0

我对状态感到很困惑，我发现查询一个带有“limit 1”的无索引列的where条件的表非常快T，尽管该列没有索引。下面是一个例子：

--1 创建包含 20000000 条数据的测试表

francs=> create table test_limit (id int4,name varchar(32));
CREATE TABLE

francs=> insert into test_limit select generate_series(1,20000000),generate_series(1,20000000) || 'a';
INSERT 0 20000000

francs=> \d test_limit;
         Table "francs.test_limit"
 Column |         Type          | Modifiers 
--------+-----------------------+-----------
 id     | integer               | 
 name   | character varying(32) |

--2 查询表

francs=> explain analyze select * from test_limit where id=1;
                                                 QUERY PLAN                                                  
-------------------------------------------------------------------------------------------------------------
 Seq Scan on test_limit  (cost=0.00..358111.05 rows=1 width=13) (actual time=0.028..3162.477 rows=1 loops=1)
   Filter: (id = 1)
 Total runtime: 3162.531 ms
(3 rows)

请注意，它大约需要 3162 毫秒，但正如我预期的那样非常慢。</p>

--3 带有“限制 1”原因的查询表

francs=> explain analyze select * from test_limit where id=1 limit 1;
                                                   QUERY PLAN                                                   
----------------------------------------------------------------------------------------------------------------
 Limit  (cost=0.00..358111.05 rows=1 width=13) (actual time=0.019..0.019 rows=1 loops=1)
   ->  Seq Scan on test_limit  (cost=0.00..358111.05 rows=1 width=13) (actual time=0.017..0.017 rows=1 loops=1)
         Filter: (id = 1)
 Total runtime: 0.047 ms
(4 rows)

注意它只需要大约 0.047 ms ms，它是如此之快，但列 id 没有索引。任何机构可以解释一下吗？多谢！

--4 附加测试

francs=> explain analyze select * from test_limit where id=2 limit 1;
                                                   QUERY PLAN                                                   
----------------------------------------------------------------------------------------------------------------
 Limit  (cost=0.00..358111.05 rows=1 width=13) (actual time=0.023..0.023 rows=1 loops=1)
   ->  Seq Scan on test_limit  (cost=0.00..358111.05 rows=1 width=13) (actual time=0.022..0.022 rows=1 loops=1)
         Filter: (id = 2)
 Total runtime: 0.066 ms
(4 rows)

francs=> explain analyze select * from test_limit where id=3 limit 1;
                                                   QUERY PLAN                                                   
----------------------------------------------------------------------------------------------------------------
 Limit  (cost=0.00..358111.05 rows=1 width=13) (actual time=0.022..0.022 rows=1 loops=1)
   ->  Seq Scan on test_limit  (cost=0.00..358111.05 rows=1 width=13) (actual time=0.021..0.021 rows=1 loops=1)
         Filter: (id = 3)
 Total runtime: 0.060 ms
(4 rows)

francs=> explain analyze select * from test_limit where id=101 limit 1;
                                                   QUERY PLAN                                                   
----------------------------------------------------------------------------------------------------------------
 Limit  (cost=0.00..358111.05 rows=1 width=13) (actual time=0.035..0.036 rows=1 loops=1)
   ->  Seq Scan on test_limit  (cost=0.00..358111.05 rows=1 width=13) (actual time=0.033..0.033 rows=1 loops=1)
         Filter: (id = 101)
 Total runtime: 0.075 ms
(4 rows)

francs=> explain analyze select * from test_limit where id=1001 limit 1;
                                                   QUERY PLAN                                                   
----------------------------------------------------------------------------------------------------------------
 Limit  (cost=0.00..358111.05 rows=1 width=13) (actual time=0.192..0.192 rows=1 loops=1)
   ->  Seq Scan on test_limit  (cost=0.00..358111.05 rows=1 width=13) (actual time=0.190..0.190 rows=1 loops=1)
         Filter: (id = 1001)
 Total runtime: 0.231 ms
(4 rows)

从加法测试中，我们可以看到它也非常快。

--5 最终测试

francs=> explain analyze select * from test_limit where id=9999999 limit 1;
                                                      QUERY PLAN                                                      
----------------------------------------------------------------------------------------------------------------------
 Limit  (cost=0.00..358111.05 rows=1 width=13) (actual time=1379.153..1379.154 rows=1 loops=1)
   ->  Seq Scan on test_limit  (cost=0.00..358111.05 rows=1 width=13) (actual time=1379.151..1379.151 rows=1 loops=1)
         Filter: (id = 9999999)
 Total runtime: 1379.206 ms
(4 rows)

从上面，我使用了后来的 id 是 9999999 ，现在很慢；我现在明白了，谢谢！

4

1 回答 1

3

也许“id = 1”在表中很早，所以当它按顺序读取表时，它会很快到达该行，并且由于你说“limit = 1”它可以在第一个结果之后停止。

或者，也可能涉及一些缓存。

于 2012-09-11T07:51:03.910 回答