> If I were switch the default, for me zstd goes first, lz4 is a close > second, and pglz is the last of its class. The only reason why we > have not chosen zstd for TOAST is the fact that we don't support it > (trickier to add support for it as we have to preserve on-disk > compatibility for inline compressed TOAST entries).
> In short, lz4 is available in many environments, but I'm also ready to > bet that zstd is equally available in these environments. Thanks for the feedback.
Yes, I did consider zstd as well. I agree that zstd can provide a better compression ratio than lz4, and if we only look at compression ratio, zstd is a very attractive choice.
The reason I started with lz4 was mainly to keep this change aligned with the recent TOAST compression change. Since lz4 has already been accepted there as the preferred default when available, I thought using the same choice for wal_compression=on would make the proposal smaller, more consistent, and easier to evaluate. so
Second, WAL compression happens on the WAL insertion path for full-page images, so compression speed and CPU overhead matter a lot. lz4 is generally a good fit for that trade-off: it still reduces WAL volume, while keeping compression latency and CPU cost lower than zstd in many workloads.
That said, I understand the argument for preferring zstd when it is available. I have prepared an alternative version where wal_compression=on selects zstd first, then lz4 if zstd is not available, and finally pglz as the fallback
