Re: custom function for converting human readable sizes to bytes

I didn't check out earlier versions of this patch, but the latest one still changes pg_size_pretty() to emit PB suffix.

I don't think it is worth it to throw a number of changes together like that.  We should focus on adding pg_size_bytes() first and make it compatible with both pg_size_pretty() and existing GUC units: that is support suffixes up to TB and make sure they have the meaning of powers of 2^10, not 10^3.  Re-using the table present in guc.c would be a plus.

Next, we could think about adding handling of PB suffix on input and output, but I don't see a big problem if that is emitted as 1024TB or the user has to specify it as 1024TB in a GUC or argument to pg_size_bytes(): an minor inconvenience only.

Last version still support BP in pg_size_pretty. It isn't big change. PB isn't issue.

We have to do significant decision - should to support SI units in pg_size_bytes? We cannot to change it later. There is disagreement for SI units in pg_size_pretty, so SI units in pg_size_bytes can be inconsistent.