Re: ALTER COLUMN to change GENERATED ALWAYS AS expression?

Ignore this...

On 2/7/23 13:05, Ron wrote:
> On 2/7/23 09:06, Adrian Klaver wrote:
>> On 2/7/23 06:09, Philip Semanchuk wrote:
>>>
>>>
>>>> On Feb 7, 2023, at 3:30 AM, Laurenz Albe <laurenz(dot)albe(at)cybertec(dot)at> wrote:
>>>>
>>>> On Mon, 2023-02-06 at 12:04 -0500, Philip Semanchuk wrote:
>>>>> I have a column defined GENERATED ALWAYS AS {my_expression} STORED.
>>>>> I’d like to change the
>>>>> {my_expression} part. After reading the documentation for ALTER TABLE
>>>>> (https://www.postgresql.org/docs/current/sql-altertable.html) and
>>>>> trying a few things that
>>>>> resulted in syntax errors, there doesn’t seem to be a way to alter the
>>>>> column’s GENERATED
>>>>> expression in place. It seems like my only option is to drop and
>>>>> re-add the column.
>>>>> Is that correct?
>>>>
>>>> I think that is correct.  But changing the expression would mean
>>>> rewriting the column
>>>> anyway.  The only downside is that a dropped column remains in the
>>>> table, and no even
>>>> a VACUUM (FULL) will get rid of it.
>>>
>>> Thanks for the confirmation. I hadn’t realized that the column would
>>> remain in the table even after a DROP + VACUUM FULL. I’m curious — its
>>> presence as a deleted column doesn't  affect performance in any
>>> meaningful way, does it?
>>
>> From docs:
>>
>> https://www.postgresql.org/docs/current/sql-altertable.html
>>
>> "The DROP COLUMN form does not physically remove the column, but simply
>> makes it invisible to SQL operations. Subsequent insert and update
>> operations in the table will store a null value for the column. Thus,
>> dropping a column is quick but it will not immediately reduce the on-disk
>> size of your table, as the space occupied by the dropped column is not
>> reclaimed. The space will be reclaimed over time as existing rows are
>> updated.
>>
>> To force immediate reclamation of space occupied by a dropped column, you
>> can execute one of the forms of ALTER TABLE that performs a rewrite of
>> the whole table. This results in reconstructing each row with the dropped
>> column replaced by a null value."
>
> VACUUM FULL doesn't rewrite the table?
>
> The doc page seems to say that it does:
> https://www.postgresql.org/docs/current/sql-vacuum.html
> "|VACUUM FULL|rewrites the entire contents of the table into a new disk
> file with no extra space".
>
> --
> Born in Arizona, moved to Babylonia.

--
Born in Arizona, moved to Babylonia.