理解 PostgreSQL 的 count 函数的行为

关于 count 函数的使用一直存在争议，尤其是在 MySQL 中，作为流行度越来越高的 PostgreSQL 是否也有类似的问题呢，我们通过实践来理解一下 PostgreSQL 中 count 函数的行为。

构建测试数据库

创建测试数据库，并创建测试表。测试表中有自增 ID、创建时间、内容三个字段，自增 ID 字段是主键。

create database performance_test;

create table test_tbl (id serial primary key, created_at timestamp, content varchar(512));

生成测试数据

使用 generate_series 函数生成自增 ID，使用 now() 函数生成 created_at 列，对于 content 列，使用了 repeat(md5(random()::text), 10) 生成 10 个 32 位长度的 md5 字符串。使用下列语句，插入 1000w 条记录用于测试。

performance_test=# insert into test_tbl select generate_series(1,10000000),now(),repeat(md5(random()::text),10);
INSERT 0 10000000
Time: 212184.223 ms (03:32.184)

由 count 语句引发的思考

默认情况下 PostgreSQL 不开启 SQL 执行时间的显示，所以需要手动开启一下，方便后面的测试对比。

\timing on

count(*) 和 count(1) 的性能区别是经常被讨论的问题，分别使用 count(*) 和 count(1) 执行一次查询。

performance_test=# select count(*) from test_tbl;
  count
----------
 10000000
(1 row)

Time: 115090.380 ms (01:55.090)

performance_test=# select count(1) from test_tbl;
  count
----------
 10000000
(1 row)

Time: 738.502 ms

可以看到两次查询的速度差别非常大，count(1) 真的有这么大的性能提升？接下来再次运行查询语句。

performance_test=# select count(*) from test_tbl;
  count
----------
 10000000
(1 row)

Time: 657.831 ms

performance_test=# select count(1) from test_tbl;
  count
----------
 10000000
(1 row)

Time: 682.157 ms

可以看到第一次查询时候会非常的慢，后面三次速度非常快并且时间相近，这里就有两个问题出现了：

• 为什么第一次查询速度这么慢？
• count(*) 和 count(1) 到底存不存在性能差别？

查询缓存

使用 explain 语句重新执行查询语句

explain (analyze,buffers,verbose) select count(*) from test_tbl;

可以看到如下输出：

 Finalize Aggregate  (cost=529273.69..529273.70 rows=1 width=8) (actual time=882.569..882.570 rows=1 loops=1)
   Output: count(*)
   Buffers: shared hit=96 read=476095
   ->  Gather  (cost=529273.48..529273.69 rows=2 width=8) (actual time=882.492..884.170 rows=3 loops=1)
         Output: (PARTIAL count(*))
         Workers Planned: 2
         Workers Launched: 2
         Buffers: shared hit=96 read=476095
         ->  Partial Aggregate  (cost=528273.48..528273.49 rows=1 width=8) (actual time=881.014..881.014 rows=1 loops=3)
               Output: PARTIAL count(*)
               Buffers: shared hit=96 read=476095
               Worker 0: actual time=880.319..880.319 rows=1 loops=1
                 Buffers: shared hit=34 read=158206
               Worker 1: actual time=880.369..880.369 rows=1 loops=1
                 Buffers: shared hit=29 read=156424
               ->  Parallel Seq Scan on public.test_tbl  (cost=0.00..517856.98 rows=4166598 width=0) (actual time=0.029..662.165 rows=3333333 loops=3)
                     Buffers: shared hit=96 read=476095
                     Worker 0: actual time=0.026..661.807 rows=3323029 loops=1
                       Buffers: shared hit=34 read=158206
                     Worker 1: actual time=0.030..660.197 rows=3285513 loops=1
                       Buffers: shared hit=29 read=156424
 Planning time: 0.043 ms
 Execution time: 884.207 ms

注意里面的 shared hit，表示命中了内存中缓存的数据，这就可以解释为什么后面的查询会比第一次快很多。接下来去掉缓存，并重启 PostgreSQL。

service postgresql stop
echo 1 > /proc/sys/vm/drop_caches
service postgresql start

重新执行 SQL 语句，速度慢了很多。

 Finalize Aggregate  (cost=529273.69..529273.70 rows=1 width=8) (actual time=50604.564..50604.564 rows=1 loops=1)
   Output: count(*)
   Buffers: shared read=476191
   ->  Gather  (cost=529273.48..529273.69 rows=2 width=8) (actual time=50604.508..50606.141 rows=3 loops=1)
         Output: (PARTIAL count(*))
         Workers Planned: 2
         Workers Launched: 2
         Buffers: shared read=476191
         ->  Partial Aggregate  (cost=528273.48..528273.49 rows=1 width=8) (actual time=50591.550..50591.551 rows=1 loops=3)
               Output: PARTIAL count(*)
               Buffers: shared read=476191
               Worker 0: actual time=50585.182..50585.182 rows=1 loops=1
                 Buffers: shared read=158122
               Worker 1: actual time=50585.181..50585.181 rows=1 loops=1
                 Buffers: shared read=161123
               ->  Parallel Seq Scan on public.test_tbl  (cost=0.00..517856.98 rows=4166598 width=0) (actual time=92.491..50369.691 rows=3333333 loops=3)
                     Buffers: shared read=476191
                     Worker 0: actual time=122.170..50362.271 rows=3320562 loops=1
                       Buffers: shared read=158122
                     Worker 1: actual time=14.020..50359.733 rows=3383583 loops=1
                       Buffers: shared read=161123
 Planning time: 11.537 ms
 Execution time: 50606.215 ms

shared read 表示没有命中缓存，通过这个现象可以推断出，上一小节的四次查询中，第一次查询没有命中缓存，剩下三次查询都命中了缓存。

count(1) 和 count(*) 的区别

接下来探究 count(1) 和 count(*) 的区别是什么，继续思考最开始的四次查询，第一次查询使用了 count(*)，第二次查询使用了 count(1) ，却依然命中了缓存，不正是说明 count(1) 和 count(*) 是一样的吗？

