Fokko opened a new pull request #28821:
URL: https://github.com/apache/spark/pull/28821
Currently, when you take an average of a Timestamp, you'll end up with a
Double, representing the seconds since epoch. This is because of old Hive
behavior.
I strongly believe that it is better to return a Timestamp.
Behaviour in Postgres:
```
root@8c4241b617ec:/# psql postgres postgres
psql (12.3 (Debian 12.3-1.pgdg100+1))
Type "help" for help.
postgres=# CREATE TABLE timestamp_demo (ts TIMESTAMP);
CREATE TABLE
postgres=# INSERT INTO timestamp_demo VALUES('2019-01-01 18:22:11');
INSERT 0 1
postgres=# INSERT INTO timestamp_demo VALUES('2018-01-01 18:22:11');
INSERT 0 1
postgres=# INSERT INTO timestamp_demo VALUES('2017-01-01 18:22:11');
INSERT 0 1
postgres=# SELECT AVG(ts) FROM timestamp_demo;
ERROR: function avg(timestamp without time zone) does not exist
LINE 1: SELECT AVG(ts) FROM timestamp_demo;
```
Behaviour in MySQL:
```
root@bab43a5731e8:/# mysql
Welcome to the MySQL monitor. Commands end with ; or \g.
Your MySQL connection id is 9
Server version: 8.0.20 MySQL Community Server - GPL
Copyright (c) 2000, 2020, Oracle and/or its affiliates. All rights reserved.
Oracle is a registered trademark of Oracle Corporation and/or its
affiliates. Other names may be trademarks of their respective
owners.
Type 'help;' or '\h' for help. Type '\c' to clear the current input
statement.
mysql> CREATE TABLE timestamp_demo (ts TIMESTAMP);
Query OK, 0 rows affected (0.05 sec)
mysql> INSERT INTO timestamp_demo VALUES('2019-01-01 18:22:11');
Query OK, 1 row affected (0.01 sec)
mysql> INSERT INTO timestamp_demo VALUES('2018-01-01 18:22:11');
Query OK, 1 row affected (0.01 sec)
mysql> INSERT INTO timestamp_demo VALUES('2017-01-01 18:22:11');
Query OK, 1 row affected (0.01 sec)
mysql> SELECT AVG(ts) FROM timestamp_demo;
+---------------------+
| AVG(ts) |
+---------------------+
| 20180101182211.0000 |
+---------------------+
1 row in set (0.00 sec)
```
Which is a YYYYMMDDHHMMSS format in double.
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