performance - 将 PostgreSQL 从 9.1 升级到 9.4 后性能下降

Question

将 Postgres 9.1 升级到 9.4 后，我的性能变得极其缓慢。这是运行速度明显更慢的两个查询的示例。

注意:我意识到这些查询可能可以被重写以更有效地工作，但是我主要担心的是在升级到更新版本的 Postgres 之后，它们的运行速度突然慢了 100 倍!我希望在我忽略的地方有一个配置变量。

在进行升级时，我使用了带有 --link 选项的 pg_upgrade 命令。配置文件在 9.4 和 9.1 之间是相同的。它不是在完全相同的硬件上运行，但它们都在 Linode 上运行，我已经尝试为新服务器使用 3 个不同的 Linode，所以我认为这不是硬件问题。

似乎在这两种情况下，9.4 使用的索引与 9.1 不同？

9.1:

EXPLAIN ANALYZE SELECT "id", "title", "timestamp", "parent", "deleted", "sunk", "closed", "sticky", "lastupdate", "views", "oldid", "editedon", "devpost", "hideblue", "totalvotes", "statustag", "forum_category_id", "account_id" FROM "forum_posts" WHERE "parent" = 882269 ORDER BY "timestamp" DESC LIMIT 1;
                                                                         QUERY PLAN                                                                      
    -----------------------------------------------------------------------------------------------------------------------------------------------------
     Limit  (cost=63.87..63.87 rows=1 width=78) (actual time=0.020..0.020 rows=0 loops=1)
       ->  Sort  (cost=63.87..63.98 rows=45 width=78) (actual time=0.018..0.018 rows=0 loops=1)
             Sort Key: "timestamp"
             Sort Method: quicksort  Memory: 17kB
             ->  Index Scan using index_forum_posts_parent on forum_posts  (cost=0.00..63.65 rows=45 width=78) (actual time=0.013..0.013 rows=0 loops=1)
                   Index Cond: (parent = 882269)
     Total runtime: 0.074 ms
    (7 rows)

9.4:

EXPLAIN ANALYZE SELECT "id", "title", "timestamp", "parent", "deleted", "sunk", "closed", "sticky", "lastupdate", "views", "oldid", "editedon", "devpost", "hideblue", "totalvotes", "statustag", "forum_category_id", "account_id" FROM "forum_posts" WHERE "parent" = 882269 ORDER BY "timestamp" DESC LIMIT 1;
                                                                              QUERY PLAN                                                                               
-----------------------------------------------------------------------------------------------------------------------------------------------------------------------
 Limit  (cost=0.42..63.48 rows=1 width=1078) (actual time=920.484..920.484 rows=0 loops=1)
   ->  Index Scan Backward using forum_posts_timestamp_index on forum_posts  (cost=0.42..182622.07 rows=2896 width=1078) (actual time=920.480..920.480 rows=0 loops=1)
         Filter: (parent = 882269)
         Rows Removed by Filter: 1576382
 Planning time: 0.166 ms
 Execution time: 920.521 ms
(6 rows)

9.1:

EXPLAIN ANALYZE SELECT "user_library_images"."id", "user_library_images"."imgsrc", "user_library_images"."library_image_id", "user_library_images"."type", "user_library_images"."is_user_uploaded", "user_library_images"."credit", "user_library_images"."orig_dimensions", "user_library_images"."account_id" FROM "user_library_images" INNER JOIN "image_tags" ON "user_library_images"."id" = "image_tags"."user_library_image_id" WHERE ("user_library_images"."account_id" = 769718 AND "image_tags"."tag" ILIKE '%stone%') GROUP BY "user_library_images"."id", "user_library_images"."imgsrc", "user_library_images"."library_image_id", "user_library_images"."type", "user_library_images"."is_user_uploaded", "user_library_images"."credit", "user_library_images"."orig_dimensions", "user_library_images"."account_id" ORDER BY "user_library_images"."id";

 Group  (cost=2015.46..2015.49 rows=1 width=247) (actual time=0.629..0.652 rows=6 loops=1)
   ->  Sort  (cost=2015.46..2015.47 rows=1 width=247) (actual time=0.626..0.632 rows=6 loops=1)
         Sort Key: user_library_images.id, user_library_images.imgsrc, user_library_images.library_image_id, user_library_images.type, user_library_images.is_user_uploaded, user_library_images.credit, user_library_images.orig_dimensions, user_library_images.account_id
         Sort Method: quicksort  Memory: 19kB
         ->  Nested Loop  (cost=0.00..2015.45 rows=1 width=247) (actual time=0.283..0.603 rows=6 loops=1)
               ->  Index Scan using index_user_library_images_account on user_library_images  (cost=0.00..445.57 rows=285 width=247) (actual time=0.076..0.273 rows=13 loops=1)
                     Index Cond: (account_id = 769718)
               ->  Index Scan using index_image_tags_user_library_image on image_tags  (cost=0.00..5.50 rows=1 width=4) (actual time=0.020..0.021 rows=0 loops=13)
                     Index Cond: (user_library_image_id = user_library_images.id)
                     Filter: (tag ~~* '%stone%'::text)
 Total runtime: 0.697 ms
(11 rows)

9.4:

