explain 真实的跑了一个select后的计划如下： 

Aggregate (cost=20.75..20.76 rows=1 width=8) (actual time=0.020..0.020 rows=1 loops=1)
Output: count(*)
Buffers: shared hit=1
-> Seq Scan on alvin.t1 (cost=0.00..18.60 rows=860 width=0) (actual time=0.009..0.010 rows=6 loops
=1)
Output: id, name, age
Buffers: shared hit=1
Planning time: 15.457 ms
Execution time: 0.090 ms

为啥Execution time比Planning time 还小？？