Github user rdblue commented on a diff in the pull request: https://github.com/apache/spark/pull/13280#discussion_r67006656 --- Diff: sql/core/src/main/scala/org/apache/spark/sql/execution/datasources/parquet/CatalystSchemaConverter.scala --- @@ -538,6 +538,22 @@ private[parquet] class CatalystSchemaConverter( private[parquet] object CatalystSchemaConverter { val SPARK_PARQUET_SCHEMA_NAME = "spark_schema" + // !! HACK ALERT !! + // + // PARQUET-363 & PARQUET-278: parquet-mr 1.8.1 doesn't allow constructing empty GroupType, + // which prevents us to avoid selecting any columns for queries like `SELECT COUNT(*) FROM t`. + // This issue has been fixed in parquet-mr 1.8.2-SNAPSHOT. + // + // To workaround this problem, here we first construct a `MessageType` with a single dummy + // field, and then remove the field to obtain an empty `MessageType`. + // + // TODO Reverts this change after upgrading parquet-mr to 1.8.2+ --- End diff -- We're close to a 1.9.0 release, just working through some performance issues with the switch to ByteBuffer APIs. No estimate on that yet. We can do a 1.8.2 if there's interest so that we can fix some things like this without pulling in all those changes.
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