My team is building an ETL process to load raw delimited text files into a Parquet based "data lake" using Spark. One of the promises of the Parquet column store is that a query will only read the necessary "column stripes".
But we're seeing unexpected columns being read for nested schema structures.
To demonstrate, here is a POC using Scala and the Spark 2.0.1 shell:
// Preliminary setup
sc.setLogLevel("INFO")
import org.apache.spark.sql.types._
import org.apache.spark.sql._
// Create a schema with nested complex structures
val schema = StructType(Seq(
StructField("F1", IntegerType),
StructField("F2", IntegerType),
StructField("Orig", StructType(Seq(
StructField("F1", StringType),
StructField("F2", StringType))))))
// Create some sample data
val data = spark.createDataFrame(
sc.parallelize(Seq(
Row(1, 2, Row("1", "2")),
Row(3, null, Row("3", "ABC")))),
schema)
// Save it
data.write.mode(SaveMode.Overwrite).parquet("data.parquet")
Then we read the file back into a DataFrame and project to a subset of columns:
// Read it back into another DataFrame
val df = spark.read.parquet("data.parquet")
// Select & show a subset of the columns
df.select($"F1", $"Orig.F1").show
When this runs we see the expected output:
+---+-------+
| F1|Orig_F1|
+---+-------+
| 1| 1|
| 3| 3|
+---+-------+
But... the query plan shows a slightly different story:
The "optimized plan" shows:
val projected = df.select($"F1", $"Orig.F1".as("Orig_F1"))
projected.queryExecution.optimizedPlan
// Project [F1#18, Orig#20.F1 AS Orig_F1#116]
// +- Relation[F1#18,F2#19,Orig#20] parquet
And "explain" shows:
projected.explain
// == Physical Plan ==
// *Project [F1#18, Orig#20.F1 AS Orig_F1#116]
// +- *Scan parquet [F1#18,Orig#20] Format: ParquetFormat, InputPaths: hdfs://sandbox.hortonworks.com:8020/user/stephenp/data.parquet, PartitionFilters: [], PushedFilters: [], ReadSchema: struct<F1:int,Orig:struct<F1:string,F2:string>>
And the INFO logs produced during execution also confirm that the Orig.F2 column is unexpectedly read:
16/10/21 15:13:15 INFO parquet.ParquetReadSupport: Going to read the following fields from the Parquet file:
Parquet form:
message spark_schema {
optional int32 F1;
optional group Orig {
optional binary F1 (UTF8);
optional binary F2 (UTF8);
}
}
Catalyst form:
StructType(StructField(F1,IntegerType,true), StructField(Orig,StructType(StructField(F1,StringType,true), StructField(F2,StringType,true)),true))
According to the Dremel paper and the Parquet documentation, columns for complex nested structures should be independently stored and independently retrievable.
Questions:
- Is this behavior a limitation of the current Spark query engine? In other words, does Parquet support optimally executing this query, but Spark's query planner is naive?
- Or, is this a limitation of the current Parquet implementation?
- Or, am I not using the Spark APIs correctly?
- Or, am I misunderstanding how Dremel/Parquet column storage is supposed to work?
Possibly related: Why does the query performance differ with nested columns in Spark SQL?
