date at time zone related syntax and semantic differences - postgresql

Question

Question: How is query 1 "semantically" different than the query 2?
Background:
To extract data from the table in a db which is at my localtime zone (AT TIME ZONE 'America/New_York').
The table has data for various time zones such as the 'America/Los_Angeles', America/North_Dakota/New_Salem and such time zones.
(Postgres stores the table data for various timezones in my local timezone)
So, everytime I retrieve data for a different location other than my localtime, I convert it to its relevant timezone for evaluation purposes..
Query 1:
test_db=# select count(id) from click_tb where date::date AT TIME ZONE 'America/Los_Angeles' = '2017-05-22'::date AT TIME ZONE 'America/Los_Angeles';
count
-------
1001
(1 row)
Query 2:
test_db=# select count(id) from click_tb where (date AT TIME ZONE 'America/Los_Angeles')::date = '2017-05-22'::date;
count
-------
5
(1 row)
Table structure:
test_db=# /d+ click_tb
Table "public.click_tb"
Column | Type | Modifiers | Storage | Stats target | Description
-----------------------------------+--------------------------+-------------------------------------------------------------+----------+--------------+-------------
id | integer | not null default nextval('click_tb_id_seq'::regclass) | plain | |
date | timestamp with time zone | | plain | |
Indexes:
"click_tb_id" UNIQUE CONSTRAINT, btree (id)
"click_tb_date_index" btree (date)
The query 1 and query 2 do not produce consistent results.
As per my tests, the below query 3, semantically addresses my requirement.
Your critical feedback is welcome.
Query 3:
test_db=# select count(id) from click_tb where ((date AT TIME ZONE 'America/Los_Angeles')::timestamp with time zone)::date = '2017-05-22'::date;

Answer 1

Do not convert the timestamp field. Instead, do a range query. Since your data is already using a timestamp with time zone type, just set the time zone of your query accordingly.
set TimeZone = 'America/Los_Angeles';
select count(id) from click_tb
where date >= '2017-01-02'
and date < '2017-01-03';
Note how this uses a half open interval of the dates (at start of day in the set time zone). If you want to compute the second date from your first date, then:
set TimeZone = 'America/Los_Angeles';
select count(id) from click_tb
where date >= '2017-01-02'
and date < (timestamp with time zone '2017-01-02' + interval '1 day');
This properly handles daylight saving time, and sargability.