Tom Lane írta:
I wrote:
I see another problem with this patch: the code added to
ATExecDropColumn is a crude hack.  It doesn't work anyway since this is
not the only possible way for columns to be dropped (another one that
comes to mind immediately is DROP TYPE ... CASCADE).  The only correct
way to handle things is to let the dependency mechanism do it.

I will try that.

Actually, the whole question of dependencies for generated columns
probably needs some thought.  What should happen if a function or
operator used in a GENERATED expression gets dropped?  The result
for a normal column's default expression is that the default expression
goes away, but the column is still there.  I suspect we don't want
that for a GENERATED column --- it would then be effectively a plain

No, I would want the DROP FUNCTION to be cancelled if used in
a GENERATED, but a DROP ... CASCADE would drop it, too.
So, DEPENDENCY_NORMAL will keep the referencing object
but DEPENDENCY_AUTO would drop it too if the referenced
object is dropped?

Along the same lines, is ALTER COLUMN DROP DEFAULT a legal operation
on a generated column?  What about just replacing the expression with

Neither of these options are legal for GENERATED columns,
AFAIK I prohibited them already.

Before you get too excited about making generated columns disappear
automatically in all these cases, consider that dropping a column
is not something to be done lightly --- it might contain irreplaceable

The standard says that the GENERATED column should be
dropped silently if either of the referenced columns is dropped.
I haven't seen anything about the expression, though.

On second thought maybe the right approach is just to allow the default
expression to be dropped the same as it would be for an ordinary column,
and to make sure that if a GENERATED column doesn't (currently) have a
default, it is treated the same as an ordinary column.

This leads to the further thought that maybe GENERATED is not a property
of a column at all, but of its default (ie, it should be stored in
pg_attrdef not pg_attribute, which would certainly make the patch less
messy).  AFAICS plain GENERATED merely indicates that the default
expression can depend on other columns, which is certainly a property
of the default --- you could imagine ALTER COLUMN SET DEFAULT GENERATED
AS ... to make a formerly plain column into a GENERATED one.  I'm not
entirely sure about ALWAYS though.

The standard says somewhere that GENERATED columns
can only be added to and dropped from a table.

My observation is: I deleted my hack from ATExecDropColumn()
and now I cannot drop the referenced column without CASCADE.
The comment in StoreAttrDefault() says the objects in the expression
will have dependencies registered. I guess "objects" also means functions?
This way, if I do explicit recordDependencyOn(, DEPENDENCY_AUTO)
on referenced columns then the standard requirement will
be satisfied, i.e. dropping columns will drop GENERATED columns
silently that reference the said column but . Am I right? Or how about using
recordDependencyOnSingleRelExpr(... , DEP_NORMAL, DEP_AUTO ) ?

                        regards, tom lane

Zoltán Böszörményi
Cybertec Geschwinde & Schönig GmbH

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