为什么 Ruby sequel 和 Postgres psql 的时区信息不同？

Question

Here is a query result from Postgres:

$ psql ... -c 'select the_date from foo where foo_id in (998,999)'
     the_date      
------------------------
 2012-03-07 09:34:47.98
 2012-03-16 11:31:25.336

the_date is "timestamp without time zone".

Here is a Ruby program:

#!/usr/bin/env ruby

require 'sequel'

@DB = Sequel.connect({...})
query = "select the_date from foo where foo_id in (998,999)"
@DB[query].each do |row|
  warn row
end

and some output:

{:the_date=>2012-03-07 09:34:47 -0600}
{:the_date=>2012-03-16 11:31:25 -0500}

Where does the -0500 and -0600 come from? That is the " Olson timezone " of the server and the client machines (US/Central), but why does Ruby add it and psql does not?

I've been reading the docs , and I'm thoroughly confused.

The server is Postgres 9.0.4, the client is psql 9.1.4, sequel is 3.33.0.

Answer 1

The column is of type 'timestamp without timezone'. Thus when Postgres displays a value in this column it just displays the timestamp with no timezone. However, Sequel wants to convert a Postgres timestamp to an instance of the Ruby Time class, and an instance of the Time class must have a timezone specified - either it's a time in the local timezone or it's a time in UTC. Thus Sequel must choose one. By default, it's choosing your local timezone.

You may configure the database and application timezone in Sequel. See https://www.rubydoc.info/gems/sequel/4.36.0/Sequel/Timezones

For example, here's the default Sequel behavior with a database I had handy:

>   c['select * from actors'].each do |row|; puts row[:created_at]; end
Thu Jul 12 20:33:17 -0400 2012

Here the timestamp is assumed to be in my local timezone (EDT).

However, if I do this:

> Sequel.database_timezone = :utc
 => :utc 
> c['select * from actors'].each do |row|; puts row[:created_at]; end
Thu Jul 12 20:33:17 UTC 2012

Then the timestamp is assumed to be in UTC.

Answer 2

I've had some strange dealings with ruby and dates/times/timestamps. It looks like the program is trying to convert the result into a known datatype and automatically casting it, which is why you receive the 5digit timezone code at the end.