When testing I stumbled upon that too, but I thought no calculation was happening in the interval field. However, it's different with the days and months etc. It seems no calculation for day and month and more:
tpch=# select extract(day from interval '86400000 seconds');
extract
---------
0
(1 row)
tpch=# select extract(month from interval '86400000 seconds');
extract
---------
0
(1 row)
tpch=# select extract(year from interval '86400000 seconds');
extract
---------
0
(1 row)
But calculation is present for hour, and minutes and seconds (90061 sec is 1 day 1 hour 1 minute 1 second):
tpch=# select extract(minute from interval '90061 seconds');
extract
---------
1
(1 row)
tpch=# select extract(hour from interval '90061 seconds');
extract
---------
25
(1 row)
tpch=# select extract(second from interval '90061 seconds');
extract
----------
1.000000
(1 row)
The docs mention The hour field (0–23) for the hours, which is not true because it's not the field at all, but the calculated amount, and the value is not 0-23.
in `9.9.1. EXTRACT, date_part`
EXTRACT(field FROM source)
I saw more inconsistencies with the doc when `source` is an interval.
the `minute` field
select extract(minute from interval '2011 year 16 month 35 day 48 hour
1005 min 71 sec 11 ms');
select extract(minute from interval '2011 year 16 month 35 day 48 hour
1005 min 2 sec 11 ms');
select extract(minute from interval '2011 year 16 month 35 day 48 hour
1005 min 2 sec 11 ms');
the `hour` field:
select extract(hour from interval '2011 year 16 month 35 day 48 hour
1005 min 71 sec 11 ms');
select extract(hour from interval '2011 year 16 month 35 day 48 hour
1005 min 2 sec 11 ms');
select extract(hour from interval '2011 year 16 month 35 day 48 hour
1005 min 71 sec 11111111111 ms');
the `quarter` field:
select extract(quarter from interval '2011 year 12 month 48 hour 1005
min 2 sec 11 ms');
SELECT EXTRACT(QUARTER FROM TIMESTAMP '2001-12-16 20:38:40');
