Query datetime as text and include different timezones - sql

Question

Short version:
I have 2 users in DB, each created in a different timezone:
User.find(1).created_at
=> Thu, 04 Aug 2016 11:15:29 IDT +03:00
User.find(33).created_at
=> Sun, 01 Jan 2017 17:50:20 IST +02:00
So my table shows 11:15, and 17:50. so for example I would like to search for 17:50, and later 11:15 as text:
search_param = '17:50'
No problem I'll just convert the date to text, then search it, But user won't be found, since the it's saved as UTC:
User.where("to_char(created_at,'DD-Mon-YYYY HH24:MI:SS') ilike ?", "%#{search_param}%").first
=> nil
To find it I'll just apply the offset to my query (adding time zone UTC+2), and indeed user was found:
User.where("to_char(created_at AT TIME ZONE 'UTC+2','DD-Mon-YYYY HH24:MI:SS') ilike ?", "%#{search_param}%").first
=> User #33 #2017-01-01 17:50:20 +0200
BUT some users are saved as UTC+3 and some as UTC+2.. I can't apply both offsets... So if I change search_param to be 11:15 I won't find user_id_1 because I will also need to change UTC+2 to UTC+3
When I use UTC+2 I will only find users which were created as +2 (like User.find(33)).
When I use UTC+3 I will only find users which were created as +3 (like User.find(1)).
My question: How to do a where query- a text search for both users' created_at hour, as they were both saved in a different timezone offset?
Or in this example a query that for a search_param 17:50 will find user_id_33 and for search_param 11:15 will find user_id_1?
More Details:
I notice in DB they are saved as UTC (I think):
User.select("created_at as created_at_db").find(33).created_at_db
=> "2017-01-01 15:50:20.903289"
User.select("created_at as created_at_db").find(1).created_at_db
=> "2016-08-04 08:15:29.171776"
Time setting:
#application.rb
config.time_zone = 'Jerusalem'
config.active_record.default_timezone = :utc
created_at column info:
User.columns.select{|table_col| table_col.name == 'created_at'}
=> [#<ActiveRecord::ConnectionAdapters::PostgreSQLColumn:0x81f9c48
#coder=nil,
#default=nil,
#limit=nil,
#name="created_at",
#null=false,
#precision=nil,
#primary=false,
#scale=nil,
#sql_type="timestamp without time zone",
#type=:datetime>]

Answer 1

You should be able to query your user entities with:
date_trunc('minute', created_at AT TIME ZONE 'UTC' AT TIME ZONE 'Asia/Jerusalem')::time = '17:50'
Or, with to_char() (but less index-friendly):
to_char(created_at AT TIME ZONE 'UTC' AT TIME ZONE 'Asia/Jerusalem', 'HH24:MI') = '17:50'
The point is that created_at is timestamp (or timestamp without time zone, which is just an alias). First, you can tell PostgreSQL, that interpret its values as they were in UTC, with the <timestamp> AT TIME ZONE <time zone> operator:
created_at AT TIME ZONE 'UTC'
Then, tell PostgreSQL to offset this value as if it were a local time in Asia/Jerusalem with the <timestamp with time zone> AT TIME ZONE <time zone> operator (which is completely different from the operator above with the same name):
created_at AT TIME ZONE 'UTC' AT TIME ZONE 'Asia/Jerusalem'
Now, you just need to truncate this value to extract only the hour & minute part.
Maybe worth mentioning that using these:
created_at AT TIME ZONE 'UTC+2'
created_at AT TIME ZONE 'UTC+3'
worked for you accidentally. These create timestamp with time zone values within the -2 and -3 UTC offsets, respectively. Then, because your config.active_record.default_timezone is set to :utc, it is sent to your client (ruby) within the UTC time zone, which means it added 2 and 3 hours, respectively.
Another issue to keep in mind is that in POSIX time zone names, positive offsets are used for locations west of Greenwich. Everywhere else, PostgreSQL follows the ISO-8601 convention that positive timezone offsets are east of Greenwich.