Re: Show various offset arrays for heap WAL records

On Tue, Jan 31, 2023 at 5:48 PM Peter Geoghegan <pg@bowt.ie> wrote:
>
> On Tue, Jan 31, 2023 at 1:52 PM Peter Geoghegan <pg@bowt.ie> wrote:
> > > I would also like to see functions like XLogRecGetBlockRefInfo() pass
> > > something more useful than a stringinfo buffer so that we could easily
> > > extract out the relfilenode in pgwalinspect.
> >
> > That does seem particularly important. It's a pain to do this from
> > SQL. In general I'm okay with focussing on pg_walinspect over
> > pg_waldump, since it'll become more important over time. Obviously
> > pg_waldump needs to still work, but I think it's okay to care less
> > about pg_waldump usability.
>
> I just realized why you mentioned XLogRecGetBlockRefInfo() -- it
> probably shouldn't even be used by pg_walinspect at all (just by
> pg_waldump). Using something like XLogRecGetBlockRefInfo() within
> pg_walinspect misses out on the opportunity to output information in a
> more descriptive tuple format, with real data types. It's not just the
> relfilenode, either -- it's the block numbers themselves. And the fork
> number.
>
> In other words, I suspect that this is out of scope for this patch,
> strictly speaking. We simply shouldn't be using
> XLogRecGetBlockRefInfo() in pg_walinspect in the first place. Rather,
> pg_walinspect should be calling some other function that ultimately
> allows the user to work with (say) an array of int8 from SQL for the
> block numbers. There is no great reason not to, AFAICT, since this
> information is completely generic -- it's not like the rmgr-specific
> output from GetRmgr(), where fine grained type information is just a
> nice-to-have, with usability issues of its own (on account of the
> details being record type specific).

Something like the attached?

start_lsn        | 0/19823390
end_lsn          | 0/19824360
prev_lsn         | 0/19821358
xid              | 1355
resource_manager | Heap
record_type      | UPDATE
record_length    | 4021
main_data_length | 14
fpi_length       | 3948
description      | off 11 xmax 1355 flags 0x00 ; new off 109 xmax 0
block_ref        |
[0:1][0:8]={{0,1663,5,17033,0,442,460,4244,0},{1,1663,5,17033,0,0,0,0,0}}

It is a bit annoying not to have information about what each block_ref
item in the array represents (previously in the string), so maybe the
format in the attached shouldn't be a replacement for what is already
displayed by pg_get_wal_records_info() and friends.

It could instead be a new function which returns information in this
format -- perhaps tuples with separate columns for each labeled block
ref field denormalized to repeat the wal record info for every block?

The one piece of information I didn't include in the new block_ref
columns is the compression type (since it is a string). Since I used the
forknum value instead of the forknum name, maybe it is defensible to
also provide a documented int value for the compression type and make
that an int too?

- Melanie