Re: Index AM API cleanup

> On Aug 26, 2024, at 5:21 AM, Peter Eisentraut <peter(at)eisentraut(dot)org> wrote:
>
> On 21.08.24 21:25, Mark Dilger wrote:
>> The next twenty patches are a mix of fixes of various layering
>> violations, such as not allowing non-core index AMs from use in replica
>> identity full, or for speculative insertion, or for foreign key
>> constraints, or as part of merge join; with updates to the "treeb" code
>> as needed. The changes to "treeb" are broken out so that they can also
>> easily be excluded from whatever gets committed.
>
> I made a first pass through this patch set.

Peter, thanks for the review!

> I think the issues it aims to address are mostly legitimate. In a few cases, we might need some more discussion and perhaps will end up slicing the APIs a bit differently. The various patches that generalize the strategy numbers appear to overlap with things being discussed at [0], so we should see that the solution covers all the use cases.
>
> [0]: https://www.postgresql.org/message-id/flat/CA+renyUApHgSZF9-nd-a0+OPGharLQLO=mDHcY4_qQ0+noCUVg(at)mail(dot)gmail(dot)com

Paul, it seems what you are doing in v39-0001-Add-stratnum-GiST-support-function.patch is similar to what I am doing in v17-0012-Convert-strategies-to-and-from-row-compare-types.patch. In particular, your function

+
+/*
+ * Returns the btree number for supported operators, otherwise invalid.
+ */
+Datum
+gist_stratnum_btree(PG_FUNCTION_ARGS)
+{
+ StrategyNumber strat = PG_GETARG_UINT16(0);
+
+ switch (strat)
+ {
+ case RTEqualStrategyNumber:
+ PG_RETURN_UINT16(BTEqualStrategyNumber);
+ case RTLessStrategyNumber:
+ PG_RETURN_UINT16(BTLessStrategyNumber);
+ case RTLessEqualStrategyNumber:
+ PG_RETURN_UINT16(BTLessEqualStrategyNumber);
+ case RTGreaterStrategyNumber:
+ PG_RETURN_UINT16(BTGreaterStrategyNumber);
+ case RTGreaterEqualStrategyNumber:
+ PG_RETURN_UINT16(BTGreaterEqualStrategyNumber);
+ default:
+ PG_RETURN_UINT16(InvalidStrategy);
+ }
+}

looks similar to the implementation of an amtranslate_rctype_function. Do you have any interest in taking a look?

> To make a dent, I picked out something that should be mostly harmless: Stop calling directly into _bt_getrootheight() (patch 0004). I think this patch is ok, but I might call the API function amgettreeheight instead of amgetrootheight. The former seems more general.

Peter, your proposed rename seems fine for the current implementation, but your suggestion below might indicate a different naming.

> I notice that _bt_getrootheight() is called only to fill in the IndexOptInfo tree_height field, which is only used by btcostestimate(), so in some sense this is btree-internal data. But making it so that btcostestimate() calls _bt_getrootheight() directly to avoid all that intermediate business seems too complicated, and there was probably a reason that the cost estimation functions don't open the index.
>
> Interestingly, the cost estimation functions for gist and spgist also look at the tree_height field but nothing ever fills it on. So with your API restructuring, someone could provide the missing API functions for those index types. Might be interesting.
>
> That said, there might be value in generalizing this a bit. If you look at the cost estimation functions in pgvector (hnswcostestimate() and ivfflatcostestimate()), they both have this pattern that btcostestimate() tries to avoid: They open the index, look up some number, close the index, then make a cost estimate computation with the number looked up. So another idea would be to generalize the tree_height field to some "index size data" or even "internal data for cost estimation". This wouldn't need to change the API much, since these are all just integer values, but we'd label the functions and fields a bit differently.

Would they be just integers? They could also be void*, with amgetrootheight_function returning data allocated in the current memory context. For btree, that would just be a four byte integer, but other indexes could return whatever they like. If you like that idea, I can code that up for v18, naming the field something like amgetcostestimateinfo_function.

—
Mark Dilger
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