Re: Add LZ4 compression in pg_dump

On Wed, Mar 01, 2023 at 05:20:05PM +0100, Tomas Vondra wrote:
> On 2/25/23 15:05, Justin Pryzby wrote:
> > On Fri, Feb 24, 2023 at 11:02:14PM -0600, Justin Pryzby wrote:
> >> I have some fixes (attached) and questions while polishing the patch for
> >> zstd compression. The fixes are small and could be integrated with the
> >> patch for zstd, but could be applied independently.
> >
> > One more - WriteDataToArchiveGzip() says:
> >
> > + if (cs->compression_spec.level == 0)
> > + pg_fatal("requested to compress the archive yet no level was specified");
> >
> > That was added at e9960732a.
> >
> > But if you specify gzip:0, the compression level is already enforced by
> > validate_compress_specification(), before hitting gzip.c:
> >
> > | pg_dump: error: invalid compression specification: compression algorithm "gzip" expects a compression level between 1 and 9 (default at -1)
> >
> > 5e73a6048 intended that to work as before, and you *can* specify -Z0:
> >
> > The change is backward-compatible, hence specifying only an integer
> > leads to no compression for a level of 0 and gzip compression when the
> > level is greater than 0.
> >
> > $ time ./src/bin/pg_dump/pg_dump -h /tmp regression -t int8_tbl -Fp --compress 0 |file -
> > /dev/stdin: ASCII text
>
> FWIW I agree we should make this backwards-compatible - accept "0" and
> treat it as no compression.
>
> Georgios, can you prepare a patch doing that?

I think maybe Tomas misunderstood. What I was trying to say is that -Z
0 *is* accepted to mean no compression. This part wasn't quoted, but I
said:

> Right now, I think that pg_fatal in gzip.c is dead code - that was first
> added in the patch version sent on 21 Dec 2022.

If you run the diff command that I've been talking about, you'll see
that InitCompressorZlib was almost unchanged - e9960732 is essentially a
refactoring. I don't think it's desirable to add a pg_fatal() in a
function that's otherwise nearly-unchanged. The fact that it's
nearly-unchanged is a good thing: it simplifies reading of what changed.
If someone wants to add a pg_fatal() in that code path, it'd be better
done in its own commit, with a separate message explaining the change.

If you insist on changing anything here, you might add an assertion (as
you said earlier) along with a comment like
/* -Z 0 uses the "None" compressor rather than zlib with no compression */

--
Justin