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Re: BUG #3431: age() gets the days wrong

From: Pelle Johansson <pelle(at)morth(dot)org>
To: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
Cc: pgsql-bugs(at)postgresql(dot)org, pgsql-hackers(at)postgresql(dot)org
Subject: Re: BUG #3431: age() gets the days wrong
Date: 2007-07-08 20:58:18
Message-ID: 5A86CA18-593F-4517-BB83-995115A6A402@morth.org (view raw or flat)
Thread:
Lists: pgsql-bugspgsql-hackerspgsql-patches
Hi Tom,

I only have one remark really, which I first thought of after sending  
the report.
If you were to use the result for subtracting from the first value,  
instead of adding to the second, the conditions are reversed. It's  
not really as obvious as I first thought whether there's 2 months and  
29 days or 2 months and 30 days between 2006-11-02 and 2007-02-01...  
If one want mathematical correctness, it will have to be defined  
precisely.
-- 
Pelle Johansson

8 jul 2007 kl. 22.07 skrev Tom Lane:

> "Pelle Johansson" <pelle(at)morth(dot)org> writes:
>> The age() function seem to work by first counting months until  
>> less than a
>> month remains to to the second argument, then counting days left.  
>> This
>> doesn't give the correct result, as shown by this example:
>
>> # select column1, age(column1, '2006-11-02'), date '2006-11-02' +
>> age(column1, '2006-11-02') from (values ('2007-01-31'::date),
>> ('2007-02-01')) as alias;
>>   column1   |      age       |      ?column?
>> ------------+----------------+---------------------
>>  2007-01-31 | 2 mons 29 days | 2007-01-31 00:00:00
>>  2007-02-01 | 2 mons 29 days | 2007-01-31 00:00:00
>> (2 rows)
>
> I took another look at this example.  I believe what is actually going
> wrong here is that when timestamp_age converts a month into an
> equivalent number of days, it uses the number of days in the first
> month of the interval it's dealing with (ie, the month containing
> the earlier of the two dates).  This is just wrong, because interval
> addition adds months first and then days.  The appropriate conversion
> to use is actually the length of the next-to-last month of the  
> interval.
>
> As an example, 8.2 and CVS HEAD produce
>
> regression=# select age('2007-03-14', '2007-02-15');
>    age
> ---------
>  27 days
> (1 row)
>
> which is reasonable, but
>
> regression=# select age('2007-04-14', '2007-02-15');
>       age
> ---------------
>  1 mon 27 days
> (1 row)
>
> is not so reasonable, nor is
>
> regression=# select age('2007-03-14', '2007-01-15');
>       age
> ---------------
>  1 mon 30 days
> (1 row)
>
> If we change the code to use the next-to-last month of the interval
> then these two cases produce '1 mon 30 days' and '1 mon 27 days'
> respectively.
>
> Another problem is that the code isn't doing the propagate-to-next- 
> field
> bit for negative fractional seconds.  Hence it produces
>
> regression=# select age('2007-02-14 01:00:00', '2007-01-15  
> 01:00:00.4');
>          age
> ----------------------
>  30 days -00:00:00.40
> (1 row)
>
> which is maybe not incorrect, but certainly fairly inconsistent with
>
> regression=# select age('2007-02-14 01:00:00', '2007-01-15 01:00:01');
>        age
> ------------------
>  29 days 23:59:59
> (1 row)
>
>
> Hence I propose the attached patch.  This does not change any existing
> regression test outputs, but it does change the example given in the
> documentation: age(timestamp '2001-04-10', timestamp '1957-06-13')
> will now produce '43 years 9 mons 28 days' not 27 days.  Which  
> actually
> is correct if you try to add back the result to timestamp  
> '1957-06-13'.
> It also appears to fix Palle's example:
>
> regression=# select column1, age(column1, '2006-11-02'), date  
> '2006-11-02' +
> age(column1, '2006-11-02') from (values ('2007-01-31'::date),
> ('2007-02-01')) as alias;
>   column1   |      age       |      ?column?
> ------------+----------------+---------------------
>  2007-01-31 | 2 mons 29 days | 2007-01-31 00:00:00
>  2007-02-01 | 2 mons 30 days | 2007-02-01 00:00:00
> (2 rows)
>
> As I said earlier, I'm worried about changing the behavior of a  
> function
> that's been around for so long, so I'm disinclined to back-patch this.
> But it seems like a reasonable change to make in 8.3.  Comments?
>
> 			regards, tom lane
>
> Index: timestamp.c
> ===================================================================
> RCS file: /cvsroot/pgsql/src/backend/utils/adt/timestamp.c,v
> retrieving revision 1.179
> diff -c -r1.179 timestamp.c
> *** timestamp.c	6 Jul 2007 04:15:59 -0000	1.179
