[SPARK-8838] [SQL] Add config to enable/disable merging part-files when merging parquet schema

JIRA: https://issues.apache.org/jira/browse/SPARK-8838

Currently all part-files are merged when merging parquet schema. However, in case there are many part-files and we can make sure that all the part-files have the same schema as their summary file. If so, we provide a configuration to disable merging part-files when merging parquet schema.

In short, we need to merge parquet schema because different summary files may contain different schema. But the part-files are confirmed to have the same schema with summary files.

Author: Liang-Chi Hsieh <viirya@appier.com>

Closes #7238 from viirya/option_partfile_merge and squashes the following commits:

71d5b5f [Liang-Chi Hsieh] Merge remote-tracking branch 'upstream/master' into option_partfile_merge
8816f44 [Liang-Chi Hsieh] For comments.
dbc8e6b [Liang-Chi Hsieh] Merge remote-tracking branch 'upstream/master' into option_partfile_merge
afc2fa1 [Liang-Chi Hsieh] Merge remote-tracking branch 'upstream/master' into option_partfile_merge
d4ed7e6 [Liang-Chi Hsieh] Merge remote-tracking branch 'upstream/master' into option_partfile_merge
df43027 [Liang-Chi Hsieh] Get dataStatuses' partitions based on all paths.
4eb2f00 [Liang-Chi Hsieh] Use given parameter.
ea8f6e5 [Liang-Chi Hsieh] Correct the code comments.
a57be0e [Liang-Chi Hsieh] Merge part-files if there are no summary files.
47df981 [Liang-Chi Hsieh] Merge remote-tracking branch 'upstream/master' into option_partfile_merge
4caf293 [Liang-Chi Hsieh] Merge remote-tracking branch 'upstream/master' into option_partfile_merge
0e734e0 [Liang-Chi Hsieh] Use correct API.
3b6be5b [Liang-Chi Hsieh] Fix key not found.
4bdd7e0 [Liang-Chi Hsieh] Don't read footer files if we can skip them.
8bbebcb [Liang-Chi Hsieh] Figure out how to test the config.
bbd4ce7 [Liang-Chi Hsieh] Add config to enable/disable merging part-files when merging parquet schema.

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

@@ -247,6 +247,13 @@ private[spark] object SQLConf {
           "otherwise the schema is picked from the summary file or a random data file " +
           "if no summary file is available.")
 
+  val PARQUET_SCHEMA_RESPECT_SUMMARIES = booleanConf("spark.sql.parquet.respectSummaryFiles",
+    defaultValue = Some(false),
+    doc = "When true, we make assumption that all part-files of Parquet are consistent with " +
+          "summary files and we will ignore them when merging schema. Otherwise, if this is " +
+          "false, which is the default, we will merge all part-files. This should be considered " +
+          "as expert-only option, and shouldn't be enabled before knowing what it means exactly.")
+
   val PARQUET_BINARY_AS_STRING = booleanConf("spark.sql.parquet.binaryAsString",
     defaultValue = Some(false),
     doc = "Some other Parquet-producing systems, in particular Impala and older versions of " +

sql/core/src/main/scala/org/apache/spark/sql/parquet/ParquetRelation.scala

@@ -124,6 +124,9 @@ private[sql] class ParquetRelation(
       .map(_.toBoolean)
       .getOrElse(sqlContext.conf.getConf(SQLConf.PARQUET_SCHEMA_MERGING_ENABLED))
 
+  private val mergeRespectSummaries =
+    sqlContext.conf.getConf(SQLConf.PARQUET_SCHEMA_RESPECT_SUMMARIES)
+
   private val maybeMetastoreSchema = parameters
     .get(ParquetRelation.METASTORE_SCHEMA)
     .map(DataType.fromJson(_).asInstanceOf[StructType])
@@ -421,7 +424,21 @@ private[sql] class ParquetRelation(
       val filesToTouch =
         if (shouldMergeSchemas) {
           // Also includes summary files, 'cause there might be empty partition directories.
-          (metadataStatuses ++ commonMetadataStatuses ++ dataStatuses).toSeq
+
+          // If mergeRespectSummaries config is true, we assume that all part-files are the same for
+          // their schema with summary files, so we ignore them when merging schema.
+          // If the config is disabled, which is the default setting, we merge all part-files.
+          // In this mode, we only need to merge schemas contained in all those summary files.
+          // You should enable this configuration only if you are very sure that for the parquet
+          // part-files to read there are corresponding summary files containing correct schema.
+
+          val needMerged: Seq[FileStatus] =
+            if (mergeRespectSummaries) {
+              Seq()
+            } else {
+              dataStatuses
+            }
+          (metadataStatuses ++ commonMetadataStatuses ++ needMerged).toSeq
         } else {
           // Tries any "_common_metadata" first. Parquet files written by old versions or Parquet
           // don't have this.

sql/core/src/test/scala/org/apache/spark/sql/parquet/ParquetQuerySuite.scala

@@ -17,10 +17,13 @@
 
 package org.apache.spark.sql.parquet
 
+import java.io.File
+
 import org.apache.hadoop.fs.Path
 
 import org.apache.spark.sql.types._
 import org.apache.spark.sql.{QueryTest, Row, SQLConf}
+import org.apache.spark.util.Utils
 
 /**
  * A test suite that tests various Parquet queries.
@@ -123,6 +126,30 @@ class ParquetQuerySuite extends QueryTest with ParquetTest {
     }
   }
 
+  test("Enabling/disabling merging partfiles when merging parquet schema") {
+    def testSchemaMerging(expectedColumnNumber: Int): Unit = {
+      withTempDir { dir =>
+        val basePath = dir.getCanonicalPath
+        sqlContext.range(0, 10).toDF("a").write.parquet(new Path(basePath, "foo=1").toString)
+        sqlContext.range(0, 10).toDF("b").write.parquet(new Path(basePath, "foo=2").toString)
+        // delete summary files, so if we don't merge part-files, one column will not be included.
+        Utils.deleteRecursively(new File(basePath + "/foo=1/_metadata"))
+        Utils.deleteRecursively(new File(basePath + "/foo=1/_common_metadata"))
+        assert(sqlContext.read.parquet(basePath).columns.length === expectedColumnNumber)
+      }
+    }
+
+    withSQLConf(SQLConf.PARQUET_SCHEMA_MERGING_ENABLED.key -> "true",
+      SQLConf.PARQUET_SCHEMA_RESPECT_SUMMARIES.key -> "true") {
+      testSchemaMerging(2)
+    }
+
+    withSQLConf(SQLConf.PARQUET_SCHEMA_MERGING_ENABLED.key -> "true",
+      SQLConf.PARQUET_SCHEMA_RESPECT_SUMMARIES.key -> "false") {
+      testSchemaMerging(3)
+    }
+  }
+
   test("Enabling/disabling schema merging") {
     def testSchemaMerging(expectedColumnNumber: Int): Unit = {
       withTempDir { dir =>