Question

我在Postgres中通过外键订购会遇到奇怪的问题。这是第二张表＆amp;通过订单而不是没有订单需要更长时间的查询。

EXPLAIN ANALYZE SELECT "spoleczniak_zdjecia"."id", "spoleczniak_zdjecia"."postac_id", "spoleczniak_zdjecia"."zdjecie", "spoleczniak_zdjecia"."opis", "spoleczniak_zdjecia"."data", "spoleczniak_zdjecia"."avatar", "spoleczniak_zdjecia"."tagi", "postac_postacie"."id", "postac_postacie"."user_id", "postac_postacie"."avatar", "postac_postacie"."ikonka", "postac_postacie"."imie", "postac_postacie"."nazwisko", "postac_postacie"."pseudonim", "postac_postacie"."plec", "postac_postacie"."wzrost", "postac_postacie"."waga", "postac_postacie"."ur_tydz", "postac_postacie"."ur_rok", "postac_postacie"."ur_miasto_id", "postac_postacie"."akt_miasto_id", "postac_postacie"."kasa", "postac_postacie"."punkty", "postac_postacie"."zmeczenie", "postac_postacie"."zdrowie", "postac_postacie"."kariera" FROM "spoleczniak_zdjecia" INNER JOIN "taggit_taggeditem" ON ("spoleczniak_zdjecia"."id" = "taggit_taggeditem"."object_id") INNER JOIN "taggit_tag" ON ("taggit_taggeditem"."tag_id" = "taggit_tag"."id") INNER JOIN "postac_postacie" ON ("spoleczniak_zdjecia"."postac_id" = "postac_postacie"."id") WHERE ("taggit_tag"."slug" = 'ja' AND "taggit_taggeditem"."content_type_id" = 922 ) ORDER BY "spoleczniak_zdjecia"."id" DESC LIMIT 28;
                                                                                QUERY PLAN
---------------------------------------------------------------------------------------------------------------------------------------------------------------------------
 Limit  (cost=27.88..27.89 rows=7 width=198) (actual time=2984.689..2984.697 rows=28 loops=1)
   ->  Sort  (cost=27.88..27.89 rows=7 width=198) (actual time=2984.688..2984.692 rows=28 loops=1)
         Sort Key: spoleczniak_zdjecia.id
         Sort Method: top-N heapsort  Memory: 32kB
         ->  Nested Loop  (cost=2.31..27.78 rows=7 width=198) (actual time=1.063..2974.901 rows=9091 loops=1)
               ->  Nested Loop  (cost=2.31..22.02 rows=7 width=109) (actual time=1.057..2899.010 rows=9091 loops=1)
                     ->  Nested Loop  (cost=2.31..19.92 rows=7 width=4) (actual time=1.046..2848.853 rows=9103 loops=1)
                           ->  Index Scan using taggit_tag_slug on taggit_tag  (cost=0.00..4.27 rows=1 width=4) (actual time=0.025..0.027 rows=1 loops=1)
                                 Index Cond: ((slug)::text = 'ja'::text)
                           ->  Bitmap Heap Scan on taggit_taggeditem  (cost=2.31..15.56 rows=7 width=8) (actual time=1.019..2847.244 rows=9103 loops=1)
                                 Recheck Cond: (tag_id = taggit_tag.id)
                                 Filter: (content_type_id = 922)
                                 ->  Bitmap Index Scan on taggit_taggeditem_tag_id  (cost=0.00..2.31 rows=7 width=0) (actual time=0.954..0.954 rows=9103 loops=1)
                                       Index Cond: (tag_id = taggit_tag.id)
                     ->  Index Scan using spoleczniak_zdjecia_pkey on spoleczniak_zdjecia  (cost=0.00..0.29 rows=1 width=109) (actual time=0.005..0.005 rows=1 loops=9103)
                           Index Cond: (id = taggit_taggeditem.object_id)
               ->  Index Scan using postac_postacie_pkey on postac_postacie  (cost=0.00..0.81 rows=1 width=89) (actual time=0.007..0.007 rows=1 loops=9091)
                     Index Cond: (id = spoleczniak_zdjecia.postac_id)
 Total runtime: 2984.760 ms

