On 1/31/2007 12:41 PM, Tom Lane wrote:
"Pavan Deolasee" <[EMAIL PROTECTED]> writes:
On 1/31/07, Tom Lane <[EMAIL PROTECTED]> wrote:
The toast code takes pains to ensure that the tuples it creates won't be
subject to re-toasting. Else it'd be an infinite recursion.
I think I found it. The toast_insert_or_update() function gets into an
unnecessary recursion because of alignment issues. It thus toasts
already toasted data. This IMHO might be causing unnecessary
overheads for each toast operation.
Interesting --- I'd never seen this because both of my usual development
machines have MAXALIGN 8, and it works out that that makes
TOAST_MAX_CHUNK_SIZE 1986, which makes the actual toasted tuple size
2030, which maxaligns to 2032, which is still less than
TOAST_TUPLE_THRESHOLD. I think the coding was implicitly assuming that
TOAST_TUPLE_THRESHOLD would itself be a maxalign'd value, but it's not
necessarily (and in fact not, with the current page header size ---
I wonder whether the bug was originally masked because the page header
size was different??)
We can't change TOAST_MAX_CHUNK_SIZE without forcing an initdb, but I
think that it would be safe to remove the MAXALIGN'ing of the tuple
size in the tests in heapam.c, that is
if (HeapTupleHasExternal(tup) ||
(MAXALIGN(tup->t_len) > TOAST_TUPLE_THRESHOLD))
heaptup = toast_insert_or_update(relation, tup, NULL);
else
heaptup = tup;
becomes
if (HeapTupleHasExternal(tup) ||
(tup->t_len > TOAST_TUPLE_THRESHOLD))
heaptup = toast_insert_or_update(relation, tup, NULL);
else
heaptup = tup;
which'll save a cycle or two as well as avoid this corner case.
It seems like a number of the uses of MAXALIGN in tuptoaster.c
are useless/bogus as well. Comments?
Can't we maxalign the page header in the calculations?
Jan
regards, tom lane
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