Subject: Dropping column from big table

From: sud <[email-masked]>
Date: Thu, 11 Jul 2024 01:43:31 +0530

Hi All,
It's postgres database version 15.4. We have a table which is daily and is
approx. ~2TB in size having a total ~90 partitions. We have a requirement
to drop columns and add new columns to this table.

I Want to understand, If this can be done online? what is the fastest way
to drop/add columns from such a big table and what will be the consequence
of this in regards to vacuum, post this activity? Or if any other issues we
may face post this?

Regards
Sud

---

From: Adrian Klaver <[email-masked]>
Date: Wed, 10 Jul 2024 14:22:40 -0700

On 7/10/24 13:13, sud wrote:
> Hi All,
> It's postgres database version 15.4. We have a table which is daily and 
> is approx. ~2TB in size having a total ~90 partitions. We have a 
> requirement to drop columns and add new columns to this table.
> 
> I Want to understand, If this can be done online? what is the fastest 
> way to drop/add columns from such a big table and what will be the 
> consequence of this in regards to vacuum, post this activity? Or if any 
> other issues we may face post this?
> 
> Regards
> Sud

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.
"

-- 
Adrian Klaver
[email-masked]

---

From: sud <[email-masked]>
Date: Thu, 11 Jul 2024 08:58:18 +0530

> dropped column replaced by a null value.
Thank you so much. When you said *"you can execute one of the forms of
ALTER TABLE that performs a rewrite*
*of the whole table."* Does it mean that post "alter table drop column" the
vacuum is going to run longer as it will try to clean up all the rows and
recreate the new rows? But then how can this be avoidable or made better
without impacting the system performance and blocking others?