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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