Re: Table AM Interface Enhancements

Hi,

On 2024-04-12 01:04:03 +0300, Alexander Korotkov wrote:
> 1) If we just apply my revert patch and leave c6fc50cb4028 and
> 041b96802ef in the tree, then we get our table AM API narrowed. As
> you expressed the current API requires block numbers to be 1:1 with
> the actual physical on-disk location [2]. Not a secret I think the
> current API is quite restrictive. And we're getting the ANALYZE
> interface narrower than it was since 737a292b5de. Frankly speaking, I
> don't think this is acceptable.

As others already pointed out, c6fc50cb4028 was committed quite a while
ago. I'm fairly unhappy about c6fc50cb4028, fwiw, but didn't realize that
until it was too late.

> In token of all of the above, is the in-tree state that bad? (if we
> abstract the way 27bc1772fc and dd1f6b0c17 were committed).

To me the 27bc1772fc doesn't make much sense on its own. You added calls
directly to heapam internals to a file in src/backend/commands/, that just
doesn't make sense.

Leaving that aside, I think the interface isn't good on its own:
table_relation_analyze() doesn't actually do anything, it just sets callbacks,
that then later are called from analyze.c, which doesn't at all fit to the
name of the callback/function. I realize that this is kinda cribbed from the
FDW code, but I don't think that is a particularly good excuse.

I don't think dd1f6b0c17 improves the situation, at all. It sets global
variables to redirect how an individual acquire_sample_rows invocation
works:
void
block_level_table_analyze(Relation relation,
AcquireSampleRowsFunc *func,
BlockNumber *totalpages,
BufferAccessStrategy bstrategy,
ScanAnalyzeNextBlockFunc scan_analyze_next_block_cb,
ScanAnalyzeNextTupleFunc scan_analyze_next_tuple_cb)
{
*func = acquire_sample_rows;
*totalpages = RelationGetNumberOfBlocks(relation);
vac_strategy = bstrategy;
scan_analyze_next_block = scan_analyze_next_block_cb;
scan_analyze_next_tuple = scan_analyze_next_tuple_cb;
}

Notably it does so within the ->relation_analyze tableam callback, which does
*NOT* not actually do anything other than returning a callback. So if
->relation_analyze() for another relation is called, the acquire_sample_rows()
for the earlier relation will do something different. Note that this isn't a
theoretical risk, acquire_inherited_sample_rows() actually collects the
acquirefunc for all the inherited relations before calling acquirefunc.

This is honestly leaving me somewhat speechless.

Greetings,

Andres Freund