Add timezone info

Author: Damien Escaich

Timezone.md

@@ -0,0 +1,135 @@
+# Time zone
+
+## Time zone parameters
+
+Teradata define time zone at three level:
+
+- Server
+
+- User
+
+- Session
+
+### Server Time zone
+
+It is set in /etc/opt/teradata/tdconfig/tdlocaledef.txt
+
+It looks like:
+
+```
+Time ZoneString {
+    "Europe Central"; "1"; "0"; "2";
+    "3"; "3"; "0"; "0"; "-1"; "02:00:00";
+    "3"; "9"; "0"; "0"; "-1"; "03:00:00";
+    "1987"; "1995"; "1"; "0"; "2"; "0";
+    "3"; "3"; "0"; "0"; "-1"; "02:00:00";
+    "3"; "10"; "0"; "0"; "-1"; "03:00:00";
+    "1996"; "9999"; "1"; "0"; "2"; "0"
+}
+```
+
+The value can be seen in dbscontrol (but not modified).
+
+### User Time zone
+
+A Time zone can be define for each user. 
+
+If the time zone is not defined, the user will use **server time zone**.
+
+> **Warning: as stated in Teradata documentation, NO specific time should be defined on users.**
+
+#### Define User time Zone
+
+During user creation, you can define a specific time zone using the parameter `Time Zone`.
+
+```sql
+create user user_name 
+    from  database_name
+    as ...
+    time zone = 'Europe Central'
+```
+
+#### Change Time zone
+
+```sql
+modify user user_name as time zone = 'Europe Central';
+-- or 
+modify user user_name as time zone = '+01:00';
+-- Remove user specific time zone
+modify user user_name as time zone = null;
+```
+
+#### Show user time zone
+
+```sql
+select 
+    username,
+    TimeZoneHour,
+    TimeZoneMinute,
+    TimeZoneString 
+from "DBC".UsersV
+```
+The time zone is defined either in TimeZoneHour + TimeZoneMinute or in TimeZoneString
+
+### Session time zone
+
+```sql
+set time zone 'Europe Central';
+-- or
+set time zone '+01:00';
+-- Set time zone to server value (not user value !)
+set time zone local;
+```
+
+### Show session time zone
+
+```sql
+select TimeZoneString from "DBC".SessionInfoVX where SessionNo = session
+```
+
+## List of the time zones
+
+[List from Teradata documentation](https://docs.teradata.com/r/w19R4KsuHIiEqyxz0WYfgA/67rSW_MQwBiVQ3siO~dt7Q)
+
+## Using time zones
+
+Let's try with examples:
+
+### Timestamp without time zone
+
+```sql
+create volatile table tb_without_tz (ts_without_tz timestamp(0)) on commit preserve rows;
+-- Here, we assume that the session time zone is the same as the server 'Europe Central' in this case.
+ins tb_without_tz values ('2021-04-28 10:00:00');
+select * from tb_without_tz;
+-- Shows 2021-04-28 10:00:00
+
+set time zone 'America Eastern';
+select * from tb_without_tz;
+-- Shows 2021-04-28 04:00:00
+```
+
+As you can see, even if the field is defined **without** time zone, the output depends on the session time zone. This can be very confusing and that's why it is recommended to keep user/session time zone unset. 
+
+#### Set a time zone different from the server before inserting
+
+```sql
+del tb_without_tz;
+set time zone 'America Eastern';
+ins tb_without_tz values ('2021-04-28 10:00:00');
+select * from tb_without_tz;
+-- Shows 2021-04-28 10:00:00
+
+-- Set back session time zone to server value
+set time zone local;
+select * from tb_without_tz;
+-- Shows 2021-04-28 16:00:00
+```
+
+#### Conclusion
+
+When working on timestamp without time zone:
+
+- when inserting, teradata convert the timestamp from the session time zone to the server time zone
+
+- when selecting, teradata convert the timestamp from the server time zone to the session time zone