Github user liancheng commented on a diff in the pull request:
https://github.com/apache/spark/pull/13280#discussion_r64947036
--- Diff:
sql/core/src/test/scala/org/apache/spark/sql/execution/datasources/parquet/ParquetSchemaSuite.scala
---
@@ -1415,6 +1425,18 @@ class ParquetSchemaSuite extends ParquetSchemaTest {
|}
""".stripMargin)
+ // !! HACK ALERT !!
+ //
+ // PARQUET-363 and PARQUET-278: Parquet 1.8.1 doesn't allow constructing
empty groups, but does
+ // support empty groups as valid projections, for cases like select
count(*). This work-around
+ // constructs a group with a dummy column and then removes it.
+ //
+ // This is fixed in Parquet so this hack should be removed when Parquet
is updated.
+ val emptySchema: MessageType = Types.buildMessage()
+ .required(PrimitiveType.PrimitiveTypeName.INT32).named("dummy")
+ .named("root")
+ emptySchema.getFields.clear(); // remove the dummy column
--- End diff --
Just a double check, so the following two are equivalent, right? I wasn't
aware of the first approach (which is a little bit confusing since there are
two consecutive `named` calls).
```scala
// (1)
Types
.buildMessage()
.required(PrimitiveType.PrimitiveTypeName.INT32).named("dummy")
.named("root")
// (2)
Types
.buildMessage()
.addField(
Types.required(PrimitiveType.PrimitiveTypeName.INT32).named("dummy")
)
.named("root")
```
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