Modifying TOAST thresholds

In another thread I wrote:
> ... One thing I was just thinking about is that it's silly to have
> the threshold constrained so strongly by a desire that tuples in toast
> tables not be toastable. It would be trivial to tweak the heapam.c
> routines so that they simply don't invoke the toaster when relkind is
> 't', and then we could have independent choices of toast-tuple size and
> main-tuple size. This would be particularly good because in the current
> scheme you can't modify toast-tuple size without an initdb, but if that
> were decoupled there'd be no reason not to allow changes in the
> main-tuple thresholds.

After thinking about this more I'm convinced that the above is a good
idea, eg in heap_insert change

if (HeapTupleHasExternal(tup) || tup->t_len > TOAST_TUPLE_THRESHOLD)
heaptup = toast_insert_or_update(relation, tup, NULL, use_wal);
else
heaptup = tup;

to

if (relation->rd_rel->relkind == RELKIND_TOASTVALUE)
{
/* toast table entries should never be recursively toasted */
Assert(!HeapTupleHasExternal(tup));
heaptup = tup;
}
else if (HeapTupleHasExternal(tup) || tup->t_len > TOAST_TUPLE_THRESHOLD)
heaptup = toast_insert_or_update(relation, tup, NULL, use_wal);
else
heaptup = tup;

I also think that we ought to add TOAST_MAX_CHUNK_SIZE to the set of
compiled-in parameters that are recorded in pg_control and checked for
compatibility at startup (like BLCKSZ) --- this will prevent anyone from
shooting themselves in the foot while experimenting.

Any objections?

regards, tom lane