Re: Compression of full-page-writes

On Tue, Dec 9, 2014 at 4:09 AM, Robert Haas <robertmhaas(at)gmail(dot)com> wrote:
>
> On Sun, Dec 7, 2014 at 9:30 PM, Simon Riggs <simon(at)2ndquadrant(dot)com> wrote:
> > * parameter should be SUSET - it doesn't *need* to be set only at
> > server start since all records are independent of each other
>
> Why not USERSET? There's no point in trying to prohibit users from
> doing things that will cause bad performance because they can do that
> anyway.

Using SUSET or USERSET has a small memory cost: we should
unconditionally palloc the buffers containing the compressed data
until WAL is written out. We could always call an equivalent of
InitXLogInsert when this parameter is updated but that would be
bug-prone IMO and it does not plead in favor of code simplicity.
Regards,
--
Michael