Cache GroupByHash raw values where appropriate #25294
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Special cases the scenario where a single FlatHashStrategy chunk is used in code generation to avoid allocating a MutableVariableWidthOffset container as part of the hashFlat logic. Also adds a specialized method signature for valueIdentical for flat vs variable width types to avoid the same MutableVariableWidthOffset allocations.
Adds logical to dynamically decide whether to cache hash values into the hash table itself based on whether the GroupByHash instance is spillable, whether container types are present in the grouping keys, the number of variable width types, and the number of types overall. Caching hash values in GroupByHash is a classic compute / memory trade-off. Caching the value costs more memory (8 extra bytes per record) but avoids relatively more expensive hash recalculation when re-hashing the table or sorting the table contents by raw hash value for spilling. Caching the hash value can also make inserting new entries cheaper by avoiding the need for relatively more expensive valueIdentical checks when the hash value can prove that the values are not identical.
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Description
Adds logical to dynamically decide whether to cache hash values into
the hash table itself based on whether the
GroupByHashinstance isspillable, container types are present in the grouping keys, there are 2 or more variable width types present, or there are 3 or more types in total.
Caching hash values in
GroupByHashis a classic compute / memorytrade-off. Caching the value costs more memory (8 extra bytes per
record) but avoids relatively more expensive hash recalculation when
re-hashing the table or sorting the table contents by raw hash value for
spilling.
Caching the hash value can also make inserting new entries
cheaper by avoiding the need for relatively more expensive
valueIdenticalchecks when the hash value can prove that the values arenot identical.
Additional context and related issues
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