What does Page Layout version mean? (Was: Re: Reducing NUMERIC size for 8.3)

Simon Riggs wrote:

> 
> The objections to applying this patch originally were:
> 1. it changes on-disk format (we've done this, so argument is void)

I'm little bit confused when we introduce new page layout version? I 
expect that new version become with changes with pageheader, tuple 
header or data encoding (varlen/TOAST ...). But in case when there is 
new data type internal implementation, there was not reason to update 
version (see inet/cidr between 8.1 -> 8.2). Can me somebody clarify this?


However, In upgrade point of view. It will be necessary to keep 
information which postgreSQL version creates page to correctly track 
data type implementation (or special structure). Information in 
pg_control file and PG_VERSION files are unusable for upgrade purpose.

		Zdenek