Github user marmbrus commented on a diff in the pull request:
https://github.com/apache/spark/pull/12008#discussion_r57617913
--- Diff:
sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/analysis/Analyzer.scala
---
@@ -1585,3 +1586,110 @@ object ResolveUpCast extends Rule[LogicalPlan] {
}
}
}
+
+/**
+ * Maps a time column to multiple time windows using the Expand operator.
Since it's non-trivial to
+ * figure out how many windows a time column can map to, we over-estimate
the number of windows and
+ * filter out the rows where the time column is not inside the time window.
+ */
+object TimeWindowing extends Rule[LogicalPlan] {
+
+ /**
+ * Depending on the operation, the TimeWindow expression may be wrapped
in an Alias (in case of
+ * projections) or be simply by itself (in case of groupBy),
+ * @param f The function that we want to apply on the TimeWindow
expression
+ * @return The user defined function applied on the TimeWindow expression
+ */
+ private def getWindowExpr[E](f: TimeWindow => E):
PartialFunction[Expression, E] = {
+ case Alias(windowExpr: TimeWindow, _) => f(windowExpr)
+ case windowExpr: TimeWindow => f(windowExpr)
+ }
+
+ /** Pass on other columns that are required either in a projection or
grouping. */
+ private def getMissingAttributes(plan: LogicalPlan):
Seq[NamedExpression] = {
+ val expressions = plan match {
+ case p: Project => p.projectList
+ case a: Aggregate => a.groupingExpressions
+ case _ => Seq.empty
+ }
+ expressions.flatMap(_.references)
+ }
+
+ /**
+ * Generates the logical plan for generating window ranges on a
timestamp column. Without
+ * knowing what the timestamp value is, it's non-trivial to figure out
deterministically how many
+ * window ranges a timestamp will map to given all possible combinations
of a window duration,
+ * slide duration and start time (offset). Therefore, we express and
over-estimate the number of
+ * windows there may be, and filter the valid windows. We use last
Project operator to group
+ * the window columns into a struct so they can be accessed as
`window.start` and `window.end`.
+ *
+ * The windows are calculated as below:
+ * maxNumOverlapping <- ceil(windowDuration / slideDuration)
+ * for (i <- 0 until maxNumOverlapping)
+ * windowId <- ceil((timestamp - startTime) / slideDuration)
+ * windowStart <- windowId * slideDuration + (i - maxNumOverlapping) *
slideDuration + startTime
+ * windowEnd <- windowStart + windowDuration
+ * return windowStart, windowEnd
+ *
+ * This behaves as follows for the given parameters for the time: 12:05.
The valid windows are
+ * marked with a +, and invalid ones are marked with a x. The invalid
ones are filtered using the
+ * Filter operator.
+ * window: 12m, slide: 5m, start: 0m :: window: 12m, slide: 5m, start: 2m
+ * 11:55 - 12:07 + 11:52 - 12:04 x
+ * 12:00 - 12:12 + 11:57 - 12:09 +
+ * 12:05 - 12:17 + 12:02 - 12:14 +
+ *
+ * @param p The logical plan
+ * @return the logical plan that will generate the time windows using
the Expand operator, with
+ * the Filter operator for correctness and Project for usability.
+ */
+ private def generateWindows(p: LogicalPlan): LogicalPlan = {
+ val windowExpr = p.expressions.collect(getWindowExpr[TimeWindow](e =>
e)).head
+ // get all expressions we need to pass on for projections
+ val otherExpressions =
+
getMissingAttributes(p).filterNot(_.children.exists(_.isInstanceOf[TimeWindow]))
+
+ val projections = Seq.tabulate(windowExpr.maxNumOverlapping + 1) { i =>
+ // windowId <- ceil((timestamp - startTime) / slideDuration)
+ val division = Ceil(Divide(Subtract(Cast(windowExpr.timeColumn,
LongType),
+ Literal(windowExpr.startTime)), Literal(windowExpr.slideDuration)))
+ // start <- (windowId + i - maxNumOverlapping) * slideDuration +
startTime
+ // the 1000000 is necessary for properly casting a LongType to a
TimestampType
+ val windowStart =
+ Multiply(
+ Add(
+ Multiply(
+ Add(division, Literal(i - windowExpr.maxNumOverlapping)),
+ Literal(windowExpr.slideDuration)),
+ Literal(windowExpr.startTime)),
+ Literal(1000000))
--- End diff --
This might be easier to read if it was written with the dsl.
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