access fields of an array within pyspark dataframe

Question

I am developing sql queries to a spark dataframe that are based on a group of ORC files. The program goes like this:

from pyspark.sql import SparkSession
spark_session = SparkSession.builder.appName("test").getOrCreate()
sdf = spark_session.read.orc("../data/")
sdf.createOrReplaceTempView("test")

Now I have a table called "test". If I do something like:

spark_session.sql("select count(*) from test")

then the result will be fine. But I need to get more columns in the query, including some of the fields in array.

In [8]: sdf.take(1)[0]["person"]
Out[8]:
[Row(name='name', value='tom'),
 Row(name='age', value='20'),
 Row(name='gender', value='m')]

I have tried something like:

spark_session.sql("select person.age, count(*) from test group by person.age")

But this does not work. My question is: how to access the fields in the "person" array?

Thanks!

EDIT:

result of sdf.printSchema()

In [3]: sdf.printSchema()
root
 |-- person: integer (nullable = true)
 |-- customtags: array (nullable = true)
 |    |-- element: struct (containsNull = true)
 |    |    |-- name: string (nullable = true)
 |    |    |-- value: string (nullable = true)

Error messages:

AnalysisException: 'No such struct field age in name, value; line 16 pos 8'

df=spark.sql(select * from test);func=udf(lambda x: x['age'],IntegerType()); df.withColumns('age',func(df.person)).groupby('age').count().show() — pratiklodha
– pratiklodha, Commented Feb 17, 2018 at 14:26

pault · Accepted Answer · 2018-02-16 22:36:32Z

I don't know how to do this using only PySpark-SQL, but here is a way to do it using PySpark DataFrames.

Basically, we can convert the struct column into a MapType() using the create_map() function. Then we can directly access the fields using string indexing.

Consider the following example:

Define Schema

schema = StructType([
        StructField('person', IntegerType()),
        StructField(
            'customtags',
            ArrayType(
                StructType(
                    [
                        StructField('name', StringType()),
                        StructField('value', StringType())
                    ]
                )
            )
        )
    ]
)

Create Example DataFrame

data = [
    (
        1, 
        [
            {'name': 'name', 'value': 'tom'},
            {'name': 'age', 'value': '20'},
            {'name': 'gender', 'value': 'm'}
        ]
    ),
    (
        2,
        [
            {'name': 'name', 'value': 'jerry'},
            {'name': 'age', 'value': '20'},
            {'name': 'gender', 'value': 'm'}
        ]
    ),
    (
        3,
        [
            {'name': 'name', 'value': 'ann'},
            {'name': 'age', 'value': '20'},
            {'name': 'gender', 'value': 'f'}
        ]
    )
]
df = sqlCtx.createDataFrame(data, schema)
df.show(truncate=False)
#+------+------------------------------------+
#|person|customtags                          |
#+------+------------------------------------+
#|1     |[[name,tom], [age,20], [gender,m]]  |
#|2     |[[name,jerry], [age,20], [gender,m]]|
#|3     |[[name,ann], [age,20], [gender,f]]  |
#+------+------------------------------------+

Convert the struct column to a map

from operator import add
import pyspark.sql.functions as f

df = df.withColumn(
        'customtags',
        f.create_map(
            *reduce(
                add, 
                [
                    [f.col('customtags')['name'][i],
                     f.col('customtags')['value'][i]] for i in range(3)
                ]
            )
        )
    )\
    .select('person', 'customtags')

df.show(truncate=False)
#+------+------------------------------------------+
#|person|customtags                                |
#+------+------------------------------------------+
#|1     |Map(name -> tom, age -> 20, gender -> m)  |
#|2     |Map(name -> jerry, age -> 20, gender -> m)|
#|3     |Map(name -> ann, age -> 20, gender -> f)  |
#+------+------------------------------------------+

The catch here is that you have to know apriori the length of the ArrayType() (in this case 3) as I don't know of a way to dynamically loop over it. This also assumes that the array has the same length for all rows.

I had to use reduce(add, ...) here because create_map() expects pairs of elements in the form of (key, value).

Group by fields in the map column

df.groupBy((f.col('customtags')['name']).alias('name')).count().show()
#+-----+-----+
#| name|count|
#+-----+-----+
#|  ann|    1|
#|jerry|    1|
#|  tom|    1|
#+-----+-----+

df.groupBy((f.col('customtags')['gender']).alias('gender')).count().show()
#+------+-----+
#|gender|count|
#+------+-----+
#|     m|    2|
#|     f|    1|
#+------+-----+

Any suggestion now on getting the length of the array dynamically?

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