Re: index prefetching

On Wed, Apr 1, 2026 at 11:51 AM Peter Geoghegan <pg(at)bowt(dot)ie> wrote:
> I plan on clarifying things in this area for v20. It will fully
> embrace the idea that IndexFetchTableData is just the table AM
> specific part of IndexScanDescData. This will allow me to move most of
> the batch-related calls currently in indexam.c over to heapam/the
> table AM. That will make it very clear that the table AM fully owns
> the batch ring buffer (there'll continue to be indexam.c calls to do
> things like take a mark, since that is implemented in a way that's
> already table AM agnostic).

Attached is v20, which does things this way. Now it's completely clear
that the scan's batch ring buffer is fully controlled by the table AM.

There are now only 2 remaining indexbatch.c functions that get called
from indexam.c. Since these are both trivial, highly generic
functions, restructuring things so that heapam would call these two
seemed unnecessary.

> One problem with the current design is that it still treats
> IndexFetchTableData as not just the table AM piece of
> IndexScanDescData. I now realize that it works that way purely so we
> can avoid changing anything about index_fetch_tuple. But, as you say,
> we really *should* do that anyway -- it shouldn't need its own
> IndexFetchTableData (or its own IndexScanDescData), since it literally
> doesn't have anything to do with index scans.

v20 also repurposes and renames index_fetch_tuple (the new name is
fetch_tid). This clarifies that it's a specialized function used only
by callers that inherently need to pass their own TID (the 2 remaining
constraint enforcement callers). These callers don't really perform an
index scan at all (the TID might come from an index, but clearly
_bt_check_unique isn't performing an index scan, by any reasonable
definition).

IndexFetchTableData is no longer used by fetch_tid or its callers. As
discussed, this change makes it possible to pass an IndexScanDescData
pointer (not a IndexFetchTableData pointer) through to heapam
functions like heapam_index_fetch_reset and heapam_index_fetch_end
(since doing so won't break index_fetch_tuple/fetch_tid, now that
their dependency on IndexFetchTableData has been broken).

One consequence of this refactoring is that we can no longer just call
heapam_index_fetch_reset from indexam.c when restoring a mark -- that
will reset the scan's batchringbuf, which isn't appropriate here (we
usually don't change the batchringbuf at all, in the happy path we
only need to override scanPos with markPos). indexam.c now calls a new
heapam_index_fetch_restrpos function (through its table AM shim)
instead -- heapam handles restoring the mark on its behalf.

The addition of this new heapam_index_fetch_restrpos function seems
strictly better to me. Restoring a mark in v19 meant resetting
xs_vm_items back to 1 -- which is only appropriate during a true
rescan. As in many other areas, the table AM can fully see what's
going on (it has all the relevant context), which allows it to do
exactly the right thing (layering that obscures useful information
about what's really going on is generally something we want to avoid
IMV).

--
Peter Geoghegan