Re: Pluggable Storage - Andres's take

I wrote a little toy implementation that just returns constant data to
play with this a little. Looks good overall.

There were a bunch of typos in the comments in tableam.h, see attached.
Some of the comments could use more copy-editing and clarification, I
think, but I stuck to fixing just typos and such for now.

index_update_stats() calls RelationGetNumberOfBlocks(<table>). If the AM
doesn't use normal data files, that won't work. I bumped into that with
my toy implementation, which wouldn't need to create any data files, if
it wasn't for this.

The comments for relation_set_new_relfilenode() callback say that the AM
can set *freezeXid and *minmulti to invalid. But when I did that, VACUUM
hits this assertion:

TRAP: FailedAssertion("!(((classForm->relfrozenxid) >= ((TransactionId)
3)))", File: "vacuum.c", Line: 1323)

There's a little bug in index-only scan executor node, where it mixes up
the slots to hold a tuple from the index, and from the table. That
doesn't cause any ill effects if the AM uses TTSOpsHeapTuple, but with
my toy AM, which uses a virtual slot, it caused warnings like this from
index-only scans:

WARNING: problem in alloc set ExecutorState: detected write past chunk
end in block 0x56419b0f88e8, chunk 0x56419b0f8f90

Attached is a patch with the toy implementation I used to test this.
I'm not suggesting we should commit that - although feel free to do that
if you think it's useful - but it shows how I bumped into these issues.
The second patch fixes the index-only-scan slot confusion (untested,
except with my toy AM).

- Heikki