Very often in old databases a timestamp column does not have the timezone info. This becomes an issue when the business gets bigger and crosses different timezones.
select bare_timestamp_field at time zone 'Asia/Shanghai' at time zone 'US/Pacific' from some_table;
The above sample query assigns the Shanghai timezone to the bare timestamp, then convert the resulting timestamp w/ timezone to the US Pacific PST timestamp.
The double ‘at time zone’ parts can be read as: from timezone #1 to timezone #2. This is due to the implicit syntax. A more explicit syntax is the following, which makes more sense similar to type casting.
select ((bare_timestamp_field at time zone 'Asia/Shanghai') at time zone 'US/Pacific') from some_table;