Yuqi Yan created CASSANDRA-21796:
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Summary: Reduce single-partition read allocations with an early
Bloom filter check
Key: CASSANDRA-21796
URL: https://issues.apache.org/jira/browse/CASSANDRA-21796
Project: Apache Cassandra
Issue Type: Improvement
Reporter: Yuqi Yan
Assignee: Yuqi Yan
Single-partition reads currently create an
{{UnfilteredRowIteratorWithLowerBound}} for every SSTable selected by the
interval tree before consulting its Bloom filter. With many overlapping
SSTables, this creates unnecessary lazy iterators and synthetic lower-bound
markers for SSTables that cannot contain the requested partition.
{{MergeIterator.ManyToOne}} wraps each input in a {{{}Candidate{}}}, whose
constructor calls {{{}lowerBound(){}}}. Non-null synthetic bounds are inserted
into the merge heap. When consumed, they are discarded without reaching the row
or range-tombstone reducer. The candidate then advances its underlying
iterator, triggering lazy initialization and eventually the Bloom-filter check
in {{{}getPosition(){}}}.
When the Bloom filter rejects the key, the source contributes no row, but the
iterator, lower-bound marker, candidate, merge structures, and associated heap
processing have already been allocated or performed.
Simply checking the Bloom filter before constructing the lower-bound iterator
prevents rejected SSTables from entering the merge-source list, reducing object
allocation and CPU overhead.
(description polished with LLM)
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