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Standard Functions for Window Aggregation (Window Functions)

Window aggregate functions (aka window functions or windowed aggregates) are functions that perform a calculation over a group of records called window that are in some relation to the current record (i.e. can be in the same partition or frame as the current row).

In other words, when executed, a window function computes a value for each and every row in a window (per window specification).

Window functions are also called over functions due to how they are applied using over operator.

Spark SQL supports three kinds of window functions:

  • ranking functions
  • analytic functions
  • aggregate functions

.Window Aggregate Functions in Spark SQL [align="center",cols="1,1,2",width="80%",options="header"] |=== | | Function | Purpose

.5+.|Ranking functions

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<>
<>
<>
<>

.3+.| Analytic functions

| <> a|

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<>
===

For aggregate functions, you can use the existing spark-sql-basic-aggregation.md[aggregate functions] as window functions, e.g. sum, avg, min, max and count.

[source, scala]

// Borrowed from 3.5. Window Functions in PostgreSQL documentation // Example of window functions using Scala API // case class Salary(depName: String, empNo: Long, salary: Long) val empsalary = Seq( Salary("sales", 1, 5000), Salary("personnel", 2, 3900), Salary("sales", 3, 4800), Salary("sales", 4, 4800), Salary("personnel", 5, 3500), Salary("develop", 7, 4200), Salary("develop", 8, 6000), Salary("develop", 9, 4500), Salary("develop", 10, 5200), Salary("develop", 11, 5200)).toDS

import org.apache.spark.sql.expressions.Window // Windows are partitions of deptName scala> val byDepName = Window.partitionBy('depName) byDepName: org.apache.spark.sql.expressions.WindowSpec = org.apache.spark.sql.expressions.WindowSpec@1a711314

scala> empsalary.withColumn("avg", avg('salary) over byDepName).show +---------+-----+------+-----------------+ | depName|empNo|salary| avg| +---------+-----+------+-----------------+ | develop| 7| 4200| 5020.0| | develop| 8| 6000| 5020.0| | develop| 9| 4500| 5020.0| | develop| 10| 5200| 5020.0| | develop| 11| 5200| 5020.0| | sales| 1| 5000|4866.666666666667| | sales| 3| 4800|4866.666666666667| | sales| 4| 4800|4866.666666666667| |personnel| 2| 3900| 3700.0| |personnel| 5| 3500| 3700.0| +---------+-----+------+-----------------+


You describe a window using the convenient factory methods in <> that create a <> that you can further refine with partitioning, ordering, and frame boundaries.

After you describe a window you can apply <> like ranking functions (e.g. RANK), analytic functions (e.g. LAG), and the regular spark-sql-basic-aggregation.md[aggregate functions], e.g. sum, avg, max.

NOTE: Window functions are supported in structured queries using <> and Column-based expressions.

Although similar to spark-sql-basic-aggregation.md[aggregate functions], a window function does not group rows into a single output row and retains their separate identities. A window function can access rows that are linked to the current row.

NOTE: The main difference between window aggregate functions and spark-sql-functions.md#aggregate-functions[aggregate functions] with spark-sql-basic-aggregation.md[grouping operators] is that the former calculate values for every row in a window while the latter gives you at most the number of input rows, one value per group.

TIP: See <> section in this document.

You can mark a function window by OVER clause after a function in SQL, e.g. avg(revenue) OVER (...) or over method on a function in the Dataset API, e.g. rank().over(...).

Note

Window functions belong to Window functions group in Spark's Scala API.

=== [[Window-object]] Window object

Window object provides functions to define windows (as WindowSpec instances).

Window object lives in org.apache.spark.sql.expressions package. Import it to use Window functions.

import org.apache.spark.sql.expressions.Window

There are two families of the functions available in Window object that create WindowSpec instance for one or many Column instances:

==== [[partitionBy]] Partitioning Records -- partitionBy Methods

[source, scala]

partitionBy(colName: String, colNames: String*): WindowSpec partitionBy(cols: Column*): WindowSpec


partitionBy creates an instance of WindowSpec with partition expression(s) defined for one or more columns.

[source, scala]

// partition records into two groups // * tokens starting with "h" // * others val byHTokens = Window.partitionBy('token startsWith "h")

// count the sum of ids in each group val result = tokens.select('*, sum('id) over byHTokens as "sum over h tokens").orderBy('id)

scala> .show +---+-----+-----------------+ | id|token|sum over h tokens| +---+-----+-----------------+ | 0|hello| 4| | 1|henry| 4| | 2| and| 2| | 3|harry| 4| +---+-----+-----------------+


==== [[orderBy]] Ordering in Windows -- orderBy Methods

[source, scala]

orderBy(colName: String, colNames: String*): WindowSpec orderBy(cols: Column*): WindowSpec


orderBy allows you to control the order of records in a window.