这里没有订单：

EXPLAIN ANALYZE SELECT "spoleczniak_zdjecia"."id", "spoleczniak_zdjecia"."postac_id", "spoleczniak_zdjecia"."zdjecie", "spoleczniak_zdjecia"."opis", "spoleczniak_zdjecia"."data", "spoleczniak_zdjecia"."avatar", "spoleczniak_zdjecia"."tagi", "postac_postacie"."id", "postac_postacie"."user_id", "postac_postacie"."avatar", "postac_postacie"."ikonka", "postac_postacie"."imie", "postac_postacie"."nazwisko", "postac_postacie"."pseudonim", "postac_postacie"."plec", "postac_postacie"."wzrost", "postac_postacie"."waga", "postac_postacie"."ur_tydz", "postac_postacie"."ur_rok", "postac_postacie"."ur_miasto_id", "postac_postacie"."akt_miasto_id", "postac_postacie"."kasa", "postac_postacie"."punkty", "postac_postacie"."zmeczenie", "postac_postacie"."zdrowie", "postac_postacie"."kariera" FROM "spoleczniak_zdjecia" INNER JOIN "taggit_taggeditem" ON ("spoleczniak_zdjecia"."id" = "taggit_taggeditem"."object_id") INNER JOIN "taggit_tag" ON ("taggit_taggeditem"."tag_id" = "taggit_tag"."id") INNER JOIN "postac_postacie" ON ("spoleczniak_zdjecia"."postac_id" = "postac_postacie"."id") WHERE ("taggit_tag"."slug" = 'ja' AND "taggit_taggeditem"."content_type_id" = 922 ) LIMIT 28;
                                                                            QUERY PLAN
-------------------------------------------------------------------------------------------------------------------------------------------------------------------
 Limit  (cost=2.31..27.78 rows=7 width=198) (actual time=1.113..1.482 rows=28 loops=1)
   ->  Nested Loop  (cost=2.31..27.78 rows=7 width=198) (actual time=1.112..1.477 rows=28 loops=1)
         ->  Nested Loop  (cost=2.31..22.02 rows=7 width=109) (actual time=1.102..1.292 rows=28 loops=1)
               ->  Nested Loop  (cost=2.31..19.92 rows=7 width=4) (actual time=1.092..1.145 rows=28 loops=1)
                     ->  Index Scan using taggit_tag_slug on taggit_tag  (cost=0.00..4.27 rows=1 width=4) (actual time=0.017..0.017 rows=1 loops=1)
                           Index Cond: ((slug)::text = 'ja'::text)
                     ->  Bitmap Heap Scan on taggit_taggeditem  (cost=2.31..15.56 rows=7 width=8) (actual time=1.072..1.118 rows=28 loops=1)
                           Recheck Cond: (tag_id = taggit_tag.id)
                           Filter: (content_type_id = 922)
                           ->  Bitmap Index Scan on taggit_taggeditem_tag_id  (cost=0.00..2.31 rows=7 width=0) (actual time=0.989..0.989 rows=9103 loops=1)
                                 Index Cond: (tag_id = taggit_tag.id)
               ->  Index Scan using spoleczniak_zdjecia_pkey on spoleczniak_zdjecia  (cost=0.00..0.29 rows=1 width=109) (actual time=0.004..0.005 rows=1 loops=28)
                     Index Cond: (id = taggit_taggeditem.object_id)
         ->  Index Scan using postac_postacie_pkey on postac_postacie  (cost=0.00..0.81 rows=1 width=89) (actual time=0.005..0.005 rows=1 loops=28)
               Index Cond: (id = spoleczniak_zdjecia.postac_id)
 Total runtime: 1.562 ms

什么可能导致问题？这是查询？配置？我应该检查任何特定的配置？在我的上一个问题中，有更复杂的查询，但该查询根本不复杂。有什么建议吗？

顺便说一下。该查询由Django生成（准确地说是django-taggit）。顺便说一下。第二部分，硬件并不差（i7,16 GB的RAM，OS 10的RAID 10 3x2和WAL的数据+ 2个RAID1磁盘，512 MB的RAID缓存+ BBU）

纯文本查询：

选择“spoleczniak_zdjecia”。“id”，“spoleczniak_zdjecia”。“postac_id”，“spoleczniak_zdjecia”。“zdjecie”，“spoleczniak_zdjecia”。“opis”，“spoleczniak_zdjecia”。“data”，“spoleczniak_zdjecia”。“avatar “，”“spoleczniak_zdjecia”。“tagi”，“postac_postacie”。“id”，“postac_postacie”。“user_id”，“postac_postacie”。“avatar”，“postac_postacie”。“ikonka”，“postac_postacie”。“imie”， “postac_postacie”。“nazwisko”，“postac_postacie”。“pseudonim”，“postac_postacie”。“plec”，“postac_postacie”。“wzrost”，“postac_postacie”。“waga”，“postac_postacie”。“ur_tydz”，“postac_postacie” “。”ur_rok“，”postac_postacie“。”ur_miasto_id“，”postac_postacie“。”akt_miasto_id“，”postac_postacie“。”kasa“，”postac_postacie“。”punkty“，”postac_postacie“。”zmeczenie“，”postac_postacie“。 “zdrowie”，“postac_postacie”。“kariera”FROM“spoleczniak_zdjecia”INNER JOIN“taggit_taggeditem”ON（“spoleczniak_zdjecia”。“id”=“taggit_taggeditem”。“object_id”）INNER JOIN“taggit_tag”ON（“taggit_taggeditem”。“ tag_id“=”taggit_tag“。”id“）INNER JOIN”postac_postaci e“ON（”spoleczniak_zdjecia“。”postac_id“=”postac_postacie“。”id“）WHERE（”taggit_tag“。”slug“='ja'AND”taggit_taggeditem“。”content_type_id“= 922）ORDER BY”spoleczniak_zdjecia“。 “id”DESC LIMIT 28;

Answer 1

区别在于EXPLAIN输出的第二行：

->  Sort  (cost=27.88..27.89 rows=7 width=198) (actual time=2984.688..2984.692 rows=28 loops=1)

请注意，“实际时间”几乎是查询的整个时间。排序不仅需要一堆比较（即排序任何东西的成本），还需要额外的数据管理，服务器需要将一些数据（行或指针指向行）复制到临时位置，以便可以对其进行排序而不会打扰其他任何内容

任何查询都需要更长的时间进行排序，除非你很幸运，你的排序与磁盘上的顺序匹配，优化器可以注意到它们匹配。

Answer 2

第二个返回无论订单如何找到的前28个记录。

首先你必须订购结果，然后返回28首记录。

如果未修改数据，则ORDER BY的查询将每次返回相同的28条记录。

但是第二个查询可以每次执行时返回28个不同的记录。结果无法保证。

Postgres按外键表现订购？

2 个答案: