Add pg_walinspect function with block info columns

Hi,

When using pg_walinspect, and calling functions like
pg_get_wal_records_info(), I often wish that the various information in
the block_ref column was separated out into columns so that I could
easily access them and pass them to various other functions to add
information -- like getting the relname from pg_class like this:

SELECT n.nspname, c.relname, wal_info.*
  FROM pg_get_wal_records_extended_info(:start_lsn, :end_lsn) wal_info
  JOIN pg_class c
    ON wal_info.relfilenode = pg_relation_filenode(c.oid) AND
      wal_info.reldatabase IN (0, (SELECT oid FROM pg_database
                            WHERE datname = current_database()))
              JOIN pg_namespace n ON n.oid = c.relnamespace;


This has been mentioned in [1] amongst other places.

So, attached is a patch with pg_get_wal_records_extended_info(). I
suspect the name is not very good. Also, it is nearly a direct copy of
pg_get_wal_fpi_infos() except for the helper called to fill in the
tuplestore, so it might be worth doing something about that.

However, I am mainly looking for feedback about whether or not others
would find this useful, and, if so, what columns they would like to see
in the returned tuplestore.

Note that I didn't include the cumulative fpi_len for all the pages
since pg_get_wal_fpi_info() now exists. I noticed that
pg_get_wal_fpi_info() doesn't list compression information (which is in
the block_ref column of pg_get_wal_records_info()). I don't know if this
is worth including in my proposed function
pg_get_wal_records_extended_info().

- Melanie

[1] https://www.postgresql.org/message-id/CAH2-Wz%3DacGKoP8cZ%2B6Af2inoai0N5cZKCY13DaqXCwQNupK8qg%40mail.gmail.com