Thread: TOAST & vacuum
FYI, TOAST is now vacuum-safe. When needed, the toaster creates a second heap tuple, containing only plain or compressed values. This one is then returned by the heap access methods to the caller, so indices will never contain external references. The changes are covered by #ifdef TOAST_INDICES, so can easily be disabled at the time we have file versioning and can recreate indices during vacuum. Jan -- #======================================================================# # It's easier to get forgiveness for being wrong than for being right. # # Let's break this rule - forgive me. # #================================================== JanWieck@Yahoo.com #
JanWieck@t-online.de (Jan Wieck) writes:
> TOAST is now vacuum-safe. When needed, the toaster creates a
> second heap tuple, containing only plain or compressed
> values. This one is then returned by the heap access methods
> to the caller, so indices will never contain external
> references.
That should be sufficient for insertions into existing indexes,
but what about CREATE INDEX on a column that already contains
toasted values? That works with fetched tuples, not ones formed
during insert/update.
I think a cleaner and more reliable short-term hack would be to twiddle
index_formtuple() to detoast any externally-stored attributes. AFAIK,
in current sources all paths for creating an index tuple go through that
routine, and it has a tupdesc handy so it knows which attributes are of
varlena type.
This way you wouldn't need to hack up the tuptoaster itself.
Also, this would work for functional indexes whereas the way you are
doing it will not (a function could return a toasted Datum extracted
from some other table, no?).
regards, tom lane
Tom Lane wrote: > JanWieck@t-online.de (Jan Wieck) writes: > > TOAST is now vacuum-safe. When needed, the toaster creates a > > second heap tuple, containing only plain or compressed > > values. This one is then returned by the heap access methods > > to the caller, so indices will never contain external > > references. > > That should be sufficient for insertions into existing indexes, > but what about CREATE INDEX on a column that already contains > toasted values? That works with fetched tuples, not ones formed > during insert/update. > > I think a cleaner and more reliable short-term hack would be to twiddle > index_formtuple() to detoast any externally-stored attributes. AFAIK, > in current sources all paths for creating an index tuple go through that > routine, and it has a tupdesc handy so it knows which attributes are of > varlena type. > > This way you wouldn't need to hack up the tuptoaster itself. You're right. Will do it that way. > Also, this would work for functional indexes whereas the way you are > doing it will not (a function could return a toasted Datum extracted > from some other table, no?). Don't know of a function that does it that way right now. But that doesn't mean no such exists - you're right again. 2 donut's for U. Jan -- #======================================================================# # It's easier to get forgiveness for being wrong than for being right. # # Let's break this rule - forgive me. # #================================================== JanWieck@Yahoo.com #
> FYI, > > TOAST is now vacuum-safe. When needed, the toaster creates a > second heap tuple, containing only plain or compressed > values. This one is then returned by the heap access methods > to the caller, so indices will never contain external > references. > > The changes are covered by #ifdef TOAST_INDICES, so can > easily be disabled at the time we have file versioning and > can recreate indices during vacuum. Seems this has some performance advantages. Un-toasting to traverse a btree index would be pretty slow. This way, it is all in the index. -- Bruce Momjian | http://candle.pha.pa.us pgman@candle.pha.pa.us | (610) 853-3000+ If your life is a hard drive, | 830 Blythe Avenue + Christ can be your backup. | Drexel Hill, Pennsylvania19026