Collection-typed slots are expensive to copy, e.g., during data
exchanges or when writing into a buffered-tuple-stream. Even worse,
such slots could be duplicated many times after unnesting in a
subplan. To alleviate this problem, this patch implements a
poor man's projection where collection-typed slots are set to NULL
inside the SubplanNode that flattens them.
The FE guarantees that the contents of an array-typed slot are never
referenced outside of the single UnnestNode that access them, so when
returning eos in UnnestNode::GetNext() we also set the unnested array
slot to NULL to avoid those expensive copies in downstream exec nodes.
The FE provides that guarantee by creating a new slot in the parent
scan for every relative CollectionTableRef. For example, for a table
't' with a collection-typed column 'c' the following query would have
two separate slots in the tuple of 't', one for 'c1' and one for 'c2':
select * from t, t.c c1, t.c c2
Change-Id: I90e5b86463019c9ed810c299945c831c744ff563
Reviewed-on: http://gerrit.cloudera.org:8080/763
Reviewed-by: Alex Behm <alex.behm@cloudera.com>
Tested-by: Internal Jenkins
deb9c6f8e6