How to convert epoch to datetime redshift?

UDF is going to be pretty slow. Checked execution time for 3 solutions and 1k rows.

The slowest -

-- using UDF from one of the answers
SELECT from_unixtime(column_with_time_in_ms/ 1000)
FROM table_name LIMIT 1000;

Execution time: 00:00:02.348062s

2nd best -

SELECT date_add('ms',column_with_time_in_ms,'1970-01-01')
FROM table_name LIMIT 1000;

Execution time: 00:00:01.112831s

And the fastest -

SELECT TIMESTAMP 'epoch' + column_with_time_in_ms/1000 *INTERVAL '1 second'
FROM table_name LIMIT 1000;

Execution time: 00:00:00.095102s

---

Execution time calculated from stl_query -

SELECT *
       ,endtime - starttime
FROM stl_query
WHERE querytxt ilike('%table_name%limit%')
ORDER BY starttime DESC;

Redshift doesn't have the from_unixtime() function. You'll need to use the below SQL query to get the timestamp. It just adds the number of seconds to epoch and return as timestamp.

select timestamp 'epoch' + your_timestamp_column * interval '1 second' AS your_column_alias
from your_table