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https://issues.apache.org/jira/browse/SPARK-21021?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16049100#comment-16049100
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Michel Lemay commented on SPARK-21021:
--------------------------------------
Indeed, the code is not broken so it's not a bug. However, it returns
unexpected results from users point of view. (at least my point of view). I
strongly encourage to follow the
[https://en.wikipedia.org/wiki/Principle_of_least_astonishment]. From my
experience, it usually results in less tickets submitted.
As for the fix/improvement itself, it's only a matter of using
{{dataSchema.merge(partitionnedSchema)}} (assuming StructTypes) instead of a
{{++}}. The former should preserve columns order of the left-hand side schema.
> Reading partitioned parquet does not respect specified schema column order
> --------------------------------------------------------------------------
>
> Key: SPARK-21021
> URL: https://issues.apache.org/jira/browse/SPARK-21021
> Project: Spark
> Issue Type: Improvement
> Components: SQL
> Affects Versions: 2.1.0
> Reporter: Michel Lemay
> Priority: Minor
>
> When reading back a partitioned parquet folder, column order gets messed up.
> Consider the following example:
> {code}
> case class Event(f1: String, f2: String, f3: String)
> val df = Seq(Event("v1", "v2", "v3")).toDF
> df.write.partitionBy("f1", "f2").parquet("out")
> val schema: StructType = StructType(StructField("f1", StringType, true) ::
> StructField("f2", StringType, true) :: StructField("f3", StringType, true) ::
> Nil)
> val dfRead = spark.read.schema(schema).parquet("out")
> dfRead.show
> +---+---+---+
> | f3| f1| f2|
> +---+---+---+
> | v3| v1| v2|
> +---+---+---+
> dfRead.columns
> Array[String] = Array(f3, f1, f2)
> schema.fields
> Array(StructField(f1,StringType,true), StructField(f2,StringType,true),
> StructField(f3,StringType,true))
> {code}
> This makes it really hard to have compatible schema when reading from
> multiple sources.
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