Re: Variable length varlena headers redux

Greg Stark <gsstark(at)mit(dot)edu> writes:
> Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> writes:
>> I'd be inclined to put the intelligence into heap_form_tuple and thereby
>> avoid getting the TOAST code involved unless there are wide fields to
>> deal with.

> And have heap_deform_tuple / heap_getattr palloc and memcpy the the datum on
> the way out? Or wait until detoast time and then do it?

No, heap_deform_tuple / heap_getattr are not responsible for palloc'ing
anything, only for computing appropriate pointers into the tuple.
Existing functions that use PG_DETOAST_DATUM would incur a palloc to
produce a 4-byte-header version of a short-header datum. We could then
work on modifying one function at a time to use some alternative macro
that doesn't force a useless palloc, but the system wouldn't be broken
meanwhile; and only the high-traffic functions would be worth fixing
at all.

The point I'm trying to get across here is to do things one small step
at a time; if you insist on a "big bang" patch then it'll never get
done. You might want to go back and review the CVS history for some
other big changes like TOAST and the version-1 function-call protocol
to see our previous uses of this approach.

regards, tom lane