Github user ioana-delaney commented on a diff in the pull request:
https://github.com/apache/spark/pull/15363#discussion_r105023746
--- Diff:
sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/optimizer/joins.scala
---
@@ -20,19 +20,340 @@ 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.{BaseTableAccess,
ExtractFiltersAndInnerJoins}
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 DetectStarSchemaJoin(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
--- End diff --
@hvanhovell A star join will indeed be represented by a left-deep tree. But
if a query has more than one star schema join, ideally you want to plan them as
a bushy-tree i.e. (star-join-1) Join (star-join2).
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