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Bug 1500

From: Lyubomir Petrov <lpetrov(at)sysmaster(dot)com>
To: pgsql-hackers(at)postgresql(dot)org
Subject: Bug 1500
Date: 2005-03-25 19:54:40
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Lists: pgsql-hackers

I have found what is causing the crash described in Bug 1500. Now I 
would like to fix it, but need opinions about what is the correct behaviour.

The bug can be easily duplicated when formatting interval in to_char() 
using 'Mon' or 'Month' in the format string.

select to_char(now() - '20011001'::date, 'YYYYMonDD');
(server process crash follows)

What happens:
1. The formatting function used is dch_date() 
(src/backend/utils/adt/formatting.c) and it works on struct pg_tm.
2. The interval2tm() (src/backend/utils/adt/timestamp.c) is used to 
convert the interval into pg_tm struct.
2a. If the Interval parameter has month != 0, then month and year are 
filled in pg_tm
2b. If not -> they are set to 0 and only  days, hours, minutes, seconds 
are filled (this is the case when the bug appears).
3. dch_date() expects the struct pg_tm to have valid 1-based month index 
and directly references the months/months_full arrays using (tm->month - 
1) as index to get the short/full name of the month.
4. SIGSEGV in the server process

This could be easily by not allowing the bad array indexing, but it 
raises a bigger problem: How is supposed the to_char() function to 
format interval datatype? What is the correct output?

Should we:
1) Try to fill the missing data (years, months) using the days (but how 
many days are in one month? hardcode 30/31? how many days in 1 year 
then...) and fix the formatting function to ignore string based 
formatting for intervals
2) Fail the entire statement (do not support interval formatting with 

Also the general to_char()  Interval formatting seems broken anyway. 
Note that the following (and similar) works now, but the result doesn't 
seem to be correct:

test=> select to_char(now() - '20011001'::date, 'YYYYDD');
(1 row)

test=> select now() - '20011001'::date;
 1271 days 12:48:18.1216260046
(1 row)

So this bug actually brings the issue of interval to_char() formatting. 

Lyubomir Petrov


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