code cleanup per rxin comments

core/src/main/scala/spark/rdd/JdbcRDD.scala

@@ -5,23 +5,27 @@ import java.sql.{Connection, ResultSet}
 import spark.{Logging, Partition, RDD, SparkContext, TaskContext}
 import spark.util.NextIterator
 
+private[spark] class JdbcPartition(idx: Int, val lower: Long, val upper: Long) extends Partition {
+  override def index = idx
+}
+
 /**
-  An RDD that executes an SQL query on a JDBC connection and reads results.
-  @param getConnection a function that returns an open Connection.
-    The RDD takes care of closing the connection.
-  @param sql the text of the query.
-    The query must contain two ? placeholders for parameters used to partition the results.
-    E.g. "select title, author from books where ? <= id and id <= ?" 
-  @param lowerBound the minimum value of the first placeholder
-  @param upperBound the maximum value of the second placeholder
-    The lower and upper bounds are inclusive.
-  @param numPartitions the number of partitions.
-    Given a lowerBound of 1, an upperBound of 20, and a numPartitions of 2,
-    the query would be executed twice, once with (1, 10) and once with (11, 20)
-  @param mapRow a function from a ResultSet to a single row of the desired result type(s).
-    This should only call getInt, getString, etc; the RDD takes care of calling next.
-    The default maps a ResultSet to an array of Object.
-*/
+ * An RDD that executes an SQL query on a JDBC connection and reads results.
+ * @param getConnection a function that returns an open Connection.
+ *   The RDD takes care of closing the connection.
+ * @param sql the text of the query.
+ *   The query must contain two ? placeholders for parameters used to partition the results.
+ *   E.g. "select title, author from books where ? <= id and id <= ?" 
+ * @param lowerBound the minimum value of the first placeholder
+ * @param upperBound the maximum value of the second placeholder
+ *   The lower and upper bounds are inclusive.
+ * @param numPartitions the number of partitions.
+ *   Given a lowerBound of 1, an upperBound of 20, and a numPartitions of 2,
+ *   the query would be executed twice, once with (1, 10) and once with (11, 20)
+ * @param mapRow a function from a ResultSet to a single row of the desired result type(s).
+ *   This should only call getInt, getString, etc; the RDD takes care of calling next.
+ *   The default maps a ResultSet to an array of Object.
+ */
 class JdbcRDD[T: ClassManifest](
     sc: SparkContext,
     getConnection: () => Connection,
@@ -29,26 +33,33 @@ class JdbcRDD[T: ClassManifest](
     lowerBound: Long,
     upperBound: Long,
     numPartitions: Int,
-    mapRow: (ResultSet) => T = JdbcRDD.resultSetToObjectArray)
+    mapRow: (ResultSet) => T = JdbcRDD.resultSetToObjectArray _)
   extends RDD[T](sc, Nil) with Logging {
 
-  override def getPartitions: Array[Partition] =
-    ParallelCollectionRDD.slice(lowerBound to upperBound, numPartitions).
-      filter(! _.isEmpty).
-      zipWithIndex.
-      map(x => new JdbcPartition(x._2, x._1.head, x._1.last)).
-      toArray
+  override def getPartitions: Array[Partition] = {
+    // bounds are inclusive, hence the + 1 here and - 1 on end
+    val length = 1 + upperBound - lowerBound
+    (0 until numPartitions).map(i => {
+      val start = lowerBound + ((i * length) / numPartitions).toLong
+      val end = lowerBound + (((i + 1) * length) / numPartitions).toLong - 1
+      new JdbcPartition(i, start, end)
+    }).toArray
+  }
 
   override def compute(thePart: Partition, context: TaskContext) = new NextIterator[T] {
     context.addOnCompleteCallback{ () => closeIfNeeded() }
     val part = thePart.asInstanceOf[JdbcPartition]
     val conn = getConnection()
     val stmt = conn.prepareStatement(sql, ResultSet.TYPE_FORWARD_ONLY, ResultSet.CONCUR_READ_ONLY)
-    // force mysql driver to stream rather than pull entire resultset into memory
+
+    // setFetchSize(Integer.MIN_VALUE) is a mysql driver specific way to force streaming results,
+    // rather than pulling entire resultset into memory.
+    // see http://dev.mysql.com/doc/refman/5.0/en/connector-j-reference-implementation-notes.html
     if (conn.getMetaData.getURL.matches("jdbc:mysql:.*")) {
       stmt.setFetchSize(Integer.MIN_VALUE)
       logInfo("statement fetch size set to: " + stmt.getFetchSize + " to force MySQL streaming ")
     }
+
     stmt.setLong(1, part.lower)
     stmt.setLong(2, part.upper)
     val rs = stmt.executeQuery()
@@ -81,14 +92,10 @@ class JdbcRDD[T: ClassManifest](
       }
     }
   }
-
-}
-
-private[spark] class JdbcPartition(idx: Int, val lower: Long, val upper: Long) extends Partition {
-  override def index = idx
 }
 
 object JdbcRDD {
-  val resultSetToObjectArray = (rs: ResultSet) =>
+  def resultSetToObjectArray(rs: ResultSet) = {
     Array.tabulate[Object](rs.getMetaData.getColumnCount)(i => rs.getObject(i + 1))
+  }
 }