Re: TOAST usage setting

> > No, you misunderstood. Bruce was suggesting changing the target to
512.
> > That means if a row is wider than ~2k, toaster will try to toast
until 
> > the base row is
> > ~512 bytes. I would not do that part for 8.3. 
> 
> OK, what do you suggest for 8.3?  Attached are my suggestion 
> to use 512 and a 4k chunk size, which I think means that 2.7k 
> is the worst values that has a loss of around 25%.

Oh, so I misunderstood you also. You are suggesting two changes:
TOAST_TUPLES_PER_PAGE	16
EXTERN_TUPLES_PER_PAGE	2

While I agree, that 2 might be a good compromise with low risc for now,
I think
that toasting all rows down to ~512 bytes is too narrowly targeted at
not reading wider columns.

When suggesting a new target, I interpreted you like so:
#define TOAST_TUPLES_PER_PAGE	4
#define TOAST_TUPLE_TARGET	\
	MAXALIGN_DOWN((BLCKSZ - \
				   MAXALIGN(sizeof(PageHeaderData) +
(TOAST_TUPLES_PER_PAGE-1) * sizeof(ItemIdData))) \
				  / 16)
So we would only toast rows wider than 2k, but once toasting, toast the
base row down to 512.  

My suggestion would be to leave TOAST_TUPLES_PER_PAGE as is, because all
else would need extensive performance testing.
#define TOAST_TUPLES_PER_PAGE	4

My next suggestion would be to leave EXTERN_TUPLES_PER_PAGE as is, but:
Split data wider than a page into page sized chunks as long as they fill
whole pages.
Split the rest with EXTERN_TUPLES_PER_PAGE (4) as now.
This would not waste more space than currently, but improve performance
for very wide columns.

I can try to do a patch if you think that is a good idea, can't do a lot
of testing though.

Andreas