Re: 'epoch'::timestamp and Daylight Savings

Thomas,

Thanks for replying so quickly.

Upon some further investigation, the conversion of the time to BST is OK on
data inserted before the upgrade (the display was just a red herring), the
problem is just on new data inserted into the table with a default value for
the timestamp fields.

The output from \d on the table is below:

capitaras_live=# \d e_application                                              Table "e_application"     Column       |
         Type           |
 
Modifiers                              
-------------------+--------------------------+-----------------------------
---------------------------------------e_app_id_num      | integer                  | not null default
nextval('"e_application_e_app_id_num_seq"'::text)e_app_cand_id_num | integer                  | e_app_job_id      |
charactervarying(30)    | not nulle_app_cid_num     | bigint                   | e_app_cts         | character
varying(30)   | e_app_mts         | character varying(30)    | e_app_print_date  | timestamp with time zone | default
 
"timestamp"('epoch'::text)e_app_xfer_date   | timestamp with time zone | default
"timestamp"('epoch'::text)e_app_completed   | character varying(6)     | e_app_xml         | text                     |
e_app_modified   | timestamp with time zone | e_app_created     | timestamp with time zone | e_app_wd_feedback | text
                 | e_app_submitted   | timestamp with time zone | 
 
Indexes: e_app_completed_idx,        e_app_job_id_and_completed_idx
Primary key: e_application_pkey
Triggers: RI_ConstraintTrigger_27961

With a bit more playing, it definitely looks like something wrong with the
way the default value has been set follwing the export & import for the
upgrade:-

Create table arnold (a    int8,b    timestamp default 'epoch'::timestamp,c    timestamp default
"timestamp"('epoch'::text)
);

capitaras_test=# \d arnold                                 Table "arnold"Column |           Type           |
      Modifiers
 

--------+--------------------------+----------------------------------------
-------a      | bigint                   | b      | timestamp with time zone | default '1970-01-01
01:00:00+01'::timestamptzc      | timestamp with time zone | default "timestamp"('epoch'::text) 

capitaras_test=# insert into arnold (a) values (1);
INSERT 182907 1
capitaras_test=# insert into arnold (a) values (2);
INSERT 182907 1

capitaras_test=# set timezone to 'Europe/London';
SET VARIABLE
capitaras_test=# select * from arnold;a |           b            |           c            
---+------------------------+------------------------1 | 1970-01-01 01:00:00+01 | 1970-01-01 00:00:00+012 | 1970-01-01
01:00:00+01| 1970-01-01 00:00:00+01
 
(2 rows)

capitaras_test=# set timezone to 'GMT';
SET VARIABLE
capitaras_test=# select * from arnold
capitaras_test-# ;a |           b            |           c            
---+------------------------+------------------------1 | 1970-01-01 00:00:00+00 | 1969-12-31 23:00:00+002 | 1970-01-01
00:00:00+00| 1969-12-31 23:00:00+00
 
(2 rows)


I think that the best way forward for us (short of re-writing the backend to
use NULL) will be to just alter the default value to the one in column b in
the test table above.

With regards the disclaimer, don't you just love lawyers ;-)

Once again, thanks for replying so quickly.

Regards

John.



-----Original Message-----
From: Thomas Lockhart [mailto:lockhart@fourpalms.org] 
Sent: 23 October 2002 16:06
To: Hosen, John
Cc: 'pgsql-hackers@postgresql.org'
Subject: Re: 'epoch'::timestamp and Daylight Savings


> We have just upgraded from 7.1.3 to 7.2.3 over the weekend, and have 
> just noticed something weird with regards 'epoch'::timestamp. In 
> 7.1.3, 'epoch'::timestamp always returned the value 1970-01-01 
> 00:00:00+00,...

I would expect 'epoch'::timestamp to always act as though the value were 
the same as Unix time zero. But it did not explicitly return that value:

lockhart=# select version();
------------------------------------------------------------- PostgreSQL 7.1.2 on i686-pc-linux-gnu, compiled by GCC
2.96

lockhart=# select 'epoch'::timestamp;
---------- epoch

lockhart=# select 'epoch'::timestamp + '0 sec'::interval;
------------------------ 1969-12-31 16:00:00-08


> following the upgrade we now get:-
>       timestamptz       
> ------------------------
>  1970-01-01 01:00:00+01

Which is consistant with the previous result in absolute time.

You will find that the most recent versions of PostgreSQL convert 
'epoch' on input, and that with- and without time zone data types are 
available:

lockhart=# select cast('epoch' as timestamp with time zone);      timestamptz
------------------------ 1969-12-31 16:00:00-08

lockhart=# select cast('epoch' as timestamp without time zone);      timestamp
--------------------- 1970-01-01 00:00:00


> Also, any fields set to 'epoch'::timestamp before the upgrade now return:-
>     e_app_xfer_date     
> ------------------------
>  1970-01-01 00:00:00+01
> If we issue a SET TIMEZONE TO 'GMT' the results are the same as for 
> the previous release.

Not sure about this one. What is the schema? Can you give an example 
where the time gets shifted by an hour?

> This is causing us a problem as we use epoch to indicate whether the 
> data has been processed, and to log the date and time of processing.

Using "special values" to indicate status can be troublesome, as you are 
finding. I'd suggest using NULL to indicate that a field is not known or 
not yet set.

> This email and any files attached to it are confidential and intended
> solely for the use of the individual or entity to whom they are
> addressed. If you have received this email in error please notify 
> the system manager.

Oops. Can you please confirm that we were the intended recipients? ;)

hth
                    - Thomas


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