Sorry, I mixed up the names. What i meant is that X_3 seems the result container created by the union, and X_19 seems the column of a (or a view a view of it).
If X_19 were really the column of a, then you could not reuse it. Most probably though X_19 is the result of the original column and the delta bats (updates,deletions). In that case i guess that yes, it could be reused as you say, but the pattern is not detected by any optimizer.
I'd advise you to post a bit more complete MAL subplan, then it will be easier for the developers to help you.
Roberto
Hi Roberto,
I wrote:
>> X_3:bat[:oid,:lng] =<tmp_321264>[0] := bat.new(nil:oid,nil:lng);
>> ..
>> # X_3 is never used before this line.
>> X_19=<tmp_331015>[23068647] := <some computation>
>> X_20=<tmp_321264>[23068647] := bat.append(X_3=<tmp_321264>:bat[:oid,:lng][23068647],
>> X_19=<tmp_331015>[23068647],true);
>> # X_3 is never used after this line.
Roberto Cornacchia <roberto.cornacchia@gmail.com> writes:
> Unless I misunderstand the problem, I think you can't just use X_3
> because that is the bat that corresponds to the physical column of
> your table a. So you can't take it and append b to it.
> Union creates a copy of a, then appends b, then c.
Thanks for your reply.
No, here X_3 is a new BAT (it is the result of bat.new).
The BAT from a physical column come from call to sql.bind.
But I'm new to MonetDB, so I might be missing some details too.
The value between brackets ([]) indicate the number of rows after the
call, and we can see that at first X_3 is empty (as expected), that X_19
contains a lot of rows (23M). And then this one is concatened to the
empty one. As a result of bat.append, we get the same BAT referenced by
X_3 (we see its internal name between <> as temp_321264), not some
another BAT created by the call itself.
Moreover, what I didn't specify, but was implied in my original post, is
that X_19 is no longer used after the call to bat.append. We see that
X_3 become of copy of X_19, and it replaces it entirely. This is
damageable for the performance.
--
Frédéric Jolliton
Sécuractive
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