Re: Add IS_INDEX macro to brin and gist index

On Wed, 14 Jan 2026 at 03:38, Andreas Karlsson <andreas(at)proxel(dot)se> wrote:
> On 1/14/26 2:56 AM, Japin Li wrote:
>> Hi, hackers,
>> While working on pageinspect [0], I noticed that brin_page_items()
>> and
>> gist_page_items() only checked the access method (IS_BRIN/IS_GIST) but did
>> not verify that the passed relation is actually an index relation.
>> To make the check more robust and consistent with other pageinspect
>> index
>> functions (like btreefuncs.c, hashfuncs.c, etc.), the attached patch:
>> 1. Defines a local helper macro IS_INDEX(r) in both brinfuncs.c and
>> gistfuncs.c.
>> 2. Updates the error check to require both: the relation must be an
>> index and
>> use the expected access method.
>> The change is very small, low-risk, and only affects two functions
>> in
>> contrib/pageinspect.
>
> Since the two functions you touch are gist_page_items() and
> brin_page_items() is there actually any harm from being able to use
> the index definition from a partitioned when parsing the page? Seems
> unnecessary to prevent people from doing so unless I am missing
> something. It is not like we have any way to prevent the wrong index
> from being used when parsing page the page so why prevent partitioned
> indexes specifically?
>

Thanks for the thoughtful question!

The reason I added the IS_INDEX check (and reject partitioned indexes) is that
a partitioned index in PostgreSQL doesn't actually store any data pages itself
it only exists as a logical parent for the partition-level indexes. So passing
a partitioned index OID to brin_page_items() or gist_page_items() would
fundamentally not make sense: there are no real pages to inspect, and trying
to read a page from it would fail.

postgres=# SELECT * FROM brin_page_items(get_raw_page('brin_test_create_at_idx', 0), 'brin_test_create_at_idx');
ERROR: cannot get raw page from relation "brin_test_create_at_idx"
DETAIL: This operation is not supported for partitioned indexes.

OTOH, this is also fully consistent with how other pageinspect functions (like
btree_page_items, hash_page_items, etc.) behave — they only accept concrete,
non-partitioned index relations that actually have physical pages.

--
Regards,
Japin Li
ChengDu WenWu Information Technology Co., Ltd.