[source, scala]

import org.apache.spark.sql.expressions.Window val byDepnameSalaryDesc = Window.partitionBy('depname).orderBy('salary desc)

// a numerical rank within the current row's partition for each distinct ORDER BY value scala> val rankByDepname = rank().over(byDepnameSalaryDesc) rankByDepname: org.apache.spark.sql.Column = RANK() OVER (PARTITION BY depname ORDER BY salary DESC UnspecifiedFrame)

scala> empsalary.select('*, rankByDepname as 'rank).show +---------+-----+------+----+ | depName|empNo|salary|rank| +---------+-----+------+----+ | develop| 8| 6000| 1| | develop| 10| 5200| 2| | develop| 11| 5200| 2| | develop| 9| 4500| 4| | develop| 7| 4200| 5| | sales| 1| 5000| 1| | sales| 3| 4800| 2| | sales| 4| 4800| 2| |personnel| 2| 3900| 1| |personnel| 5| 3500| 2| +---------+-----+------+----+


==== [[rangeBetween]] rangeBetween Method

[source, scala]

rangeBetween(start: Long, end: Long): WindowSpec

rangeBetween creates a <> with the frame boundaries from start (inclusive) to end (inclusive).

NOTE: It is recommended to use Window.unboundedPreceding, Window.unboundedFollowing and Window.currentRow to describe the frame boundaries when a frame is unbounded preceding, unbounded following and at current row, respectively.

[source, scala]

import org.apache.spark.sql.expressions.Window import org.apache.spark.sql.expressions.WindowSpec val spec: WindowSpec = Window.rangeBetween(Window.unboundedPreceding, Window.currentRow)


Internally, rangeBetween creates a WindowSpec with SpecifiedWindowFrame and RangeFrame type.

=== [[frame]] Frame

At its core, a window function calculates a return value for every input row of a table based on a group of rows, called the frame. Every input row can have a unique frame associated with it.

When you define a frame you have to specify three components of a frame specification - the start and end boundaries, and the type.

Types of boundaries (two positions and three offsets):

  • UNBOUNDED PRECEDING - the first row of the partition
  • UNBOUNDED FOLLOWING - the last row of the partition
  • CURRENT ROW
  • <value> PRECEDING
  • <value> FOLLOWING

Offsets specify the offset from the current input row.

Types of frames:

  • ROW - based on physical offsets from the position of the current input row
  • RANGE - based on logical offsets from the position of the current input row

In the current implementation of <> you can use two methods to define a frame:

  • rowsBetween
  • rangeBetween

See <> for their coverage.

=== [[sql]] Window Operators in SQL Queries

The grammar of windows operators in SQL accepts the following:

  1. CLUSTER BY or PARTITION BY or DISTRIBUTE BY for partitions,

  2. ORDER BY or SORT BY for sorting order,

  3. RANGE, ROWS, RANGE BETWEEN, and ROWS BETWEEN for window frame types,

  4. UNBOUNDED PRECEDING, UNBOUNDED FOLLOWING, CURRENT ROW for frame bounds.

TIP: Consult sql/AstBuilder.md#withWindows[withWindows] helper in AstBuilder.

=== [[examples]] Examples

==== [[example-top-n]] Top N per Group

Top N per Group is useful when you need to compute the first and second best-sellers in category.

NOTE: This example is borrowed from an excellent article https://databricks.com/blog/2015/07/15/introducing-window-functions-in-spark-sql.html[Introducing Window Functions in Spark SQL].

.Table PRODUCT_REVENUE [align="center",width="80%",options="header"] |=== |product |category |revenue | Thin|cell phone| 6000 | Normal| tablet| 1500 | Mini| tablet| 5500 |Ultra thin|cell phone| 5000 | Very thin|cell phone| 6000 | Big| tablet| 2500 | Bendable|cell phone| 3000 | Foldable|cell phone| 3000 | Pro| tablet| 4500 | Pro2| tablet| 6500 |===

Question: What are the best-selling and the second best-selling products in every category?

