[SPARK-12546][SQL] Change default number of open parquet files

A common problem that users encounter with Spark 1.6.0 is that writing to a partitioned parquet table OOMs.  The root cause is that parquet allocates a significant amount of memory that is not accounted for by our own mechanisms.  As a workaround, we can ensure that only a single file is open per task unless the user explicitly asks for more.

Author: Michael Armbrust <michael@databricks.com>

Closes #11308 from marmbrus/parquetWriteOOM.

sql/core/src/main/scala/org/apache/spark/sql/SQLConf.scala

@@ -430,7 +430,7 @@ private[spark] object SQLConf {
 
   val PARTITION_MAX_FILES =
     intConf("spark.sql.sources.maxConcurrentWrites",
-      defaultValue = Some(5),
+      defaultValue = Some(1),
       doc = "The maximum number of concurrent files to open before falling back on sorting when " +
             "writing out files using dynamic partitioning.")