Conversion from WITH TIME ZONE to WITHOUT TIME ZONE types should drop the time
zone instead of use the session time zone
------------------------------------------------------------------------------------------------------------------------
Key: CORE-6276
URL: http://tracker.firebirdsql.org/browse/CORE-6276
Project: Firebird Core
Issue Type: Bug
Components: Engine
Affects Versions: 4.0 Beta 1
Reporter: Adriano dos Santos Fernandes
Firebird (as well PostgreSQL) uses the session time zone when converting from
WITH-TZ to WITHOUT-TZ types.
Example:
set time zone '-03:00';
SQL> select cast(timestamp '2020-04-04 10:00:00 -05:00' as timestamp) from
rdb$database;
CAST
=========================
2020-04-04 12:00:00.0000
When converting it back to TIMESTAMP WITH-TZ (not altering the session time
zone), we get an equal (equivalent) timestamp (which different offset):
SQL> select cast(timestamp '2020-04-04 12:00:00.0000' as timestamp with time
zone) from rdb$database;
CAST
=========================================================
2020-04-04 12:00:00.0000 -03:00
SQL> select timestamp '2020-04-04 12:00:00.0000 -03:00' = timestamp '2020-04-04
10:00:00 -05:00' from rdb$database;
=======
<true>
However the standard SQL says that conversion from TIMESTAMP WITH-TZ to
TIMESTAMP WITHOUT-TZ should be "SV.UTC + SV.TZ" (SV is the source value). In
practice this means to drop the time zone.
Oracle implementation also respects (and documents in
https://docs.oracle.com/cd/B19306_01/server.102/b14225/ch4datetime.htm) this
behavior: "Insert the same date and time as a TIMESTAMP WITH TIME ZONE literal.
Oracle converts it to a TIMESTAMP value, which means that the time zone
information is dropped.".
Also accordingly to the standard the conversion from WITHOUT-TZ to WITH-TZ is
"TV.UTC = SV - STZD; TV.TZ = STZD" (TV is target value; STZD is the session
time zone). So in this respect we are correct and then converting WITH-TZ ->
WITHOUT-TZ -> WITH-TZ does not necessarily produces an equivalent value.
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