Thread: Default Values Section

I have a problem reading the "5.3 Default Values" Section of
Postgresql8.0.0-rc1 documentation. In this section there are nothing about
using 'now' constant. I think which there are a conflict with a note about
'now' in section "9.9. Date/Time Functions and Operators" which show the way
to achieve the desired behavior. See:

5.3. Default Values

...

The default value may be a scalar expression, which will be evaluated
whenever the default value is inserted (not when the table is created).


9.9. Date/Time Functions and Operators

...

SELECT CURRENT_TIMESTAMP;
SELECT now();
SELECT TIMESTAMP 'now';
Note: You do not want to use the third form when specifying a DEFAULT clause
while creating a table. The system will convert now to a timestamp as soon
as the constant is parsed, so that when the default value is needed, the
time of the table creation would be used! The first two forms will not be
evaluated until the default value is used, because they are function calls.
Thus they will give the desired behavior of defaulting to the time of row
insertion.

I think which a simple reference to any unpredictable behavior must be
placed here to avoid confusion when anybody don't reach the desired behavior
when use default values.

What think you about my suggestion?

Wait replies, Tanks.

Lourival Oliveira da Silva

PS: Sorry by my poor English.

OK, I updated the documentation to add a comment on that line:

    SELECT CURRENT_TIMESTAMP;
    SELECT now();
    SELECT TIMESTAMP 'now';  -- incorrect for use with DEFAULT


---------------------------------------------------------------------------

Lourival Oliveira wrote:
> I have a problem reading the "5.3 Default Values" Section of
> Postgresql8.0.0-rc1 documentation. In this section there are nothing about
> using 'now' constant. I think which there are a conflict with a note about
> 'now' in section "9.9. Date/Time Functions and Operators" which show the way
> to achieve the desired behavior. See:
>
> 5.3. Default Values
>
> ...
>
> The default value may be a scalar expression, which will be evaluated
> whenever the default value is inserted (not when the table is created).
>
>
> 9.9. Date/Time Functions and Operators
>
> ...
>
> SELECT CURRENT_TIMESTAMP;
> SELECT now();
> SELECT TIMESTAMP 'now';
> Note: You do not want to use the third form when specifying a DEFAULT clause
> while creating a table. The system will convert now to a timestamp as soon
> as the constant is parsed, so that when the default value is needed, the
> time of the table creation would be used! The first two forms will not be
> evaluated until the default value is used, because they are function calls.
> Thus they will give the desired behavior of defaulting to the time of row
> insertion.
>
> I think which a simple reference to any unpredictable behavior must be
> placed here to avoid confusion when anybody don't reach the desired behavior
> when use default values.
>
> What think you about my suggestion?
>
> Wait replies, Tanks.
>
> Lourival Oliveira da Silva
>
> PS: Sorry by my poor English.
>
>
>
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