Re: Dropping column from big table

On Monday, July 15, 2024, David G. Johnston <david(dot)g(dot)johnston(at)gmail(dot)com>
wrote:

> On Monday, July 15, 2024, sud <suds1434(at)gmail(dot)com> wrote:
>
>>
>> However even with "vacuum full", the old rows will be removed completely
>> from the storage , but the new rows will always be there with the 'dropped'
>> column still existing under the hood along with the table storage, with
>> just carrying "null" values in it. […] Is this understanding correct?
>>
>
> No. The table rewrite process involves creating new tuples that exactly
> conform to the current row specification. The potentially non-null data
> present in live tuples for columns that have been dropped are not copied
> into the newly constructed tuples.
>
> https://github.com/postgres/postgres/blob/d2b74882cab84b9f4fdce0f2f32e89
> 2ba9164f5c/src/backend/access/heap/heapam_handler.c#L2499
>
>
My bad, stopped at the code comment. Apparently the data is just nulled,
not removed, the current row descriptor contains those columns with “is
dropped” and since this behavior doesn’t change the catalogs in this way
the new ones must as well. We just get the space back.

David J.