Re: Fix bug with indexes on whole-row expressions

ywgrit <yw987194828@gmail.com> writes:
> I forbid to create indexes on whole-row expression in the following patch.
> I'd like to hear your opinions.

As I said in the previous thread, I don't think this can possibly
be acceptable.  Surely there are people depending on the capability.
I'm not worried so much about the exact case of an index column
being a whole-row Var --- I agree that that's pretty useless ---
but an index column that is a function on a whole-row Var seems
quite useful.  (Your patch fails to detect that, BTW, which means
it does not block the case presented in bug #18244.)

I thought about extending the ALTER TABLE logic to disallow changes
in composite types that appear in index expressions.  We already have
find_composite_type_dependencies(), and it turns out that this already
blocks ALTER for the case you want to forbid, but we concluded that we
didn't need to prevent it for the bug #18244 case:

         * If objsubid identifies a specific column, refer to that in error
         * messages.  Otherwise, search to see if there's a user column of the
         * type.  (We assume system columns are never of interesting types.)
         * The search is needed because an index containing an expression
         * column of the target type will just be recorded as a whole-relation
         * dependency.  If we do not find a column of the type, the dependency
         * must indicate that the type is transiently referenced in an index
         * expression but not stored on disk, which we assume is OK, just as
         * we do for references in views.  (It could also be that the target
         * type is embedded in some container type that is stored in an index
         * column, but the previous recursion should catch such cases.)

Perhaps a reasonable answer would be to issue a WARNING (not error)
in the case where an index has this kind of dependency.  The index
might need to be reindexed --- but it might not, too, and in any case
I doubt that flat-out forbidding the ALTER is a helpful idea.

                        regards, tom lane