On Thu, Feb 4, 2021 at 10:41 PM Robert Haas <robertmhaas@gmail.com> wrote:
> On Thu, Feb 4, 2021 at 4:33 AM Amit Langote <amitlangote09@gmail.com> wrote:
> > To be clear, the new refetch in ExecModifyTable() is to fill in the
> > unchanged columns in the new tuple. If we rejigger the
> > table_tuple_update() API to receive a partial tuple (essentially
> > what's in 'planSlot' passed to ExecUpdate) as opposed to the full
> > tuple, we wouldn't need the refetch.
>
> I don't think we should assume that every AM needs the unmodified
> columns. Imagine a table AM that's a columnar store. Imagine that each
> column is stored completely separately, so you have to look up the TID
> once per column and then stick in the new values. Well, clearly you
> want to skip this completely for columns that don't need to be
> modified. If someone gives you all the columns it actually sucks,
> because now you have to look them all up again just to figure out
> which ones you need to change, whereas if they gave you only the
> unmodified columns you could just do nothing for those and save a
> bunch of work.
Right, that's the idea in case I wasn't clear. Currently, a slot
containing the full tuple is passed to the table AM, with or without
the patch. The API:
static inline TM_Result
table_tuple_update(Relation rel, ItemPointer otid, TupleTableSlot *slot, ...
describes its 'slot' parameter as:
* slot - newly constructed tuple data to store
We could, possibly in a follow-on patch, adjust the
table_tuple_update() API to only accept the changed values through the
slot.
> zheap, though, is always going to need to take another look at the
> tuple to do the update, unless you can pass up some values through
> hidden columns. I'm not exactly sure how expensive that really is,
> though.
I guess it would depend on how many of those hidden columns there need
to be (in addition to the existing "ctid" hidden column) and how many
levels of the plan tree they would need to climb through when bubbling
up.
--
Amit Langote
EDB: http://www.enterprisedb.com