Performance 18254756429: Improve hash grouping aggregation parallelism #2729
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Reference Issues/PRs
18254756429
What does this implement or fix?
Poor quality hash implementations of integral types, including at least some implementations of
std::hashare basically a static cast. e.g.std::hash<int64_t>{}(100) == 100. This is fast, but leads to poor distributions in our bucketing, where we mod the hash with the number of buckets. In particular, if performing a grouping hash on a timeseries where the time points are dates results in all of the rows being partitioned into bucket zero, which then results in no parallelism in the aggregation clause.Swap to using a consistent hash function across all supported platforms with improved uniformity.