Github user mengxr commented on a diff in the pull request:
https://github.com/apache/spark/pull/11325#discussion_r53824820
--- Diff:
sql/core/src/main/scala/org/apache/spark/sql/DataFrameStatFunctions.scala ---
@@ -37,13 +37,27 @@ import org.apache.spark.util.sketch.{BloomFilter,
CountMinSketch}
final class DataFrameStatFunctions private[sql](df: DataFrame) {
/**
- * Calculate the approximate quantile of numerical column of a DataFrame.
+ * Calculates the approximate quantiles of a numerical column of a
DataFrame.
+ *
+ * Note on the target error:
+ *
+ * The result of this algorithm has the following deterministic bound:
+ * if the DataFrame has N elements and if we request the quantile `phi`
up to error `epsi`,
+ * then the algorithm will return a sample `x` from the DataFrame so
that the *exact* rank
+ * of `x` is close to (phi * N). More precisely:
+ *
+ * floor((phi - epsi) * N) <= rank(x) <= ceil((phi + epsi) * N)
+ *
+ *
* @param col the name of the column
- * @param quantile the quantile number
- * @return the approximate quantile
+ * @param quantiles a list of quantiles to approximate. Each number must
belong to [0, 1]. For example 0 is the
--- End diff --
* Please rename `quantiles` to `probabilities`. For the documentation, we
can take R's doc as reference: http://www.inside-r.org/r-doc/stats/quantile.
* This line too long. Please run `dev/lint-scala` to check style.
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