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Re: BUG #1871: operations with data types

From: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
To: Michael Fuhr <mike(at)fuhr(dot)org>
Cc: anris(at)polynet(dot)lviv(dot)ua, pgsql-bugs(at)postgresql(dot)org
Subject: Re: BUG #1871: operations with data types
Date: 2005-09-11 07:03:57
Message-ID: 3050.1126422237@sss.pgh.pa.us (view raw or flat)
Thread:
Lists: pgsql-bugs
Michael Fuhr <mike(at)fuhr(dot)org> writes:
> [ Mike's unhappy that ] these give different answers:

> select '2005-08-30'::date + '1 day'::interval + '1 month'::interval,
>        '2005-08-30'::date + '1 month'::interval + '1 day'::interval;

This is certainly an arena in which you can't expect the commutative
or associative laws to hold.  For instance:

	('2005-02-28' + '1 day') + '1 month'

You can't possibly argue that that produces anything but
'2005-03-01' + '1 month' and thence '2005-04-01'.  On the other
hand, you can't possibly argue that

	('2005-02-28' + '1 month') + '1 day'

produces anything but '2005-03-28' + '1 day' and thence '2005-03-29'.
So the original complaint can at best be described as lacking thought.

> Wait a minute, here's proof that it *must* be wrong ;-)
> mysql> select date_add('2005-08-31', interval 1 month);
> | 2005-09-30                               |

rotfl ;-)

> What do other DBMSs do?

This is a fair question.  Given that the SQL committee hasn't provided
any useful leadership, what are other groups doing?

			regards, tom lane

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Next:From: Bob FriesenhahnDate: 2005-09-11 22:02:43
Subject: Re: BUG #1865: isinf wrongly dectected under Solaris 9
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Subject: Re: BUG #1871: operations with data types

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