Re: Pgstattuple on Sequences: Seeking Community Feedback on Potential Patch

On Mon, Aug 26, 2024 at 09:14:27PM +0530, Ayush Vatsa wrote:
> Given the situation, I see two potential paths forward:
> *1/ Reintroduce Support for Sequences in pgstattuple*: This would be a
> relatively small change. However, it's important to note that the purpose
> of pgstattuple is to provide statistics like the number of tuples, dead
> tuples, and free space in a relation. Sequences, on the other hand, return
> only one value at a time and don´t have attributes like dead tuples.
>
> [...]
>
> *2/ Explicitly Block Sequence Support in pgstattuple*: We could align
> sequences with other unsupported objects, such as foreign tables, by
> providing a more explicit error message.

While it is apparently pretty uncommon to use pgstattuple on sequences,
this is arguably a bug that should be fixed and back-patched. I've CC'd
Michael Paquier, who is working on sequence AMs and may have thoughts. I
haven't looked at his patch set for that, but I'm assuming that it might
fill in pg_class.relam for sequences, which would have the same effect as
option 1.

I see a couple of other places we might want to look into as part of this
thread. Besides pgstattuple, I see that pageinspect and pg_surgery follow
a similar pattern. pgrowlocks does, too, but that one seems intentionally
limited to RELKIND_RELATION. I also see that amcheck explicitly allows
sequences:

/*
* Sequences always use heap AM, but they don't show that in the catalogs.
* Other relkinds might be using a different AM, so check.
*/
if (ctx.rel->rd_rel->relkind != RELKIND_SEQUENCE &&
ctx.rel->rd_rel->relam != HEAP_TABLE_AM_OID)
ereport(ERROR,
(errcode(ERRCODE_FEATURE_NOT_SUPPORTED),
errmsg("only heap AM is supported")));

IMHO it would be good to establish some level of consistency here.

--
nathan