> --- timestamp.c	8 Jul 2007 19:45:04 -0000
> ***************
> *** 3044,3050 ****
>   	if (timestamp2tm(dt1, NULL, tm1, &fsec1, NULL, NULL) == 0 &&
>   		timestamp2tm(dt2, NULL, tm2, &fsec2, NULL, NULL) == 0)
>   	{
> ! 		fsec = (fsec1 - fsec2);
>   		tm->tm_sec = tm1->tm_sec - tm2->tm_sec;
>   		tm->tm_min = tm1->tm_min - tm2->tm_min;
>   		tm->tm_hour = tm1->tm_hour - tm2->tm_hour;
> --- 3044,3051 ----
>   	if (timestamp2tm(dt1, NULL, tm1, &fsec1, NULL, NULL) == 0 &&
>   		timestamp2tm(dt2, NULL, tm2, &fsec2, NULL, NULL) == 0)
>   	{
> ! 		/* form the symbolic difference */
> ! 		fsec = fsec1 - fsec2;
>   		tm->tm_sec = tm1->tm_sec - tm2->tm_sec;
>   		tm->tm_min = tm1->tm_min - tm2->tm_min;
>   		tm->tm_hour = tm1->tm_hour - tm2->tm_hour;
> ***************
> *** 3064,3069 ****
> --- 3065,3081 ----
>   			tm->tm_year = -tm->tm_year;
>   		}
>
> + 		/* propagate any negative fields into the next higher field */
> + 		while (fsec < 0)
> + 		{
> + #ifdef HAVE_INT64_TIMESTAMP
> + 			fsec += USECS_PER_SEC;
> + #else
> + 			fsec += 1.0;
> + #endif
> + 			tm->tm_sec--;
> + 		}
> +
>   		while (tm->tm_sec < 0)
>   		{
>   			tm->tm_sec += SECS_PER_MINUTE;
> ***************
> *** 3082,3097 ****
>   			tm->tm_mday--;
>   		}
>
> ! 		while (tm->tm_mday < 0)
>   		{
>   			if (dt1 < dt2)
>   			{
> ! 				tm->tm_mday += day_tab[isleap(tm1->tm_year)][tm1->tm_mon - 1];
> ! 				tm->tm_mon--;
>   			}
>   			else
>   			{
> ! 				tm->tm_mday += day_tab[isleap(tm2->tm_year)][tm2->tm_mon - 1];
>   				tm->tm_mon--;
>   			}
>   		}
> --- 3094,3130 ----
>   			tm->tm_mday--;
>   		}
>
> ! 		/*
> ! 		 * day-to-month conversion is tricky because variable.  For each
> ! 		 * decrement in tm_mon, we should adjust tm_mday by the length of
> ! 		 * the next-to-last month(s) of the original time interval.
> ! 		 * This corresponds to the notion that interval addition will add
> ! 		 * months first, then days.
> ! 		 */
> ! 		if (tm->tm_mday < 0)
>   		{
> + 			int		end_year;
> + 			int		end_mon;
> +
>   			if (dt1 < dt2)
>   			{
> ! 				end_year = tm2->tm_year;
> ! 				end_mon = tm2->tm_mon;
>   			}
>   			else
>   			{
> ! 				end_year = tm1->tm_year;
> ! 				end_mon = tm1->tm_mon;
> ! 			}
> !
> ! 			while (tm->tm_mday < 0)
> ! 			{
> ! 				if (--end_mon <= 0)
> ! 				{
> ! 					end_mon = MONTHS_PER_YEAR;
> ! 					end_year--;
> ! 				}
> ! 				tm->tm_mday += day_tab[isleap(end_year)][end_mon - 1];
>   				tm->tm_mon--;
>   			}
>   		}
> ***************
> *** 3158,3163 ****
> --- 3191,3197 ----
>   	if (timestamp2tm(dt1, &tz1, tm1, &fsec1, &tzn, NULL) == 0 &&
>   		timestamp2tm(dt2, &tz2, tm2, &fsec2, &tzn, NULL) == 0)
>   	{
> + 		/* form the symbolic difference */
>   		fsec = fsec1 - fsec2;
>   		tm->tm_sec = tm1->tm_sec - tm2->tm_sec;
>   		tm->tm_min = tm1->tm_min - tm2->tm_min;
> ***************
> *** 3178,3183 ****
> --- 3212,3228 ----
>   			tm->tm_year = -tm->tm_year;
>   		}
>
> + 		/* propagate any negative fields into the next higher field */
> + 		while (fsec < 0)
> + 		{
> + #ifdef HAVE_INT64_TIMESTAMP
> + 			fsec += USECS_PER_SEC;
> + #else
> + 			fsec += 1.0;
> + #endif
> + 			tm->tm_sec--;
> + 		}
> +
>   		while (tm->tm_sec < 0)
>   		{
>   			tm->tm_sec += SECS_PER_MINUTE;
> ***************
> *** 3196,3211 ****
>   			tm->tm_mday--;
>   		}
>
> ! 		while (tm->tm_mday < 0)
>   		{
>   			if (dt1 < dt2)
>   			{
> ! 				tm->tm_mday += day_tab[isleap(tm1->tm_year)][tm1->tm_mon - 1];
> ! 				tm->tm_mon--;
>   			}
>   			else
>   			{
> ! 				tm->tm_mday += day_tab[isleap(tm2->tm_year)][tm2->tm_mon - 1];
>   				tm->tm_mon--;
>   			}
>   		}
> --- 3241,3277 ----
>   			tm->tm_mday--;
>   		}
>
> ! 		/*
> ! 		 * day-to-month conversion is tricky because variable.  For each
> ! 		 * decrement in tm_mon, we should adjust tm_mday by the length of
> ! 		 * the next-to-last month(s) of the original time interval.
> ! 		 * This corresponds to the notion that interval addition will add
> ! 		 * months first, then days.
> ! 		 */
> ! 		if (tm->tm_mday < 0)
>   		{
> + 			int		end_year;
> + 			int		end_mon;
> +
>   			if (dt1 < dt2)
>   			{
> ! 				end_year = tm2->tm_year;
> ! 				end_mon = tm2->tm_mon;
>   			}
>   			else
>   			{
> ! 				end_year = tm1->tm_year;
> ! 				end_mon = tm1->tm_mon;
> ! 			}
> !
> ! 			while (tm->tm_mday < 0)
> ! 			{
> ! 				if (--end_mon <= 0)
> ! 				{
> ! 					end_mon = MONTHS_PER_YEAR;
> ! 					end_year--;
> ! 				}
> ! 				tm->tm_mday += day_tab[isleap(end_year)][end_mon - 1];
>   				tm->tm_mon--;
>   			}
>   		}


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