Fix interval_transform so it doesn't throw away non-no-op casts.

interval_transform() contained two separate bugs that caused it to
sometimes mistakenly decide that a cast from interval to restricted
interval is a no-op and throw it away.

First, it was wrong to rely on dt.h's field type macros to have an
ordering consistent with the field's significance; in one case they do
not.  This led to mistakenly treating YEAR as less significant than MONTH,
so that a cast from INTERVAL MONTH to INTERVAL YEAR was incorrectly
discarded.

Second, fls(1<<k) produces k+1 not k, so comparing its output directly
to SECOND was wrong.  This led to supposing that a cast to INTERVAL
MINUTE was really a cast to INTERVAL SECOND and so could be discarded.

To fix, get rid of the use of fls(), and make a function based on
intervaltypmodout to produce a field ID code adapted to the need here.

Per bug #14479 from Piotr Stefaniak.  Back-patch to 9.2 where transform
functions were introduced, because this code was born broken.

Discussion: https://postgr.es/m/20161227172307.10135.7747@wrigleys.postgresql.org

2 files changed, 23 insertions, 0 deletions

diff --git a/src/test/regress/expected/interval.out b/src/test/regress/expected/interval.out
index c873a99bd94..946c97ad924 100644
--- a/src/test/regress/expected/interval.out
+++ b/src/test/regress/expected/interval.out
@@ -682,6 +682,24 @@ SELECT interval '1 2:03:04.5678' minute to second(2);
  1 day 02:03:04.57
 (1 row)
 
+-- test casting to restricted precision (bug #14479)
+SELECT f1, f1::INTERVAL DAY TO MINUTE AS "minutes",
+  (f1 + INTERVAL '1 month')::INTERVAL MONTH::INTERVAL YEAR AS "years"
+  FROM interval_tbl;
+       f1        |     minutes     |  years   
+-----------------+-----------------+----------
+ 00:01:00        | 00:01:00        | 00:00:00
+ 05:00:00        | 05:00:00        | 00:00:00
+ 10 days         | 10 days         | 00:00:00
+ 34 years        | 34 years        | 34 years
+ 3 mons          | 3 mons          | 00:00:00
+ -00:00:14       | 00:00:00        | 00:00:00
+ 1 day 02:03:04  | 1 day 02:03:00  | 00:00:00
+ 6 years         | 6 years         | 6 years
+ 5 mons          | 5 mons          | 00:00:00
+ 5 mons 12:00:00 | 5 mons 12:00:00 | 00:00:00
+(10 rows)
+
 -- test inputting and outputting SQL standard interval literals
 SET IntervalStyle TO sql_standard;
 SELECT  interval '0'                       AS "zero",
diff --git a/src/test/regress/sql/interval.sql b/src/test/regress/sql/interval.sql
index 789c3de4403..cff9adab321 100644
--- a/src/test/regress/sql/interval.sql
+++ b/src/test/regress/sql/interval.sql
@@ -196,6 +196,11 @@ SELECT interval '1 2.3456' minute to second(2);
 SELECT interval '1 2:03.5678' minute to second(2);
 SELECT interval '1 2:03:04.5678' minute to second(2);
 
+-- test casting to restricted precision (bug #14479)
+SELECT f1, f1::INTERVAL DAY TO MINUTE AS "minutes",
+  (f1 + INTERVAL '1 month')::INTERVAL MONTH::INTERVAL YEAR AS "years"
+  FROM interval_tbl;
+
 -- test inputting and outputting SQL standard interval literals
 SET IntervalStyle TO sql_standard;
 SELECT  interval '0'                       AS "zero",