Why does a \timing always show faster times when run from within a script than on the terminal?

Question

Let's say I have a very simple command like

SELECT count(*) FROM ( SELECT generate_series(1,100000) ) AS t;

If I open up a terminal and I run \timing and then the above query, I'll consitantly get massively slower times then if I run

$ psql --command '\timing' --command 'SELECT count(*) FROM ( SELECT generate_series(1,100000) ) AS t;'

It's not even close. Is there any explanation of this behavior and variance in \timing

Output of Benchmark

When I run it in one go, I never get more than 5 ms

$ psql --command '\timing' --command 'SELECT count(*) FROM ( SELECT generate_series(1,100000) ) AS t;'

Timing is on.
 count  
--------
 100000
(1 row)

Time: 4.497 ms

$ psql --command '\timing' --command 'SELECT count(*) FROM ( SELECT generate_series(1,100000) ) AS t;'

Timing is on.
 count  
--------
 100000
(1 row)

Time: 4.437 ms

With the copy-paste into terminal I'm showing sometimes close to 20ms.

=# \timing
Timing is on.
ecarroll=# SELECT count(*) FROM ( SELECT generate_series(1,100000) ) AS t;
 count  
--------
 100000
(1 row)

Time: 9.500 ms

=# SELECT count(*) FROM ( SELECT generate_series(1,100000) ) AS t;
 count  
--------
 100000
(1 row)

Time: 19.017 ms

It doesn't seem to matter if I use Unix Domains Sockets or TCP.

Psql version

psql (PostgreSQL) 13.4 (Debian 13.4-0+deb11u1)

Answer 1

(Not enough space to write a comment, so posting as an answer instead -- I'll change it when I hear back from the author)

Can you provide some more details about the "massively slower times" when using psql? What version are you using? In my tests, they look similar (3x in each method):

edb=# SELECT count(*) FROM ( SELECT generate_series(1,100000) ) AS t;
 count  
--------
 100000
(1 row)

Time: 13.328 ms
edb=# SELECT count(*) FROM ( SELECT generate_series(1,100000) ) AS t;
 count  
--------
 100000
(1 row)

Time: 12.400 ms
edb=# SELECT count(*) FROM ( SELECT generate_series(1,100000) ) AS t;
 count  
--------
 100000
(1 row)

Time: 12.242 ms
edb=# \q
[root@mdn /]# psql --command '\timing' --command 'SELECT count(*) FROM ( SELECT generate_series(1,100000) ) AS t;'
Timing is on.
 count  
--------
 100000
(1 row)

Time: 13.369 ms
[root@mdn /]# psql --command '\timing' --command 'SELECT count(*) FROM ( SELECT generate_series(1,100000) ) AS t;'
Timing is on.
 count  
--------
 100000
(1 row)

Time: 13.325 ms
[root@mdn /]# psql --command '\timing' --command 'SELECT count(*) FROM ( SELECT generate_series(1,100000) ) AS t;'
Timing is on.
 count  
--------
 100000
(1 row)

Time: 13.257 ms