[FEA] Support exploding nested type columns

[beckernick]

When processing a nested type column, I'd like to be able to explode the column into a non-nested type column, like in Spark-sql or pandas. Spark API doc.

Pyspark:

import pandas as pd

from pyspark import SparkContext
from pyspark.sql import SQLContext
from pyspark.sql import functions as F

sc = SparkContext()
sqlContext = SQLContext(sc)

df = pd.DataFrame()
df['col1'] = [0, 1]
df['col2'] = [[12,12,38], [42,93]]

sdf = sqlContext.createDataFrame(df)
sdf.withColumn("exploded", F.explode("col2")).select("exploded").show()
+--------+
|exploded|
+--------+
|      12|
|      12|
|      38|
|      42|
|      93|
+--------+

Pandas:

import pandas as pd

df = pd.DataFrame()
df['col1'] = [0, 1]
df['col2'] = [[12,12,38], [42,93]]

df.col2.explode()
0    12
0    12
0    38
1    42
1    93
Name: col2, dtype: object

[kkraus14]

Relevant Pandas API: https://pandas.pydata.org/pandas-docs/stable/reference/api/pandas.Series.explode.html

[kkraus14]

Relevant libcudf PR that implemented the functionality: https://github.com/rapidsai/cudf/pull/7140/files

[skirui-source]

Looks like Marlene is already working on this issue:
#7227