On Wed, Dec 13, 2023 at 7:01 AM Tom Lane <t...@sss.pgh.pa.us> wrote:
> ywgrit <yw987194...@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 > WARNING can be easily overlooked. Users of mobile/web apps don't see Postgres WARNINGs. Forbidding ALTER sounds more reasonable. Do you see any good use cases for whole-row indexes? And for such cases, wouldn't it be reasonable for users to specify all columns explicitly? E.g.: create index on t using btree(row(c1, c2, c3));
