Re: TOAST usage setting

> 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.

I have a PoC patch running, but it is larger than expected because of
the size checks during read 
(toast_fetch_datum_slice not done, but would be straight forward).
Also the pg_control variable toast_max_chunk_size would need to be
renamed and reflect the
EXTERN_TUPLES_PER_PAGE (4) number and the fact that fullpage chunks are
used
(else the chunk size checks and slice could not work like now).

Should I pursue, keep for 8.4, dump it ?

The downside of this concept is, that chunks smaller than fullpage still
get split into the smaller pieces.
And the < ~8k chunks may well outnumber the > ~8k on real data. 
The up side is, that I do not see a better solution that would keep
slice cheap and still lower the overhead even for pathological cases.

Andreas