Group  (cost=166708.13..166709.46 rows=59 width=1241) (actual time=9677.052..9677.052 rows=0 loops=1)
   Group Key: user_library_images.id, user_library_images.imgsrc, user_library_images.library_image_id, user_library_images.type, user_library_images.is_user_uploaded, user_library_images.credit, user_library_images.orig_dimensions, user_library_images.account_id
   ->  Sort  (cost=166708.13..166708.28 rows=59 width=1241) (actual time=9677.049..9677.049 rows=0 loops=1)
         Sort Key: user_library_images.id, user_library_images.imgsrc, user_library_images.library_image_id, user_library_images.type, user_library_images.is_user_uploaded, user_library_images.credit, user_library_images.orig_dimensions, user_library_images.account_id
         Sort Method: quicksort  Memory: 17kB
         ->  Hash Join  (cost=10113.22..166706.39 rows=59 width=1241) (actual time=9677.035..9677.035 rows=0 loops=1)
               Hash Cond: (image_tags.user_library_image_id = user_library_images.id)
               ->  Seq Scan on image_tags  (cost=0.00..156488.85 rows=11855 width=4) (actual time=0.301..9592.048 rows=63868 loops=1)
                     Filter: (tag ~~* '%stone%'::text)
                     Rows Removed by Filter: 9370406
               ->  Hash  (cost=10045.97..10045.97 rows=5380 width=1241) (actual time=0.047..0.047 rows=4 loops=1)
                     Buckets: 1024  Batches: 1  Memory Usage: 1kB
                     ->  Bitmap Heap Scan on user_library_images  (cost=288.12..10045.97 rows=5380 width=1241) (actual time=0.027..0.037 rows=4 loops=1)
                           Recheck Cond: (account_id = 769718)
                           Heap Blocks: exact=4
                           ->  Bitmap Index Scan on index_user_library_images_account  (cost=0.00..286.78 rows=5380 width=0) (actual time=0.019..0.019 rows=4 loops=1)
                                 Index Cond: (account_id = 769718)
 Planning time: 0.223 ms
 Execution time: 9677.109 ms
(19 rows)

====

运行分析脚本后(见下面的答案)，问题解决了。作为引用，这是新的 ANALYZE 输出(针对 9.4):

 Group  (cost=2062.82..2062.91 rows=4 width=248) (actual time=8.775..8.801 rows=7 loops=1)
   Group Key: user_library_images.id, user_library_images.imgsrc, user_library_images.library_image_id, user_library_images.type, user_library_images.is_user_uploaded, user_library_images.credit, user_library_images.orig_dimensions, user_library_images.account_id
   ->  Sort  (cost=2062.82..2062.83 rows=4 width=248) (actual time=8.771..8.780 rows=7 loops=1)
         Sort Key: user_library_images.id, user_library_images.imgsrc, user_library_images.library_image_id, user_library_images.type, user_library_images.is_user_uploaded, user_library_images.credit, user_library_images.orig_dimensions, user_library_images.account_id
         Sort Method: quicksort  Memory: 19kB
         ->  Nested Loop  (cost=0.87..2062.78 rows=4 width=248) (actual time=4.156..8.685 rows=7 loops=1)
               ->  Index Scan using index_user_library_images_account on user_library_images  (cost=0.43..469.62 rows=304 width=248) (actual time=0.319..2.528 rows=363 loops=1)
                     Index Cond: (account_id = 769718)
               ->  Index Scan using index_image_tags_user_library_image on image_tags  (cost=0.43..5.23 rows=1 width=4) (actual time=0.014..0.014 rows=0 loops=363)
                     Index Cond: (user_library_image_id = user_library_images.id)
                     Filter: (tag ~~* '%stone%'::text)
                     Rows Removed by Filter: 2
 Planning time: 2.956 ms
 Execution time: 8.907 ms
(14 rows)



Limit  (cost=65.81..65.81 rows=1 width=77) (actual time=0.256..0.256 rows=0 loops=1)
   ->  Sort  (cost=65.81..65.92 rows=47 width=77) (actual time=0.252..0.252 rows=0 loops=1)
         Sort Key: "timestamp"
         Sort Method: quicksort  Memory: 17kB
         ->  Index Scan using index_forum_posts_parent on forum_posts  (cost=0.43..65.57 rows=47 width=77) (actual time=0.211..0.211 rows=0 loops=1)
               Index Cond: (parent = 882269)
 Planning time: 2.978 ms
 Execution time: 0.380 ms
(8 rows)

Answer 1

pg_upgrade 不会复制(或迁移)数据库的统计信息。

因此您需要分析您的表，以便更新迁移数据库中的统计信息。 pg_upgrade 将创建一个名为 analyze_new_cluster 的批处理文件/shell 脚本，可用于此目的。

或者，您可以手动使用 vacuum analyze 来实现相同的目的。

可以通过查看执行计划来检测丢失的统计信息。预期行数与实际行数相差太大:

(cost=0.00..286.78 rows=5380 width=0) (actual time=0.019..0.019 rows=4 loops=1)

==> 5380 与 4 行

或

(cost=0.00..156488.85 rows=11855 width=4) (actual time=0.301..9592.048 rows=63868 loops=1)

==> 11855 与 63868 行