几乎相同查询的非常不同的查询计划

Question

我正在使用 PostgreSQL 10 + pg_trgm 扩展。

表格布局：

    Column    |       Type        | Collation | Nullable |        Default        | Storage  |
--------------+-------------------+-----------+----------+-----------------------+----------+
 id           | integer           |           | not null |                       | plain    | 
 reps         | integer           |           |          | 1                     | plain    | 
 user         | integer           |           |          |                       | plain    | 
 ip           | character varying |           | not null | ''::character varying | extended | 
 visittime    | integer           |           |          |                       | plain    | 
 domain       | character varying |           |          |                       | extended | 
 address      | text              |           |          |                       | extended | 
 method       | character varying |           |          |                       | extended | 
 mime         | character varying |           |          |                       | extended | 
 duration     | integer           |           |          |                       | plain    | 
 size         | bigint            |           |          |                       | plain    | 
 req_status   | integer           |           |          |                       | plain    | 
 http_status  | integer           |           |          |                       | plain    | 
 xproxymeta   | integer           |           |          |                       | plain    | 
Indexes:
    "http_requests_pkey" PRIMARY KEY, btree (id)
    "http_trgm_idx" gin (address gin_trgm_ops)
    "md_userid_idx" btree ("user" DESC)
    "md_visittime_idx" btree (visittime DESC)
Foreign-key constraints:
    "http_requests_user_fkey" FOREIGN KEY ("user") REFERENCES username_id(id)
Triggers:
    add_occupied_space_record_num AFTER INSERT ON http_requests FOR EACH ROW EXECUTE PROCEDURE add_occupied_space_record_num_func()
    count_repeated_records BEFORE INSERT ON http_requests FOR EACH ROW EXECUTE PROCEDURE count_repeated_records_func()
    delete_occupied_space_record_num AFTER DELETE ON http_requests FOR EACH ROW EXECUTE PROCEDURE delete_occupied_space_record_num_func()

请注意，address 列上有一个 GIN Trigram 全文索引。 该表目前有大约 1000 万条记录。

现在这两个相同的查询导致了非常不同的计划。 第一个需要 56 毫秒，第二个需要大约 25 秒。

# explain analyze select * from http_requests where address ilike '%abc%' order by visittime desc limit 10;
                                                                    QUERY PLAN                                                                     
---------------------------------------------------------------------------------------------------------------------------------------------------
 Limit  (cost=0.43..60.93 rows=10 width=209) (actual time=2.862..21.725 rows=10 loops=1)
   ->  Index Scan using md_visittime_idx on http_requests  (cost=0.43..500074.21 rows=82654 width=209) (actual time=2.861..21.719 rows=10 loops=1)
         Filter: (address ~~* '%abc%'::text)
         Rows Removed by Filter: 6663
 Planning time: 0.279 ms
 Execution time: 21.751 ms
(6 rows)

现在是相同的查询，只是搜索模式不同：xyz

# explain analyze select * from http_requests where address ilike '%xyz%' order by visittime desc limit 10;
                                                                  QUERY PLAN                                                                  
----------------------------------------------------------------------------------------------------------------------------------------------
 Limit  (cost=5246.47..5246.50 rows=10 width=209) (actual time=23367.849..23367.870 rows=10 loops=1)
   ->  Sort  (cost=5246.47..5248.52 rows=818 width=209) (actual time=23367.846..23367.848 rows=10 loops=1)
         Sort Key: visittime DESC
         Sort Method: top-N heapsort  Memory: 34kB
         ->  Bitmap Heap Scan on http_requests  (cost=2090.34..5228.79 rows=818 width=209) (actual time=17.202..23352.607 rows=18926 loops=1)
               Recheck Cond: (address ~~* '%xyz%'::text)
               Heap Blocks: exact=18243
               ->  Bitmap Index Scan on http_trgm_idx  (cost=0.00..2090.14 rows=818 width=0) (actual time=12.342..12.342 rows=18926 loops=1)
                     Index Cond: (address ~~* '%xyz%'::text)
 Planning time: 0.190 ms
 Execution time: 23368.164 ms
(11 rows)

为什么计划如此不同，我该如何解决慢查询？

Answer 1

PostgreSQL 知道有很多行包含三元组abc，但很少有行包含三元组xyz。

因此它正确地决定在第一种情况下，使用支持ORDER BY 子句的索引并丢弃与ILIKE 条件不匹配的行，直到找到 10 个结果行。

该策略在第二种情况下效果不佳，因为 PostgreSQL 在找到 10 个结果之前必须搜索表的更大部分，因此它决定使用三元索引并对少数结果进行排序。

不幸的是 PostgreSQL 的估计值已关闭（18926 行而不是估计的 818 行），因此位图堆扫描需要很长时间。

尝试“分析”表格，也许使用更高的default_statistics_target，以获得更好的估计。

如果全部失败，将条件更改为类似

WHERE address || '' ILIKE '%abc%'

防止 PostgreSQL 使用三元组索引。