execution-plan