PostgreSQL format lag column

Question

This query works great for finding the difference between successive rows:

select id, created_at, created_at - lag(created_at, 1) 
over (order by created_at) as diff
from fistbumps where bumper_id = 2543 
and created_at between '2012-01-11' and '2012-01-12' order by created_at;

...but the results come out like:

   id   |         created_at         |      diff       
--------+----------------------------+-----------------
 197230 | 2012-01-11 00:04:31.774426 | 
 197231 | 2012-01-11 00:04:32.279181 | 00:00:00.504755
 197232 | 2012-01-11 00:04:33.961665 | 00:00:01.682484
 197233 | 2012-01-11 00:04:36.506685 | 00:00:02.54502

What would be really groovy is if I could format that diff column to just seconds and millis (e.g. 2.54502). I tried using date_trunc() and extract(), but I can't seem to get the syntax right.

Answer 1

The result of created_at - lag(create_at) is a value of type interval.

You can get the seconds of an interval using extract(epoch from interval_value)

So in your case it would be:

extract(epoch from (created_at - lag(created_at, 1)) )