Github user mengxr commented on a diff in the pull request: https://github.com/apache/spark/pull/1025#discussion_r14694251 --- Diff: core/src/main/scala/org/apache/spark/rdd/PairRDDFunctions.scala --- @@ -195,6 +193,37 @@ class PairRDDFunctions[K, V](self: RDD[(K, V)]) } /** + * Return a subset of this RDD sampled by key (via stratified sampling). + * + * If exact set to true, we guarantee, with high probability, a sample size = + * math.ceil(fraction * S_i), where S_i is the size of the ith stratum (collection of entries + * that share the same key). When sampling without replacement, we need one additional pass over + * the RDD to guarantee sample size with a 99.99% confidence; when sampling with replacement, we + * need two additional passes. + * + * @param withReplacement whether to sample with or without replacement + * @param fractions map of specific keys to sampling rates + * @param seed seed for the random number generator + * @param exact whether sample size needs to be exactly math.ceil(fraction * size) per stratum + * @return RDD containing the sampled subset + */ + def sampleByKey(withReplacement: Boolean, + fractions: Map[K, Double], + exact: Boolean = true, + seed: Long = Utils.random.nextLong): RDD[(K, V)]= { + + require(fractions.forall {case(k, v) => v >= 0.0}, "Invalid sampling rates.") + + val samplingFunc = if (withReplacement) { + val counts = if (exact) Some(this.countByKey()) else None --- End diff -- `counts` is not used outside `getPoissionSamplingFunction`. Shall we move it into `getPoissonSamplingFunction`?
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