Re: Determine potential change in table size after a column dropped?

I need only drop the column and VACUUM FULL the table, and not the entire
DB, right?

On Sat, Jan 22, 2022 at 5:38 AM Ron <ronljohnsonjr(at)gmail(dot)com> wrote:

> On 1/22/22 1:43 AM, Vijaykumar Jain wrote:
>
>
>
> On Sat, Jan 22, 2022, 12:47 PM Wells Oliver <wells(dot)oliver(at)gmail(dot)com>
> wrote:
>
>> I have a large large large table with many many many rows, and it's a
>> certain size in pg_relation_size -- there's a timestamp with tz column on
>> this table that's mostly kind of useless, and I want to figure out how much
>> space it would free if we just dropped it. Can I easily do this?
>>
>
> 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.
>
>
> What about VACUUM FULL?
>
> --
> Angular momentum makes the world go 'round.
>

--
Wells Oliver
wells(dot)oliver(at)gmail(dot)com <wellsoliver(at)gmail(dot)com>