> You should forbid it. Unless you can decompose the command into multiple SQL
> commands to make it a safe operation for logical replication.
>
> Let's say you want to add a column with a volatile default.
>
> ALTER TABLE foo ADD COLUMN bar double precision DEFAULT random();
>
> If you replicate the DDL command as is, you will have different data
> downstream. You should forbid it. However, this operation can be supported if
> the DDL command is decomposed in multiple steps.
>
> -- add a new column without DEFAULT to avoid rewrite
> ALTER TABLE foo ADD COLUMN bar double precision;
>
> -- future rows could use the DEFAULT expression
> -- it also doesn't rewrite the table
> ALTER TABLE foo ALTER COLUMN bar SET DEFAULT random();
>
> -- it effectively rewrites the table
> -- all rows are built from one source node
> -- data will be the same on all nodes
> UPDATE foo SET bar = random();
>
> The ALTER TABLE ... ALTER COLUMN ... TYPE has a similar issue. This DDL command
> can be decomposed to avoid the rewrite. If you are changing the data type, in
> general, you add a new column and updates all rows doing the proper conversion.
> (If you are updating in batches, you usually add a trigger to automatically
> adjust the new column value for INSERTs and UPDATEs. Another case is when you
> are reducing the the typmod (for example, varchar(100) to varchar(20)). In this
> case, the DDL command can be decomposed removing the typmod information (ALTER
> TABLE ... ALTER COLUMN ... TYPE varchar) and replacing it with a CHECK
> constraint.
>
> I didn't review this patch in depth but we certainly need to impose some DDL
> restrictions if we are replicating DDLs. There are other cases that should be
> treated accordingly such as a TABLESPACE specification or a custom data type.
This is helpful. Thanks.
Zheng
