On Thu, Feb 4, 2021 at 4:33 AM Amit Langote <amitlangote09@gmail.com> wrote:
> So would zheap refetch a tuple using the "ctid" column in the plan's
> output tuple and then use some other columns from the fetched tuple to
> actually do the update?
Yes.
> 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.
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.
--
Robert Haas
EDB: http://www.enterprisedb.com