> 3) To maintain crash recovery chance and reduce the amount of 
> archive log, removal of  unnecessary full page writes from 
> archive logs is a good choice.

Definitely, yes. pg_compresslog could even move the full pages written
during backup out of WAL and put them in a different file that needs to
be applied before replay of the corresponding WAL after a physical
restore. This would further help reduce log shipping volume.

> To do this, we need both logical log and full page writes in WAL.

This is only true in the sense, that it allows a less complex
implementation of pg_compresslog.

Basically a WAL record consists of info about what happened and
currently eighter per tuple new data or a full page image. The info of
"what happened" together with the full page image is sufficient to
reconstruct the "per tuple new data". There might be a few WAL record
types (e.g. in btree split ?) where this is not so, but we could eighter
fix those or not compress those.

This is why I don't like Josh's suggested name of wal_compressable
WAL is compressable eighter way, only pg_compresslog would need to be
more complex if you don't turn off the full page optimization. I think a
good name would tell that you are turning off an optimization.
(thus my wal_fullpage_optimization on/off)


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