Re: Add pg_walinspect function with block info columns

On Thu, Mar 16, 2023 at 2:19 AM Bharath Rupireddy
<bharath.rupireddyforpostgres@gmail.com> wrote:
> On Wed, Mar 15, 2023 at 12:20 PM Michael Paquier <michael@paquier.xyz> wrote:
> > I am not sure to get the concern here.  As long as one is smart enough
> > with SQL, there is no need to perform a double scan of the contents of
> > pg_wal with a large scan on the start LSN.  If one wishes to only
> > extract some block for a given record type, or for a filter of your
> > choice, it is possible to use a LATERAL on pg_get_wal_block_info(),
> > say:
> > SELECT r.start_lsn, b.blockid
> >   FROM pg_get_wal_records_info('0/01000028', '0/1911AA8') AS r,
> >   LATERAL pg_get_wal_block_info(start_lsn, end_lsn) as b
> >   WHERE r.resource_manager = 'Heap2';
> >
> > This will extract the block information that you'd want for a given
> > record type.

The same information *already* appears in pg_get_wal_records_info()'s
block_ref output! Why should the user be expected to use a LATERAL
join (or any type of join) to get _the same information_, just in a
usable form?

> IIUC, the concern raised so far in this thread is not just on the
> performance of JOIN queries to get both block info and record level
> info, but on ease of using pg_walinspect functions. If
> pg_get_wal_block_info emits the record level information too (which
> turns out to be 50 LOC more), one doesn't have to be expert at writing
> JOIN queries or such, but just can run the function, which actually
> takes way less time (3sec) to scan the same 5mn WAL records [3].

That's exactly my concern, yes. As you say, it's not just the
performance aspect. Requiring users to write a needlessly ornamental
query is actively misleading. It suggests that block_ref is distinct
information from the blocks output by pg_get_wal_block_info().

-- 
Peter Geoghegan