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- F.approx_count_distinct (HyperLogLog++-based) gives fast, fixed-memory approximate distinct counts — ideal on billions of rows. The camelCase F.approxCountDistinct is only a deprecated alias (deprecated since Spark 2.1.0) that internally calls approx_count_distinct; use the snake_case name in current code. countDistinct is exact but slower. percentile_approx is for quantiles. sum is additive aggregation.