Re: WAL compression setting after PostgreSQL LZ4 default change

Re: Andrey Borodin
> FWIW on 19beta2, one client: a seqscan setting hint bits on a freshly
> checkpointed 380MB table with wal_log_hints on, so ~47k full-page images
> and little else.  Best of three: off 765ms, pglz 1402ms, lz4 444ms, zstd
> 797ms, writing 382/105/107/67 MB of WAL (fsync off, so the I/O saving is
> understated).  lz4 is the only setting faster than no compression at all,
> while pglz costs nearly double for the size lz4 gives away for free.  zstd
> compresses a third better but pays for it in CPU, partly by allocating a
> ZSTD_CCtx per FPI - see the nearby thread "Compression of bigger WAL
> records".  +1 for "on" meaning lz4 where available.

v4 moves lz4 first.

(I did not include any reason in the commit message about choosing lz4
over zstd yet.)

One minor bit: The patch now has

#if defined(USE_LZ4)
#define DEFAULT_WAL_COMPRESSION·WAL_COMPRESSION_LZ4
#elif defined(USE_ZSTD)
#define DEFAULT_WAL_COMPRESSION·WAL_COMPRESSION_ZSTD

when the old one had
#ifdef USE_ZSTD
#elif defined(USE_LZ4)

I left it at "#if defined(USE_LZ4)" because that looks more
symmetrical.

Christoph