Gabriella Lotz created KUDU-3782:
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Summary: TServer aborts (SIGABRT / std::length_error) on COUNT(*)
scans with predicates
Key: KUDU-3782
URL: https://issues.apache.org/jira/browse/KUDU-3782
Project: Kudu
Issue Type: Bug
Components: tserver
Reporter: Gabriella Lotz
h3. Summary
When a tablet server serves a counting scan (COUNT(*) or COUNT(<constant>),
which uses an empty projection) that also carries column predicates,
MaterializingIterator::Init (in src/kudu/common/generic_iterators.cc) computes
the number of projected columns minus the number of predicates using mismatched
signed/unsigned types. When the projected column count is smaller than the
predicate count, the subtraction underflows to a very large value, which is
passed to std::vector::reserve, throwing an uncaught std::length_error and
aborting the process.
This happens because the projection for a counting scan is tiny, while during
initialization the rowset iterator (CFileSet::Iterator::OptimizePKPredicates →
LiftPrimaryKeyBounds) re-adds predicates derived from the rowset's primary key
bounds on key columns that are not in the projection. The guarding DCHECK_GE is
compiled out in release builds, so release builds reach the underflow. It is
intermittent because it depends on the rowset's on-disk key layout.
Trigger: a COUNT(*)/COUNT(<const>) query with predicates on a non-contiguous
subset of a composite primary key, against a rowset whose primary key bounds
share a long constant prefix.
h3. Fix (in MaterializingIterator::Init)
Remove the invalid DCHECK_GE, compute the reservation so it cannot underflow,
and skip predicates whose column is not in the projection (these are
rowset-derived and already enforced by the key range) instead of failing the
scan. A regression test covering a projection smaller than the predicate set
passes with this change, and the existing generic_iterators-test suite
continues to pass.
h3. Workaround
Rewrite affected counting queries so the scan projects real columns, for
example by wrapping the count in a sub-query that selects actual rows. A
non-empty projection avoids the code path entirely. Only COUNT(...) queries
that include a WHERE clause can trigger this.
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