Re: When exactly is a TIMESTAMPTZ converted to the sessions time zone?

On 08/09/2018 01:49 PM, Bruce Momjian wrote:
> On Tue, Jul 3, 2018 at 05:00:17PM -0400, Tom Lane wrote:
>> Thomas Kellerer <spam_eater(at)gmx(dot)net> writes:
>>> A recent discussion around timestamptz behaviour has lead me to question my own understanding on how a TIMESTAMPTZ is converted to the session's time zone.
>>> I assumed this conversion happens *on the server* before the value is sent to the client.
>>
>> It's done in the datatype's output conversion function.
>>
>>> A co-worker of mine claims that this is purely a client side thing, and that the server will always send the "plain" UTC value that is stored in a timestamptz column.
>>
>> Your co-worker is certainly wrong so far as text output is concerned.
>> If you retrieve data in binary format, though, it looks like you get
>> the raw (un-rotated) timestamp value, so that any conversion would have
>> to be done on the client side.
>
> Wow, I am kind of surprised by that. Do any other data types have this
> behavior?

This isn't related to binary-vs-string format, but I think it's often
overlooked that timestamptz considers your timezone not just to
stringify the value, but also to truncate it:

db=> create table t (ts timestamp, tstz timestamptz);
CREATE TABLE
Time: 3.154 ms
db=> set timezone='America/Los_Angeles';
SET
Time: 0.303 ms
db=> insert into t (ts, tstz) values ('2018-06-09 19:00:00', '2018-06-09
19:00:00');
INSERT 0 1
Time: 2.653 ms
db=> select ts, date_trunc('day', ts), tstz, date_trunc('day', tstz) from t;
ts | date_trunc | tstz |
date_trunc
---------------------+---------------------+------------------------+------------------------
2018-06-09 19:00:00 | 2018-06-09 00:00:00 | 2018-06-09 19:00:00-07 |
2018-06-09 00:00:00-07
(1 row)

Time: 0.438 ms
db=> set timezone='UTC';
SET
Time: 0.227 ms
db=> select ts, date_trunc('day', ts), tstz, date_trunc('day', tstz) from t;
ts | date_trunc | tstz |
date_trunc
---------------------+---------------------+------------------------+------------------------
2018-06-09 19:00:00 | 2018-06-09 00:00:00 | 2018-06-10 02:00:00+00 |
2018-06-10 00:00:00+00
(1 row)

Are there any other places where timestamptz consults your timezone?

--
Paul ~{:-)
pj(at)illuminatedcomputing(dot)com