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JSONB索引比本机索引慢吗?

postgresql-9.6 postgresql

p_mcp · 技术社区 · 6 年前

我有一张大桌子(30米排),有大约10个座位 jsonb B树索引。

当我添加更多的条件,特别是一个稀疏 jsonb公司

我想知道 jsonb公司 索引比本机索引慢?我是否希望通过切换到本机列而不是JSON来提高性能?

表定义:

id  integer 
type    text    
data    jsonb   
company_index   ARRAY   
exchange_index  ARRAY   
eligible boolean

查询示例:

SELECT id, data, type 
FROM collection.bundles    
WHERE ( (ARRAY['.X'] && bundles.exchange_index)  AND   
type IN ('discussion') AND  
( ((data->>'sentiment_score')::bigint > 0 AND 
(data->'display_tweet'->'stocktwit'->'id') IS NOT NULL) )  AND  
(  eligible = true  )  AND  
((data->'display_tweet'->'stocktwit')->>'id')::bigint IS NULL )  
ORDER BY id DESC   
LIMIT 50

输出:

Limit  (cost=0.56..16197.56 rows=50 width=212) (actual time=31900.874..31900.874 rows=0 loops=1)
  Buffers: shared hit=13713180 read=1267819 dirtied=34 written=713
  I/O Timings: read=7644.206 write=7.294
  ->  Index Scan using bundles2_id_desc_idx on bundles  (cost=0.56..2401044.17 rows=7412 width=212) (actual time=31900.871..31900.871 rows=0 loops=1)
        Filter: (eligible AND ('{.X}'::text[] && exchange_index) AND (type = 'discussion'::text) AND ((((data -> 'display_tweet'::text) -> 'stocktwit'::text) -> 'id'::text) IS NOT NULL) AND (((data ->> 'sentiment_score'::text))::bigint > 0) AND (((((data -> 'display_tweet'::text) -> 'stocktwit'::text) ->> 'id'::text))::bigint IS NULL))
        Rows Removed by Filter: 16093269
        Buffers: shared hit=13713180 read=1267819 dirtied=34 written=713
        I/O Timings: read=7644.206 write=7.294
Planning time: 0.366 ms
Execution time: 31900.909 ms

注: jsonb公司 jsonb公司 此查询中使用的条件。 exchange_index 和 company_index 有杜松子酒索引。

更新在Laurenz更改查询之后:

Limit  (cost=150634.15..150634.27 rows=50 width=211) (actual time=15925.828..15925.828 rows=0 loops=1)
  Buffers: shared hit=1137490 read=680349 written=2
  I/O Timings: read=2896.702 write=0.038
  ->  Sort  (cost=150634.15..150652.53 rows=7352 width=211) (actual time=15925.827..15925.827 rows=0 loops=1)
        Sort Key: bundles.id DESC
        Sort Method: quicksort  Memory: 25kB
        Buffers: shared hit=1137490 read=680349 written=2
        I/O Timings: read=2896.702 write=0.038
        ->  Bitmap Heap Scan on bundles  (cost=56666.15..150316.40 rows=7352 width=211) (actual time=15925.816..15925.816 rows=0 loops=1)
              Recheck Cond: (('{.X}'::text[] && exchange_index) AND (type = 'discussion'::text))
              Filter: (eligible AND ((((data -> 'display_tweet'::text) -> 'stocktwit'::text) -> 'id'::text) IS NOT NULL) AND (((data ->> 'sentiment_score'::text))::bigint > 0) AND (((((data -> 'display_tweet'::text) -> 'stocktwit'::text) ->> 'id'::text))::bigint IS NULL))
              Rows Removed by Filter: 273230
              Heap Blocks: exact=175975
              Buffers: shared hit=1137490 read=680349 written=2
              I/O Timings: read=2896.702 write=0.038
              ->  BitmapAnd  (cost=56666.15..56666.15 rows=23817 width=0) (actual time=1895.890..1895.890 rows=0 loops=1)
                    Buffers: shared hit=37488 read=85559
                    I/O Timings: read=325.535
                    ->  Bitmap Index Scan on bundles2_exchange_index_ops_idx  (cost=0.00..6515.57 rows=863703 width=0) (actual time=218.690..218.690 rows=892669 loops=1)
                          Index Cond: ('{.X}'::text[] && exchange_index)
                          Buffers: shared hit=7 read=313
                          I/O Timings: read=1.458
                    ->  Bitmap Index Scan on bundles_eligible_idx  (cost=0.00..23561.74 rows=2476877 width=0) (actual time=436.719..436.719 rows=2569331 loops=1)
                          Index Cond: (eligible = true)
                          Buffers: shared hit=37473
                    ->  Bitmap Index Scan on bundles2_type_idx  (cost=0.00..26582.83 rows=2706276 width=0) (actual time=1052.267..1052.267 rows=2794517 loops=1)
                          Index Cond: (type = 'discussion'::text)
                          Buffers: shared hit=8 read=85246
                          I/O Timings: read=324.077
Planning time: 0.433 ms
Execution time: 15928.959 ms

1 回复 | 直到 6 年前

Laurenz Albe 6 年前

所有你喜欢的索引都没有被使用,所以问题不在于它们是否快速。

这里有几件事在起作用:

看到 dirtied 以及 written 在索引扫描期间,我怀疑表中有很多“死元组”。当索引扫描访问它们并注意到它们已死亡时,它会“杀死”这些索引项,以便后续的索引扫描不必重复该工作。

你可以通过跑步来减少这个问题 VACUUM 放在桌子上,或确保自动吸尘器经常处理桌子。
然而,你的主要问题是 LIMIT

不幸的是,它必须扫描16093319行,直到找到50个点击。“高处”的一排 id “表尾不符合条件。PostgreSQL不知道这种相关性。

解决方案是阻止PostgreSQL走这条路。最简单的方法是删除但鉴于它的名字,这可能是不可行的。

另一种方法是防止PostgreSQL“看到” 限制 条款计划扫描时:
```
SELECT id, data, type
FROM (SELECT id, data, type
      FROM collection.bundles
      WHERE /* all your complicated conditions */
      OFFSET 0) subquery
ORDER BY id DESC
LIMIT 50;
```

备注: 你没有显示你的索引定义,但是听起来你有很多,可能太多了。索引是昂贵的,所以请确保只定义那些给您带来明显好处的索引。