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

On Wed, Dec 13, 2023 at 7:01 AM Tom Lane <tgl@sss.pgh.pa.us> wrote:

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

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));

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 <tgl@sss.pgh.pa.us> 于2023年12月13日周三 23:01写道：