use random seeds for bernoulli sample when parallel is enabled #16223
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use random seeds for bernoulli sample when parallel is enabled (duckdb/duckdb#16223)
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fixes https://github.com/duckdblabs/duckdb-internal/issues/4203
fixes #16175
For smaller data sizes and a parallel Bernoulli sample, you could get skewed results. This is because the same seed was used for all threads. So if you have 10 threads and a data size of 1000, then every thread gets ~100 rows. In the example, the sample size was 1%. It's possible the random engine doesn't produce a value <0.01 for the first 100 randomly generated values. This means none of the threads return a value for the result.
The fix is to assign every thread a random seed, but if repeatable is set, then we set
ParallelSinkto false and we use the seed, guaranteeing a repeatable result.This PR is similar to how reservoir sampling currently behaves.
related PR
https://github.com/duckdb/duckdb/pull/14797/files