Re: ALTER COLUMN to change GENERATED ALWAYS AS expression?

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".

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