Re: Interval "1 month" is equals to interval "30 days" - WHY?

Dmitry Koterov wrote:
>> I've just discovered a very strange thing:
>>
>> SELECT '1 mon'::interval = '30 days'::interval --> TRUE???
>>
>> This returns TRUE (also affected when I create an unique index using
an
>> interval column). Why?
>>
>> I know that Postgres stores monthes, days and seconds in interval
values
>> separately. So how to make "=" to compare intervals "part-by-part"
and not
>> treat "1 mon" as "30 days"?
>>
>> P.S.
>> Reproduced at least in 8.4 and 9.1.

> ...and even worse:
>
> SELECT ('1 year'::interval) = ('360 days'::interval); --> TRUE :-)
> SELECT ('1 year'::interval) = ('365 days'::interval); --> FALSE :-)

Intervals are internally stored in three fields: months, days
and microseconds. A year has 12 months.

PostgreSQL converts intervals into microseconds before comparing them:
a month is converted to 30 days, and a day is converted to 24 hours.

Of course this is not always correct.
But what should the result of
INTERVAL '1 month' = INTERVAL '30 days'
be? FALSE would be just as wrong.

Yours,
Laurenz Albe