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[SPARK-24953] [SQL] Prune a branch in CaseWhen if previously seen #21904

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dbtsai wants to merge 3 commits into from
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[SPARK-24953] [SQL] Prune a branch in CaseWhen if previously seen #21904

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4ac3d1e
prune casewhen branch
dbtsai Jul 27, 2018
4244991
temp
dbtsai Jul 30, 2018
6915647
refactoring
dbtsai Jul 30, 2018
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40 changes: 40 additions & 0 deletions sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/optimizer/expressions.scala
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Original file line number Diff line number Diff line change
Expand Up @@ -385,6 +385,40 @@ object SimplifyConditionals extends Rule[LogicalPlan] with PredicateHelper {
case _ => false
}

// If a condition in a branch is previously seen, this branch can be pruned.
// TODO: In fact, if a condition is a sub-condition of the previous one,
// TODO: it can be pruned. This is less strict and can be implemented
// TODO: by decomposing the seen conditions.
private def pruneSeenBranches(branches: Seq[(Expression, Expression)])
: Option[Seq[(Expression, Expression)]] = {
val newBranches = branches.foldLeft(new ArrayBuffer[(Expression, Expression)]()) {
case (newBranches, branch) if newBranches.exists(_._1.semanticEquals(branch._1)) =>
newBranches
case (newBranches, branch) => newBranches += branch
}
if (newBranches.length < branches.length) {
Some(newBranches)
} else {
None
}
}

// If the outputs of two adjacent branches are the same, two branches can be combined.
private def combineAdjacentBranches(branches: Seq[(Expression, Expression)])
: Option[Seq[(Expression, Expression)]] = {
val newBranches = branches.foldLeft(new ArrayBuffer[(Expression, Expression)]()) {
case (newBranches, branch)
if newBranches.nonEmpty && newBranches.last._2.semanticEquals(branch._2) =>
newBranches.init += ((Or(newBranches.last._1, branch._1), newBranches.last._2))
case (newBranches, branch) => newBranches += branch
}
if (newBranches.length < branches.length) {
Some(newBranches)
} else {
None
}
}

def apply(plan: LogicalPlan): LogicalPlan = plan transform {
case q: LogicalPlan => q transformExpressionsUp {
case If(TrueLiteral, trueValue, _) => trueValue
Expand Down Expand Up @@ -416,6 +450,12 @@ object SimplifyConditionals extends Rule[LogicalPlan] with PredicateHelper {
// these branches can be pruned away
val (h, t) = branches.span(_._1 != TrueLiteral)
CaseWhen( h :+ t.head, None)

case e @ CaseWhen(branches, _) if pruneSeenBranches(branches).nonEmpty =>
e.copy(branches = pruneSeenBranches(branches).get)

case e @ CaseWhen(branches, _) if combineAdjacentBranches(branches).nonEmpty =>
e.copy(branches = combineAdjacentBranches(branches).get)
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@viirya viirya Jul 30, 2018

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Maybe:

case e @ CaseWhen(branches, _) =>
  val prunedBranches = pruneSeenBranches(branches).getOrElse(branches)
  val newBranches = combineAdjacentBranches(prunedBranches).getOrElse(prunedBranches)
  if (newBranches.length < branches.length) {
    e.copy(branches = newBranches)
  } else {
    e
  }

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@viirya viirya Jul 30, 2018

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If pruneSeenBranches and combineAdjacentBranches both return input branches if no changes applied, this maybe more simplified:

case e @ CaseWhen(branches, _) =>
  val prunedBranches = pruneSeenBranches(branches)
  val newBranches = combineAdjacentBranches(prunedBranches)
  if (newBranches.length < branches.length) {
    e.copy(branches = newBranches)
  } else {
    e
  }

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@dbtsai dbtsai Jul 30, 2018

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This will work. But my only concern is that this will be caught all the CaseWhen cases; as a result, no more rule can be added after this.

}
}
}
Expand Down
54 changes: 53 additions & 1 deletion ...yst/src/test/scala/org/apache/spark/sql/catalyst/optimizer/SimplifyConditionalSuite.scala
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Expand Up @@ -46,7 +46,9 @@ class SimplifyConditionalSuite extends PlanTest with PredicateHelper {
private val unreachableBranch = (FalseLiteral, Literal(20))
private val nullBranch = (Literal.create(null, NullType), Literal(30))

private val testRelation = LocalRelation('a.int)
val isNotNullCond = IsNotNull(UnresolvedAttribute("a"))
val isNullCond = IsNull(UnresolvedAttribute("a"))
val notCond = Not(UnresolvedAttribute("c"))

test("simplify if") {
assertEquivalent(
Expand Down Expand Up @@ -122,4 +124,54 @@ class SimplifyConditionalSuite extends PlanTest with PredicateHelper {
None),
CaseWhen(normalBranch :: trueBranch :: Nil, None))
}

test("remove a branch in CaseWhen if a cond in this branch is previously seen") {
assertEquivalent(
CaseWhen((GreaterThan(Rand(0), Literal(0.5)), Literal(1)) ::
(GreaterThan(Rand(0), Literal(0.5)), Literal(2)) ::
(NonFoldableLiteral(true), Literal(3)) ::
(LessThan(Rand(1), Literal(0.5)), Literal(4)) ::
(NonFoldableLiteral(true), Literal(5)) ::
(NonFoldableLiteral(false), Literal(6)) ::
(NonFoldableLiteral(false), Literal(7)) ::
Nil,
None),
CaseWhen((GreaterThan(Rand(0), Literal(0.5)), Literal(1)) ::
(GreaterThan(Rand(0), Literal(0.5)), Literal(2)) ::
(NonFoldableLiteral(true), Literal(3)) ::
(LessThan(Rand(1), Literal(0.5)), Literal(4)) ::
(NonFoldableLiteral(false), Literal(6)) ::
Nil,
None)
)
}

test("combine two adjacent branches in CaseWhen if they have the same output values") {
assertEquivalent(
CaseWhen((GreaterThan(Rand(0), Literal(0.5)), Literal(1)) ::
(NonFoldableLiteral(true), Literal(1)) ::
(LessThan(Rand(1), Literal(0.5)), Literal(3)) ::
(NonFoldableLiteral(true), Literal(3)) ::
(NonFoldableLiteral(false), Literal(4)) ::
Nil,
None),
CaseWhen((Or(GreaterThan(Rand(0), Literal(0.5)), NonFoldableLiteral(true)), Literal(1)) ::
(LessThan(Rand(1), Literal(0.5)), Literal(3)) ::
(NonFoldableLiteral(false), Literal(4)) ::
Nil,
None)
)

// The first two conditions can be combined, and then the optimizer uses rule in `Or`
// to be optimized into `TrueLiteral`. Thus, the entire `CaseWhen` can be removed.
assertEquivalent(
CaseWhen((UnresolvedAttribute("a"), Literal(1)) ::
(Not(UnresolvedAttribute("a")), Literal(1)) ::
(LessThan(Rand(1), Literal(0.5)), Literal(3)) ::
(NonFoldableLiteral(true), Literal(4)) ::
(NonFoldableLiteral(false), Literal(5)) ::
Nil,
None),
Literal(1))
}
}