Thanks, tom. Considering the scenario where the indexed column is a function Var on a whole expression, it's really not a good idea to disable creating index on whole expression. I tried find_composite_type_dependencies, it seems that this function can only detect dependencies created by statements such as 'CREATE INDEX test_tbl1_idx ON test_tbl1((row(x,y)::test_type1))', and cannot detect dependencies created by statements such as 'CREATE INDEX test_tbl1_idx ON test_tbl1((test _tbl1))'. After the execution of the former sql statement, 4 rows are added to the pg_depend table, one of which is the index -> pg_type dependency. After the latter sql statement is executed, only one row is added to the pg_depend table, and there is no index -> pg_type dependency, so I guess this function doesn't detect all cases of index on whole-row expression. And I would suggest to do the detection when the index is created, because then we can get the details of the index and give a warning in the way you mentioned.
Tom Lane <t...@sss.pgh.pa.us> 于2023年12月13日周三 23:01写道： > 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 >
