Tools to convert timestamp data to another time zone in PostgreSQL

On 6/13/22 2:52 PM, Joel Rabinovitch wrote:

Hi,

We have recently modified our application to work with PostgreSQL
databases and schemas. We also support Oracle and SQL Server Databases.

Along with adding support for PostgreSQL, we have upgraded our
infrastructure such that all environments are configured to use the UTC
time zone. Previously, the environments were configured to use the time
zone where the database server and application server were installed.

As a result, we have hit an issue where we need to convert data in
timestamp columns in existing records to reflect that the time is in
UTC. The timezone is not specified in our timestamp columns (i.e. they
are defined as timezone without time zone). We need to do this for
interoperability between the database engines we support.

After a bit of searching, we found we can write an SQL similar to the
one below to do the conversion:

update client

�� set create_stamp = (create_stamp at time zone 'America/New_York' at
time zone 'UTC')

where client_code = 'HOANA';

This does work correctly. However, we have some limitations in terms
using SQL statements like this.

- We would need to identify the timestamp columns that would be affected
across many tables and multiple schemas.

select table_schema, table_name, column_name from
information_schema.columns where data_type = 'timestamp without time zone';

- We also store date-only information in timestamp without time zone
columns. This was done as a result of migrating our application from
Oracle where the DATE data type was used at the time (Oracle now
supports timestamp columns).

A date stored in a timestamp field is going to be a timestamp at midnight:

timestamp_test
Table "public.timestamp_test"
Column | Type | Collation | Nullable | Default
--------+-----------------------------+-----------+----------+---------
ts | timestamp without time zone | | |
tsz | timestamp with time zone |

insert into timestamp_test values (current_date, current_date);

ts | tsz
-------------------------+----------------------------
2022-06-13 00:00:00 | 2022-06-13 00:00:00-07

I was wondering if you are aware of any open source and/or commercial
tools that could allow us to easily identify the affected columns,
exclude columns if necessary, and apply the necessary conversion. If
not, we would have to write a utility that does this for us, which could
be a lengthy process.

Thanks,

Joel

--
Adrian Klaver
adrian.klaver@aklaver.com

On Mon, Jun 13, 2022 at 09:52:00PM +0000, Joel Rabinovitch wrote:

Hi,

We have recently modified our application to work with PostgreSQL
databases and schemas. We also support Oracle and SQL Server Databases.

[skipped]

As a result, we have hit an issue where we need to convert data in
timestamp columns in existing records to reflect that the time is in UTC.
The timezone is not specified in our timestamp columns (i.e. they are
defined as timezone without time zone). We need to do this for

btw, it's not specified in timestamptz either.
timestamptz always stores time in UTC microseconds, and displays
it in timezone according to the session settings.

interoperability between the database engines we support.

It's better to use timestamptz type

https://wiki.postgresql.org/wiki/Don%27t_Do_This#Don.27t_use_timestamp_.28without_time_zone.29

and I think it's a good time to do that change.

[skipped]

Setting timezone in session to 'America/New_York' and converting
column to timestamptz should do it fine

set timezone = 'America/New_York';
ALTER TABLE <tablename> ALTER COLUMN create_stamp TYPE timestamptz;

should do it just fine (on a reasonably sized tables).
Other possibilities, like creating a new column and renaming it
after the proper feel in, are possible.