sql - B树索引好像没有用？

Question

我是 Postgres 的新手，我想更多地了解索引。我使用的是 12.5 版，这是我的代码:

CREATE TABLE textfun(content TEXT);
CREATE UNIQUE INDEX text_b ON textfun(content);

INSERT INTO textfun (content)
SELECT (CASE WHEN (random()<=0.3) THEN 'https://mywebsite/nanana/'
WHEN (random()<=0.6) THEN 'https://mywebsite/friendy/'
ELSE 'https://mywebsite/mina/' END) || generate_series(1000000,2000000);

这里，我创建了一百万条记录，希望看到索引的效果。

当我尝试获取查询计划时:

explain analyze
SELECT content FROM textfun WHERE content LIKE 'mina%'; 

我得到了这个:

Gather  (cost=1000.00..14300.34 rows=100 width=32) (actual time=77.574..80.054 rows=0 loops=1)
  Workers Planned: 2    
  Workers Launched: 2
  Parallel Seq Scan on textfun  (cost=0.00..13290.34 rows=42 width=32) (actual time=69.022..69.022 rows=0 loops=3)
     Filter: (content ~~ 'mina%'::text)
     Rows Removed by Filter: 333334  
Planning Time: 0.254 ms  
Execution Time: 80.071 ms 
(8 rows)

我期待并行索引扫描。

我试过:

explain analyze
SELECT content FROM textfun WHERE content LIKE '1500000%';

和:

explain analyze
SELECT content FROM textfun WHERE content LIKE '%mina';

但两者都给我一个顺序扫描计划。

我是否缺少此处的详细信息，为什么我没有进行索引扫描？

Answer 1

要支持 LIKE 条件，您需要使用 text_pattern_ops 创建索引

CREATE UNIQUE INDEX text_b ON textfun(content text_pattern_ops);

这样，结果就是下面的执行计划:

Bitmap Heap Scan on textfun  (cost=191.68..7654.53 rows=5000 width=32) (actual time=2.553..2.554 rows=0 loops=1)
  Filter: (content ~~ '1500000%'::text)
  ->  Bitmap Index Scan on text_b  (cost=0.00..190.43 rows=5000 width=0) (actual time=2.550..2.550 rows=0 loops=1)
        Index Cond: ((content ~>=~ '1500000'::text) AND (content ~<~ '1500001'::text))
Planning Time: 6.247 ms
Execution Time: 6.809 ms

Online example