select specific columns in Spark DataFrames from Array of Struct

问题

I have a Spark DataFrame df with the following Schema:

root
 |-- k: integer (nullable = false)
 |-- v: array (nullable = true)
 |    |-- element: struct (containsNull = true)
 |    |    |-- a: integer (nullable = false)
 |    |    |-- b: double (nullable = false)
 |    |    |-- c: string (nullable = true)

Is it possible to just select a, c in v from df without doing a map? In particular, df is loaded from a Parquet file and I don't want the values for c to even be loaded/read.

回答1:

It depends on exactly what you expect as an output, which is not clear from your question. Let me clarify. You can do

df.select($"v.a",$"v.b").show()

however, the result may be not what you want, since v is an array, it will yield an array for a and one per b. What you may want to do is explode the array v then select from the exploded dataframe:

df.select(explode($"v").as("v" :: Nil )).select($"v.a", $"v.b").show()

this will flatten v to a table with all its values flattened. In either case, spark/parquet should be smart enough to use predicate push down and not load c at all.

来源：https://stackoverflow.com/questions/37172254/select-specific-columns-in-spark-dataframes-from-array-of-struct