事实上，PostgreSQL 官方对于 is there a difference performance-wise between select count(1) and select count(*)? 问题的回复也证实了这一点：

Nope. In fact, the latter is converted to the former during parsing.[2]

既然 count(1) 在性能上没有比 count(*) 更好，那么使用 count(*) 就是更好的选择。

sequence scan 和 index scan

接下来测试一下，在不同数据量大小的情况下 count(*) 的速度，将查询语句写在 count.sql 文件中，使用 pgbench 进行测试。

pgbench -c 5 -t 20 performance_test -r -f count.sql

分别测试 200w - 1000w 数据量下的 count 语句耗时

数据大小	count耗时(ms)
200w	738.758
300w	1035.846
400w	1426.183
500w	1799.866
600w	2117.247
700w	2514.691
800w	2526.441
900w	2568.240
1000w	2650.434

绘制成耗时曲线

曲线的趋势在 600w - 700w 数据量之间出现了转折，200w - 600w 是线性增长，600w 之后 count 的耗时就基本相同了。使用 explain 语句分别查看 600w 和 700w 数据时的 count 语句执行。

700w：

 Finalize Aggregate  (cost=502185.93..502185.94 rows=1 width=8) (actual time=894.361..894.361 rows=1 loops=1)
   Output: count(*)
   Buffers: shared hit=16344 read=352463
   ->  Gather  (cost=502185.72..502185.93 rows=2 width=8) (actual time=894.232..899.763 rows=3 loops=1)
         Output: (PARTIAL count(*))
         Workers Planned: 2
         Workers Launched: 2
         Buffers: shared hit=16344 read=352463
         ->  Partial Aggregate  (cost=501185.72..501185.73 rows=1 width=8) (actual time=889.371..889.371 rows=1 loops=3)
               Output: PARTIAL count(*)
               Buffers: shared hit=16344 read=352463
               Worker 0: actual time=887.112..887.112 rows=1 loops=1
                 Buffers: shared hit=5459 read=118070
               Worker 1: actual time=887.120..887.120 rows=1 loops=1
                 Buffers: shared hit=5601 read=117051
               ->  Parallel Index Only Scan using test_tbl_pkey on public.test_tbl  (cost=0.43..493863.32 rows=2928960 width=0) (actual time=0.112..736.376 rows=2333333 loops=3)
                     Index Cond: (test_tbl.id < 7000000)
                     Heap Fetches: 2328492
                     Buffers: shared hit=16344 read=352463
                     Worker 0: actual time=0.107..737.180 rows=2344479 loops=1
                       Buffers: shared hit=5459 read=118070
                     Worker 1: actual time=0.133..737.960 rows=2327028 loops=1
                       Buffers: shared hit=5601 read=117051
 Planning time: 0.165 ms
 Execution time: 899.857 ms

600w：

 Finalize Aggregate  (cost=429990.94..429990.95 rows=1 width=8) (actual time=765.575..765.575 rows=1 loops=1)
   Output: count(*)
   Buffers: shared hit=13999 read=302112
   ->  Gather  (cost=429990.72..429990.93 rows=2 width=8) (actual time=765.557..770.889 rows=3 loops=1)
         Output: (PARTIAL count(*))
         Workers Planned: 2
         Workers Launched: 2
         Buffers: shared hit=13999 read=302112
         ->  Partial Aggregate  (cost=428990.72..428990.73 rows=1 width=8) (actual time=763.821..763.821 rows=1 loops=3)
               Output: PARTIAL count(*)
               Buffers: shared hit=13999 read=302112
               Worker 0: actual time=762.742..762.742 rows=1 loops=1
                 Buffers: shared hit=4638 read=98875
               Worker 1: actual time=763.308..763.308 rows=1 loops=1
                 Buffers: shared hit=4696 read=101570
               ->  Parallel Index Only Scan using test_tbl_pkey on public.test_tbl  (cost=0.43..422723.16 rows=2507026 width=0) (actual time=0.053..632.199 rows=2000000 loops=3)
                     Index Cond: (test_tbl.id < 6000000)
                     Heap Fetches: 2018490
                     Buffers: shared hit=13999 read=302112
                     Worker 0: actual time=0.059..633.156 rows=1964483 loops=1
                       Buffers: shared hit=4638 read=98875
                     Worker 1: actual time=0.038..634.271 rows=2017026 loops=1
                       Buffers: shared hit=4696 read=101570
 Planning time: 0.055 ms
 Execution time: 770.921 ms

根据以上现象推断，PostgreSQL 似乎在 count 的数据量小于数据表长度的某一比例时，才使用 index scan，通过查看官方 wiki 也可以看到相关描述：

It is important to realise that the planner is concerned with minimising the total cost of the query. With databases, the cost of I/O typically dominates. For that reason, "count(*) without any predicate" queries will only use an index-only scan if the index is significantly smaller than its table. This typically only happens when the table's row width is much wider than some indexes'.[3]

根据 Stackoverflow 上的回答，count 语句查询的数量大于表大小的 3/4 时候就会用使用全表扫描代替索引扫描[4]。

结论

1. 不要用 count(1) 或 count(列名) 代替 count(*)
2. count 本身是非常耗时的
3. count 可能是 index scan 也可能是 sequence scan，取决于 count 数量占表大小的比例

参考资料

[1] 深入理解Postgres中的cache

[2] Re: performance difference in count(1) vs. count(*)?

[3] Is "count(*)" much faster now?

[4] PostgreSQL not using index during count(*)