Github user ioana-delaney commented on a diff in the pull request:
https://github.com/apache/spark/pull/15363#discussion_r106790425
--- Diff:
sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/optimizer/joins.scala
---
@@ -20,19 +20,347 @@ package org.apache.spark.sql.catalyst.optimizer
import scala.annotation.tailrec
import org.apache.spark.sql.catalyst.expressions._
-import org.apache.spark.sql.catalyst.planning.ExtractFiltersAndInnerJoins
+import
org.apache.spark.sql.catalyst.planning.{ExtractFiltersAndInnerJoins,
PhysicalOperation}
import org.apache.spark.sql.catalyst.plans._
import org.apache.spark.sql.catalyst.plans.logical._
import org.apache.spark.sql.catalyst.rules._
+import org.apache.spark.sql.catalyst.CatalystConf
+
+/**
+ * Encapsulates star-schema join detection.
+ */
+case class StarSchemaDetection(conf: CatalystConf) extends PredicateHelper
{
+
+ /**
+ * Star schema consists of one or more fact tables referencing a number
of dimension
+ * tables. In general, star-schema joins are detected using the
following conditions:
+ * 1. Informational RI constraints (reliable detection)
+ * + Dimension contains a primary key that is being joined to the
fact table.
+ * + Fact table contains foreign keys referencing multiple dimension
tables.
+ * 2. Cardinality based heuristics
+ * + Usually, the table with the highest cardinality is the fact
table.
+ * + Table being joined with the most number of tables is the fact
table.
+ *
+ * To detect star joins, the algorithm uses a combination of the above
two conditions.
+ * The fact table is chosen based on the cardinality heuristics, and the
dimension
+ * tables are chosen based on the RI constraints. A star join will
consist of the largest
+ * fact table joined with the dimension tables on their primary keys. To
detect that a
+ * column is a primary key, the algorithm uses table and column
statistics.
+ *
+ * Since Catalyst only supports left-deep tree plans, the algorithm
currently returns only
+ * the star join with the largest fact table. Choosing the largest fact
table on the
+ * driving arm to avoid large inners is in general a good heuristic.
This restriction can
+ * be lifted with support for bushy tree plans.
+ *
+ * The highlights of the algorithm are the following:
+ *
+ * Given a set of joined tables/plans, the algorithm first verifies if
they are eligible
+ * for star join detection. An eligible plan is a base table access with
valid statistics.
+ * A base table access represents Project or Filter operators above a
LeafNode. Conservatively,
+ * the algorithm only considers base table access as part of a star join
since they provide
+ * reliable statistics.
+ *
+ * If some of the plans are not base table access, or statistics are not
available, the algorithm
+ * returns an empty star join plan since, in the absence of statistics,
it cannot make
+ * good planning decisions. Otherwise, the algorithm finds the table
with the largest cardinality
+ * (number of rows), which is assumed to be a fact table.
+ *
+ * Next, it computes the set of dimension tables for the current fact
table. A dimension table
+ * is assumed to be in a RI relationship with a fact table. To infer
column uniqueness,
+ * the algorithm compares the number of distinct values with the total
number of rows in the
+ * table. If their relative difference is within certain limits (i.e.
ndvMaxError * 2, adjusted
+ * based on 1TB TPC-DS data), the column is assumed to be unique.
+ */
+ def findStarJoins(
+ input: Seq[LogicalPlan],
+ conditions: Seq[Expression]): Seq[Seq[LogicalPlan]] = {
+
+ val emptyStarJoinPlan = Seq.empty[Seq[LogicalPlan]]
+
+ if (!conf.starSchemaDetection || input.size < 2) {
+ emptyStarJoinPlan
+ } else {
+ // Find if the input plans are eligible for star join detection.
+ // An eligible plan is a base table access with valid statistics.
+ val foundEligibleJoin = input.forall {
+ case PhysicalOperation(_, _, t: LeafNode) if
t.stats(conf).rowCount.isDefined => true
+ case _ => false
+ }
+
+ if (!foundEligibleJoin) {
+ // Some plans don't have stats or are complex plans.
Conservatively,
+ // return an empty star join. This restriction can be lifted
+ // once statistics are propagated in the plan.
+ emptyStarJoinPlan
+ } else {
+ // Find the fact table using cardinality based heuristics i.e.
+ // the table with the largest number of rows.
+ val sortedFactTables = input.map { plan =>
+ TableAccessCardinality(plan, getTableAccessCardinality(plan))
+ }.collect { case t @ TableAccessCardinality(_, Some(_)) =>
+ t
+ }.sortBy(_.size)(implicitly[Ordering[Option[BigInt]]].reverse)
+
+ sortedFactTables match {
+ case Nil =>
+ emptyStarJoinPlan
+ case table1 :: table2 :: _
+ if table2.size.get.toDouble > conf.starSchemaFTRatio *
table1.size.get.toDouble =>
+ // If the top largest tables have comparable number of rows,
return an empty star plan.
+ // This restriction will be lifted when the algorithm is
generalized
+ // to return multiple star plans.
+ emptyStarJoinPlan
+ case TableAccessCardinality(factTable, _) :: _ =>
+ // Find the fact table joins.
+ val allFactJoins = input.filterNot { plan =>
+ plan eq factTable
+ }.filter { plan =>
+ val joinCond = findJoinConditions(factTable, plan,
conditions)
+ joinCond.nonEmpty
+ }
+
+ // Find the corresponding join conditions.
+ val allFactJoinCond = allFactJoins.flatMap { plan =>
+ val joinCond = findJoinConditions(factTable, plan,
conditions)
+ joinCond
+ }
+
+ // Verify if the join columns have valid statistics
+ val areStatsAvailable = allFactJoins.forall { dimTable =>
+ allFactJoinCond.exists {
+ case BinaryComparison(lhs: AttributeReference, rhs:
AttributeReference) =>
+ val dimCol = if (dimTable.outputSet.contains(lhs)) lhs
else rhs
+ val factCol = if (factTable.outputSet.contains(lhs)) lhs
else rhs
+ hasStatistics(dimCol, dimTable) &&
hasStatistics(factCol, factTable)
+ case _ => false
+ }
+ }
+
+ if (!areStatsAvailable) {
+ emptyStarJoinPlan
+ } else {
+ // Find the subset of dimension tables. A dimension table is
assumed to be in
+ // RI relationship with the fact table. Also, only consider
equi-joins
+ // between a fact and a dimension table.
+ val eligibleDimPlans = allFactJoins.filter { dimTable =>
+ allFactJoinCond.exists {
+ case cond @ BinaryComparison(lhs: AttributeReference,
rhs: AttributeReference)
--- End diff --
@cloud-fan Done. Thank you.
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