Re: [HACKERS] generated columns

On Tue, Oct 30, 2018 at 09:35:18AM +0100, Peter Eisentraut wrote:
> 1. (current implementation) New column attgenerated contains 's' for
> STORED, 'v' for VIRTUAL, '\0' for nothing.  atthasdef means "there is
> something in pg_attrdef for this column".
> 
> 2. Alternative: A generated column has attgenerated = s/v but atthasdef
> = false, so that atthasdef means specifically "column has a default".
> Then a column would have a pg_attrdef entry for either attgenerated !=
> '\0' or atthasdef = true.
> 
> 3. Radical alternative: Collapse everything into one new column.  We
> could combine atthasdef and attgenerated and even attidentity into a new
> column.  (Only one of the three can be the case.)  This would give
> client code a clean break, which may or may not be good.  The
> implementation would be uglier than #1 but probably cleaner than #2.  We
> could also get 4 bytes back per pg_attribute row.
> 
> I'm happy with the current choice #1, but it's worth thinking about.

#3 looks very appealing in my opinion as those columns have no overlap,
so it would take five possible values:
- generated always
- generated by default
- default value
- stored expression
- virtual expression

Obviously this requires a first patch to combine the catalog
representation of the existing columns atthasdef and attidentity.

+                   ereport(ERROR,
+                           (errcode(ERRCODE_FEATURE_NOT_SUPPORTED),
+                            errmsg("generated colums are not supported
on typed tables")));
s/colums/columns/.
--
Michael