Postgres：为什么添加索引会减慢正则表达式查询的速度？

Question

我在 Postgres 中有一个 TEXT keyvalues 列：

select * from test5 limit 5;

 id |                      keyvalues
----+------------------------------------------------------
  1 | ^ first 1 | second 3
  2 | ^ first 1 | second 2 ^ first 2 | second 3
  3 | ^ first 1 | second 2 | second 3
  4 | ^ first 2 | second 3 ^ first 1 | second 2 | second 2
  5 | ^ first 2 | second 3 ^ first 1 | second 3

我的查询必须从匹配中间排除 ^ 字符，所以我使用的是正则表达式：

explain analyze select count(*) from test5 where keyvalues ~* '\^ first 1[^\^]+second 0';

                                                              QUERY PLAN
--------------------------------------------------------------------------------------------------------------------------------------
 Finalize Aggregate  (cost=78383.31..78383.32 rows=1 width=8) (actual time=7332.030..7332.030 rows=1 loops=1)
   ->  Gather  (cost=78383.10..78383.30 rows=2 width=8) (actual time=7332.021..7337.138 rows=3 loops=1)
         Workers Planned: 2
         Workers Launched: 2
         ->  Partial Aggregate  (cost=77383.10..77383.10 rows=1 width=8) (actual time=7328.155..7328.156 rows=1 loops=3)
               ->  Parallel Seq Scan on test5  (cost=0.00..77382.50 rows=238 width=0) (actual time=7328.146..7328.146 rows=0 loops=3)
                     Filter: (keyvalues ~* '\^ first 1[^\^]+second 0'::text)
                     Rows Removed by Filter: 1666668
 Planning Time: 0.068 ms
 Execution Time: 7337.184 ms

查询有效（零行匹配），但速度太慢，超过 7 秒。

我认为用八卦索引会有帮助，但运气不好:

create extension if not exists pg_trgm;
create index on test5 using gin (keyvalues gin_trgm_ops);

explain analyze select count(*) from test5 where keyvalues ~* '\^ first 1[^\^]+second 0';
                                                                   QUERY PLAN
-------------------------------------------------------------------------------------------------------------------------------------------------
 Aggregate  (cost=1484.02..1484.03 rows=1 width=8) (actual time=23734.646..23734.646 rows=1 loops=1)
   ->  Bitmap Heap Scan on test5  (cost=1480.00..1484.01 rows=1 width=0) (actual time=23734.641..23734.641 rows=0 loops=1)
         Recheck Cond: (keyvalues ~* '\^ first 1[^\^]+second 0'::text)
         Rows Removed by Index Recheck: 5000005
         Heap Blocks: exact=47620
         ->  Bitmap Index Scan on test5_keyvalues_idx  (cost=0.00..1480.00 rows=1 width=0) (actual time=1756.158..1756.158 rows=5000005 loops=1)
               Index Cond: (keyvalues ~* '\^ first 1[^\^]+second 0'::text)
 Planning Time: 0.412 ms
 Execution Time: 23734.722 ms

使用三元组索引的查询慢了 3 倍！它仍然是 returns 正确的结果（零行）。我希望三字母索引能够立即找出任何地方都没有 second 0 字符串，并且速度非常快。

（动机：我想避免将 keyvalues 规范化为 , so I'm looking to encode the matching logic in a single TEXT field using text indexing and regexps instead. The logic works, but is too slow, 。）

Answer 1

如您所见，这不适用于八卦。 Trigrams 不匹配 space 边界，因此如果您的所有数据都包含相同的词，则索引将匹配每一行。

这可能会让事情更清楚：

with data as (select * from (values ('^ first 1 | second 3'), 
                                    ('^ first 1 | second 2 ^ first 2 | second 3'), 
                                    ('^ first 1 | second 2 | second 3'), 
                                    ('^ first 2 | second 3 ^ first 1 | second 2 | second 2'), 
                                    ('^ first 2 | second 3 ^ first 1 | second 3')
                             ) v(keyvalues)
)
select keyvalues, show_trgm(keyvalues) from data;
                      keyvalues                       |                                               show_trgm
------------------------------------------------------+-------------------------------------------------------------------------------------------------------
 ^ first 1 | second 3                                 | {"  1","  3","  f","  s"," 1 "," 3 "," fi"," se",con,eco,fir,irs,"nd ",ond,rst,sec,"st "}
 ^ first 1 | second 2 ^ first 2 | second 3            | {"  1","  2","  3","  f","  s"," 1 "," 2 "," 3 "," fi"," se",con,eco,fir,irs,"nd ",ond,rst,sec,"st "}
 ^ first 1 | second 2 | second 3                      | {"  1","  2","  3","  f","  s"," 1 "," 2 "," 3 "," fi"," se",con,eco,fir,irs,"nd ",ond,rst,sec,"st "}
 ^ first 2 | second 3 ^ first 1 | second 2 | second 2 | {"  1","  2","  3","  f","  s"," 1 "," 2 "," 3 "," fi"," se",con,eco,fir,irs,"nd ",ond,rst,sec,"st "}
 ^ first 2 | second 3 ^ first 1 | second 3            | {"  1","  2","  3","  f","  s"," 1 "," 2 "," 3 "," fi"," se",con,eco,fir,irs,"nd ",ond,rst,sec,"st "}

能否使用部分索引来排除中间带有 ^ 的行？

Answer 2

根据 OP，用户@jjanes 在 here 上 DBA.SE 给出了正确答案：

I expected the trigram index to figure out immediately there's no second 0 string anywhere

'second' 和 '0' 是单独的词，因此它无法检测到它们的联合缺失。它似乎可以检测到“0”的缺失，但来自 "contrib/pg_trgm/trgm_regexp.c" 的评论似乎是相关的：
     * Note: Using again the example "foo bar", we will not consider the
     * trigram "  b", though this trigram would be found by the trigram
     * extraction code.  Since we will find " ba", it doesn't seem worth
     * trying to hack the algorithm to generate the additional trigram.
由于0是模式串的最后一个字符，所以也不会出现"0a"形式的卦，所以它就错过了这个机会。

即使不是这个限制，你的方法也显得非常脆弱。

Postgres：为什么添加索引会减慢正则表达式查询的速度？

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