Gabriella Lotz created KUDU-3784:
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Summary: Support scan predicates on columns that are not in the
projection
Key: KUDU-3784
URL: https://issues.apache.org/jira/browse/KUDU-3784
Project: Kudu
Issue Type: Improvement
Reporter: Gabriella Lotz
When a scan projects no (or few) columns but has predicates on columns outside
the projection, those predicates cannot be evaluated by the
MaterializingIterator. The most common case is a counting scan (SELECT
count(...)) with predicates that are not folded into the key range.
Today these out-of-projection predicates are skipped (see the TODO in
MaterializingIterator::Init). This is safe on the server scan path because
TabletServiceImpl adds predicate columns to the projection via
ScanSpec::GetMissingColumns(), so the only skipped predicates are redundant
ones lifted from the rowset's primary key bounds. But the lower-level scan path
(no missing-column expansion) has no such guarantee, so a genuine predicate
could be silently dropped.
Proper fix: materialize the needed columns (or otherwise evaluate the
predicate) instead of skipping, so predicates on non-projected columns are
honored generally. The disabled case TestPushdownIntValueRange (count-only
path) in tablet-pushdown-test.cc should be re-enabled as part of this.
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