Github user cloud-fan commented on a diff in the pull request: https://github.com/apache/spark/pull/15959#discussion_r89257721 --- Diff: sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/plans/logical/Statistics.scala --- @@ -58,60 +61,170 @@ case class Statistics( } } + /** - * Statistics for a column. + * Statistics collected for a column. + * + * 1. Supported data types are defined in `ColumnStat.supportsType`. + * 2. The JVM data type stored in min/max is the external data type (used in Row) for the + * corresponding Catalyst data type. For example, for DateType we store java.sql.Date, and for + * TimestampType we store java.sql.Timestamp. + * 3. For integral types, they are all upcasted to longs, i.e. shorts are stored as longs. + * 4. There is no guarantee that the statistics collected are accurate. Approximation algorithms + * (sketches) might have been used, and the data collected can also be stale. + * + * @param distinctCount number of distinct values + * @param min minimum value + * @param max maximum value + * @param nullCount number of nulls + * @param avgLen average length of the values. For fixed-length types, this should be a constant. + * @param maxLen maximum length of the values. For fixed-length types, this should be a constant. */ -case class ColumnStat(statRow: InternalRow) { +case class ColumnStat( + distinctCount: BigInt, + min: Option[Any], + max: Option[Any], + nullCount: BigInt, + avgLen: Long, + maxLen: Long) { - def forNumeric[T <: AtomicType](dataType: T): NumericColumnStat[T] = { - NumericColumnStat(statRow, dataType) - } - def forString: StringColumnStat = StringColumnStat(statRow) - def forBinary: BinaryColumnStat = BinaryColumnStat(statRow) - def forBoolean: BooleanColumnStat = BooleanColumnStat(statRow) + /** + * Returns a map from string to string that can be used to serialize the column stats. + * The key is the name of the field (e.g. "ndv" or "min"), and the value is the string + * representation for the value. The deserialization side is defined in [[ColumnStat.fromMap]]. + * + * As part of the protocol, the returned map always contains a key called "version". + * In the case min/max values are null (None), they will be stored as "<null>". --- End diff -- I may miss the discussion, why not just remove the `max`, `min` entries in the map if they are null?
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