Question

在排序操作中，主要的瓶颈是磁盘I / O，即从磁盘中获取所有数据。

所以我想用Postgres的IMCS和内存中的柱状插件进行实验。

以下是常规查询计划。（我没有包含用于学习目的的索引）

perf_test=# explain (analyze,buffers) select * from users_act_demo order by userid limit 10;
                                                                QUERY PLAN
-------------------------------------------------------------------------------------------------------------------------------------------
Limit  (cost=410011.04..410011.06 rows=10 width=14) (actual time=4328.393..4328.396 rows=10 loops=1)
   Buffers: shared hit=3 read=59875
   ->  Sort  (cost=410011.04..437703.22 rows=11076875 width=14) (actual time=4328.387..4328.389 rows=10 loops=1)
         Sort Key: userid
         Sort Method: top-N heapsort  Memory: 25kB
         Buffers: shared hit=3 read=59875
         ->  Seq Scan on users_act_demo  (cost=0.00..170643.75 rows=11076875 width=14) (actual time=0.042..1938.043 rows=11076719 loops=1)
               Buffers: shared read=59875
Planning time: 42.055 ms
Execution time: 4328.482 ms
(10 rows)

执行cs_load后，这是一个将所有页面移动到内存中的函数。之后，下面是查询计划。

perf_test=# explain (analyze,buffers) select * from users_act_demo order by userid limit 10;
                                                                QUERY PLAN
-------------------------------------------------------------------------------------------------------------------------------------------
Limit  (cost=410011.04..410011.06 rows=10 width=14) (actual time=3902.765..3902.767 rows=10 loops=1)
   Buffers: shared hit=59875
   ->  Sort  (cost=410011.04..437703.22 rows=11076875 width=14) (actual time=3902.763..3902.763 rows=10 loops=1)
         Sort Key: userid
         Sort Method: top-N heapsort  Memory: 25kB
         Buffers: shared hit=59875
         ->  Seq Scan on users_act_demo  (cost=0.00..170643.75 rows=11076875 width=14) (actual time=0.028..1520.772 rows=11076719 loops=1)
               Buffers: shared hit=59875
Planning time: 0.145 ms
Execution time: 3902.827 ms

我的问题是，为什么排序不快？如果所有数据都不受I / O限制，那么它是不是受CPU限制？为什么它变慢了，这里的成本到底是什么？

Postgres完全从内存中排序似乎是CPU绑定的？

0 个答案: