fujii.y...@df.mitsubishielectric.co.jp писал 2022-11-30 13:01:
Hi Mr.Pyhalov.
1) In previous version of the patch aggregates, which had
partialaggfn, were ok
to push down. And it was a definite sign that aggregate can be pushed
down.
Now we allow pushing down an aggregate, which prorettype is not
internal and
aggfinalfn is not defined. Is it safe for all user-defined (or
builtin) aggregates,
even if they are generally shippable? Aggcombinefn is executed locally
and we
check that aggregate function itself is shippable. Is it enough?
Perhaps, we
could use partialagg_minversion (like aggregates with
partialagg_minversion
== -1 should not be pushed down) or introduce separate explicit flag?
In what case partial aggregate pushdown is unsafe for aggregate which
has not internal aggtranstype
and has no aggfinalfn?
By reading [1], I believe that if aggcombinefn of such aggregate
recieves return values of original
aggregate functions in each remote then it must produce same value
that would have resulted
from scanning all the input in a single operation.
One more issue I started to think about - now we don't check
partialagg_minversion for "simple" aggregates at all. Is it correct? It
seems that , for example, we could try to pushdown bit_or(int8) to old
servers, but it didn't exist, for example, in 8.4. I think it's a
broader issue (it would be also the case already if we push down
aggregates) and shouldn't be fixed here. But there is an issue -
is_shippable() is too optimistic.
--
Best regards,
Alexander Pyhalov,
Postgres Professional
