Hyukjin Kwon created SPARK-30185:
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Summary: Implement Dataset.tail API
Key: SPARK-30185
URL: https://issues.apache.org/jira/browse/SPARK-30185
Project: Spark
Issue Type: New Feature
Components: SQL
Affects Versions: 3.0.0
Reporter: Hyukjin Kwon
Assignee: Hyukjin Kwon
I would like to propose an API called DataFrame.tail.
*Background & Motivation*
Many other systems support the way to take data from the end, for instance,
pandas[1] and
Python[2][3]. Scala collections APIs also have head and tail
On the other hand, in Spark, we only provide a way to take data from the start
(e.g., DataFrame.head). This has been requested multiple times here and there
in Spark
user mailing list[4], StackOverFlow[5][6], JIRA[7] and other third party
projects such as
Koalas[8].
It seems we're missing non-trivial use case in Spark and this motivated me to
propose this
API.
*Proposal*
I would like to propose an API against DataFrame called tail that collects rows
from the
end in contrast with head.
Namely, as below:
scala> spark.range(10).head(5)
res1: Array[Long] = Array(0, 1, 2, 3, 4)
scala> spark.range(10).tail(5)
res2: Array[Long] = Array(5, 6, 7, 8, 9)
Implementation details will be similar with head but it will be reversed:
Run the job against the last partition and collect rows. If this is enough,
return as is.
If this is not enough, calculate the number of partitions to select more based
upon
‘spark.sql.limit.scaleUpFactor’
Run more jobs against more partitions (in a reversed order compared to head)
as many as the number calculated from 2.
Go to 2.
[1]
[https://pandas.pydata.org/pandas-docs/stable/reference/api/pandas.DataFrame.tail.html?highlight=tail#pandas.DataFrame.tail]
[2] [https://stackoverflow.com/questions/10532473/head-and-tail-in-one-line]
[3]
[https://stackoverflow.com/questions/646644/how-to-get-last-items-of-a-list-in-python]
[4] [http://apache-spark-user-list.1001560.n3.nabble.com/RDD-tail-td4217.html]
[5]
[https://stackoverflow.com/questions/39544796/how-to-select-last-row-and-also-how-to-access-pyspark-dataframe-by-index]
[6]
[https://stackoverflow.com/questions/45406762/how-to-get-the-last-row-from-dataframe]
[7] https://issues.apache.org/jira/browse/SPARK-26433
[8] [https://github.com/databricks/koalas/issues/343]
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