Re: Column Filtering in Logical Replication

On 12/14/21 17:43, Alvaro Herrera wrote:
> On 2021-Dec-13, Alvaro Herrera wrote:
> 
>> I think this means we need a new OBJECT_PUBLICATION_REL_COLUMN value in
>> the ObjectType (paralelling OBJECT_COLUMN), and no new ObjectClass
>> value.  Looking now to confirm.
> 
> After working on this a little bit more, I realized that this is a bad
> idea overall.  It causes lots of complications and it's just not worth
> it.  So I'm back at my original thought that we need to throw an ERROR
> at ALTER TABLE .. DROP COLUMN time if the column is part of a
> replication column filter, and suggest the user to remove the column
> from the filter first and reattempt the DROP COLUMN.
> 
> This means that we need to support changing the column list of a table
> in a publication.  I'm looking at implementing some form of ALTER
> PUBLICATION for that.
> 

Yeah. I think it's not clear if this should behave more like an index or 
a view. When an indexed column gets dropped we simply drop the index. 
But if you drop a column referenced by a view, we fail with an error. I 
think we should handle this more like a view, because publications are 
externally visible objects too (while indexes are pretty much just an 
implementation detail).

But why would it be easier not to add new object type? We still need to 
check there is no publication referencing the column - either you do 
that automatically through a dependency, or you do that by custom code. 
Using a dependency seems better to me, but I don't know what are the 
complications you mentioned.

regards

-- 
Tomas Vondra
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