DB Tsai created SPARK-22231:
-------------------------------
Summary: Support of map, filter, withColumn, dropColumn in nested
list of structures
Key: SPARK-22231
URL: https://issues.apache.org/jira/browse/SPARK-22231
Project: Spark
Issue Type: New Feature
Components: SQL
Affects Versions: 2.2.0
Reporter: DB Tsai
At Netflix's algorithm team, we work on ranking problems to find the great
content to fulfill the unique tastes of our members. Before building a
recommendation algorithms, we need to prepare the training, testing, and
validation datasets in Apache Spark. Due to the nature of ranking problems, we
have a nested list of items to be ranked in one column, and the top level is
the contexts describing the setting for where a model is to be used (e.g.
profiles, country, time, device, etc.) Here is a blog post describing the
details, [Distributed Time Travel for Feature
Generation|https://medium.com/netflix-techblog/distributed-time-travel-for-feature-generation-389cccdd3907]
To be more concrete, for the ranks of videos for a given profile_id at a given
country, our data schema can be looked like this,
{code:java}
root
|-- profile_id: long (nullable = true)
|-- country_iso_code: string (nullable = true)
|-- items: array (nullable = false)
| |-- element: struct (containsNull = false)
| | |-- title_id: integer (nullable = true)
| | |-- scores: double (nullable = true)
...
{code}
We oftentimes need to work on the nested level by applying some functions in
it, even dropping, or adding new columns in the nested level. Currently, there
is no easy solution in open source Apache Spark to perform those operations
using SQL primitives; many people we talked to just converting the data into
RDD to work on the nested level of data, and then reconstruct the new
dataframe. This is extremely inefficient because all the optimizations like
predicate pushdown in SQL can not be performed, we can not leverage on the
columnar format, and the serialization and deserialization cost becomes really
huge even we just want to add a new column in the nested level.
We built a solution internally at Netflix which we're very happy with. We plan
to make it open source in Spark upstream, and we would like to socialize the
API design to see if we miss any use-case.
The first API we added is *mapItems* on dataframe which take a function from
*Column* to *Column*, and then apply the function on nested dataframe. Here is
an example,
{code:java}
case class Data(foo: Int, bar: Double, items: Seq[Double])
val df: Dataset[Data] = spark.createDataset(Seq(
Data(10, 10.0, Seq(10.1, 10.2, 10.3, 10.4)),
Data(20, 20.0, Seq(20.1, 20.2, 20.3, 20.4))
))
val result = df.mapItems("items") {
item => item * 2.0
}
result.printSchema()
// root
// |-- foo: integer (nullable = false)
// |-- bar: double (nullable = false)
// |-- items: array (nullable = true)
// | |-- element: double (containsNull = true)
result.show()
// +---+----+--------------------+
// |foo| bar| items|
// +---+----+--------------------+
// | 10|10.0|[20.2, 20.4, 20.6...|
// | 20|20.0|[40.2, 40.4, 40.6...|
// +---+----+--------------------+
{code}
Now, with the ability of applying a function in the nested dataframe, we can
add a new function, *withColumn* in *Column* to add or replace the existing
column that has the same name in the nested list of struct. Here is two
examples demonstrating the API together with *mapItems*; the first one replaces
the existing column,
{code:java}
case class Item(a: Int, b: Double)
case class Data(foo: Int, bar: Double, items: Seq[Item])
val df: Dataset[Data] = spark.createDataset(Seq(
Data(10, 10.0, Seq(Item(10, 10.0), Item(11, 11.0))),
Data(20, 20.0, Seq(Item(20, 20.0), Item(21, 21.0)))
))
val result = df.mapItems("items") {
item => item.withColumn(item("b") + 1 as "b")
}
result.printSchema
root
// |-- foo: integer (nullable = false)
// |-- bar: double (nullable = false)
// |-- items: array (nullable = true)
// | |-- element: struct (containsNull = true)
// | | |-- a: integer (nullable = true)
// | | |-- b: double (nullable = true)
result.show(false)
// +---+----+----------------------+
// |foo|bar |items |
// +---+----+----------------------+
// |10 |10.0|[[10,11.0], [11,12.0]]|
// |20 |20.0|[[20,21.0], [21,22.0]]|
// +---+----+----------------------+
{code}
and the second one adds a new column in the nested dataframe.
{code:java}
val df: Dataset[Data] = spark.createDataset(Seq(
Data(10, 10.0, Seq(Item(10, 10.0), Item(11, 11.0))),
Data(20, 20.0, Seq(Item(20, 20.0), Item(21, 21.0)))
))
val result = df.mapItems("items") {
item => item.withColumn(item("b") + 1 as "b")
}
result.printSchema
root
// |-- foo: integer (nullable = false)
// |-- bar: double (nullable = false)
// |-- items: array (nullable = true)
// | |-- element: struct (containsNull = true)
// | | |-- a: integer (nullable = true)
// | | |-- b: double (nullable = true)
// | | |-- c: double (nullable = true)
result.show(false)
// +---+----+--------------------------------+
// |foo|bar |items |
// +---+----+--------------------------------+
// |10 |10.0|[[10,10.0,11.0], [11,11.0,12.0]]|
// |20 |20.0|[[20,20.0,21.0], [21,21.0,22.0]]|
// +---+----+--------------------------------+
{code}
We also implement a filter predicate to nested list of struct, and it will
return those items which matched the predicate. The following is the API
example,
{code:java}
val df: Dataset[Data] = spark.createDataset(Seq(
Data(10, 10.0, Seq(Item(10, 10.0), Item(11, 11.0))),
Data(20, 20.0, Seq(Item(20, 20.0), Item(21, 21.0)))
))
val result = df.filterItems("items") {
item => item("a") < 20
}
// +---+----+----------------------+
// |foo|bar |items |
// +---+----+----------------------+
// |10 |10.0|[[10,10.0], [11,11.0]]|
// |20 |20.0|[] |
// +---+----+----------------------+
{code}
Dropping a column in the nested list of struct can be achieved by similar API
to *withColumn*. We add *drop* method to *Column* to implement this. Here is an
example,
{code:java}
val df: Dataset[Data] = spark.createDataset(Seq(
Data(10, 10.0, Seq(Item(10, 10.0), Item(11, 11.0))),
Data(20, 20.0, Seq(Item(20, 20.0), Item(21, 21.0)))
))
val result = df.mapItems("items") {
item => item.drop("b")
}
result.printSchema
root
// |-- foo: integer (nullable = false)
// |-- bar: double (nullable = false)
// |-- items: array (nullable = true)
// | |-- element: struct (containsNull = true)
// | | |-- a: integer (nullable = true)
result.show(false)
// +---+----+------------+
// |foo|bar |items |
// +---+----+------------+
// |10 |10.0|[[10], [11]]|
// |20 |20.0|[[20], [21]]|
// +---+----+------------+
{code}
As you can see, those APIs are not opaque to Spark optimizers, and can fully
take advantage of columnar data structure.
We're looking forward to the community feedback and suggestion! Thanks.
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