problem creating rtree index on timestamptz

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Hello,

I was trying to create a functional index on a timestamptz column and
had the following problem. It seems subraction of an interval from a
timestamptz is not immutable. Should this be the case?

foo=> create table timestamp_tz_test ( start timestamptz primary key );
NOTICE: CREATE TABLE / PRIMARY KEY will create implicit index
"timestamp_tz_test_pkey" for table "timestamp_tz_test"
CREATE TABLE
 
foo=> create INDEX idx_timestamptz_rtree on timestamp_tz_test using
rtree ((start - '15 minutes'::INTERVAL));
ERROR: functions in index expression must be marked IMMUTABLE

I then tried a similar task with just at timestamp column and got this
error:

foo=> create table timestamp_test ( start timestamp primary key );
NOTICE: CREATE TABLE / PRIMARY KEY will create implicit index
"timestamp_test_pkey" for table "timestamp_test"
CREATE TABLE
 
foo=> create INDEX idx_timestamp_rtree on timestamp_test using rtree
((start - '15 minutes'::INTERVAL));
ERROR: data type timestamp without time zone has no default operator
class for access method "rtree"
HINT: You must specify an operator class for the index or define a
default operator class for the data type.

Should there be a default for this?

Thanks in advance,

- --hussein
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