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

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
> <mailto: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
> <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.