val dataset = Seq(
  ("Thin",       "cell phone", 6000),
  ("Normal",     "tablet",     1500),
  ("Mini",       "tablet",     5500),
  ("Ultra thin", "cell phone", 5000),
  ("Very thin",  "cell phone", 6000),
  ("Big",        "tablet",     2500),
  ("Bendable",   "cell phone", 3000),
  ("Foldable",   "cell phone", 3000),
  ("Pro",        "tablet",     4500),
  ("Pro2",       "tablet",     6500))
  .toDF("product", "category", "revenue")

scala> dataset.show
+----------+----------+-------+
|   product|  category|revenue|
+----------+----------+-------+
|      Thin|cell phone|   6000|
|    Normal|    tablet|   1500|
|      Mini|    tablet|   5500|
|Ultra thin|cell phone|   5000|
| Very thin|cell phone|   6000|
|       Big|    tablet|   2500|
|  Bendable|cell phone|   3000|
|  Foldable|cell phone|   3000|
|       Pro|    tablet|   4500|
|      Pro2|    tablet|   6500|
+----------+----------+-------+

scala> data.where('category === "tablet").show
+-------+--------+-------+
|product|category|revenue|
+-------+--------+-------+
| Normal|  tablet|   1500|
|   Mini|  tablet|   5500|
|    Big|  tablet|   2500|
|    Pro|  tablet|   4500|
|   Pro2|  tablet|   6500|
+-------+--------+-------+

The question boils down to ranking products in a category based on their revenue, and to pick the best selling and the second best-selling products based the ranking.

import org.apache.spark.sql.expressions.Window
val overCategory = Window.partitionBy('category).orderBy('revenue.desc)

val ranked = data.withColumn("rank", dense_rank.over(overCategory))

scala> ranked.show
+----------+----------+-------+----+
|   product|  category|revenue|rank|
+----------+----------+-------+----+
|      Pro2|    tablet|   6500|   1|
|      Mini|    tablet|   5500|   2|
|       Pro|    tablet|   4500|   3|
|       Big|    tablet|   2500|   4|
|    Normal|    tablet|   1500|   5|
|      Thin|cell phone|   6000|   1|
| Very thin|cell phone|   6000|   1|
|Ultra thin|cell phone|   5000|   2|
|  Bendable|cell phone|   3000|   3|
|  Foldable|cell phone|   3000|   3|
+----------+----------+-------+----+

scala> ranked.where('rank <= 2).show
+----------+----------+-------+----+
|   product|  category|revenue|rank|
+----------+----------+-------+----+
|      Pro2|    tablet|   6500|   1|
|      Mini|    tablet|   5500|   2|
|      Thin|cell phone|   6000|   1|
| Very thin|cell phone|   6000|   1|
|Ultra thin|cell phone|   5000|   2|
+----------+----------+-------+----+

==== Revenue Difference per Category

NOTE: This example is the 2nd example from an excellent article https://databricks.com/blog/2015/07/15/introducing-window-functions-in-spark-sql.html[Introducing Window Functions in Spark SQL].

import org.apache.spark.sql.expressions.Window
val reveDesc = Window.partitionBy('category).orderBy('revenue.desc)
val reveDiff = max('revenue).over(reveDesc) - 'revenue

scala> data.select('*, reveDiff as 'revenue_diff).show
+----------+----------+-------+------------+
|   product|  category|revenue|revenue_diff|
+----------+----------+-------+------------+
|      Pro2|    tablet|   6500|           0|
|      Mini|    tablet|   5500|        1000|
|       Pro|    tablet|   4500|        2000|
|       Big|    tablet|   2500|        4000|
|    Normal|    tablet|   1500|        5000|
|      Thin|cell phone|   6000|           0|
| Very thin|cell phone|   6000|           0|
|Ultra thin|cell phone|   5000|        1000|
|  Bendable|cell phone|   3000|        3000|
|  Foldable|cell phone|   3000|        3000|
+----------+----------+-------+------------+

==== Difference on Column

Compute a difference between values in rows in a column.

val pairs = for {
  x <- 1 to 5
  y <- 1 to 2
} yield (x, 10 * x * y)
val ds = pairs.toDF("ns", "tens")

scala> ds.show
+---+----+
| ns|tens|
+---+----+
|  1|  10|
|  1|  20|
|  2|  20|
|  2|  40|
|  3|  30|
|  3|  60|
|  4|  40|
|  4|  80|
|  5|  50|
|  5| 100|
+---+----+

import org.apache.spark.sql.expressions.Window
val overNs = Window.partitionBy('ns).orderBy('tens)
val diff = lead('tens, 1).over(overNs)

scala> ds.withColumn("diff", diff - 'tens).show
+---+----+----+
| ns|tens|diff|
+---+----+----+
|  1|  10|  10|
|  1|  20|null|
|  3|  30|  30|
|  3|  60|null|
|  5|  50|  50|
|  5| 100|null|
|  4|  40|  40|
|  4|  80|null|
|  2|  20|  20|
|  2|  40|null|
+---+----+----+

Please note that http://stackoverflow.com/a/32379437/1305344[Why do Window functions fail with "Window function X does not take a frame specification"?]

The key here is to remember that DataFrames are RDDs under the covers and hence aggregation like grouping by a key in DataFrames is RDD's groupBy (or worse, reduceByKey or aggregateByKey transformations).

==== [[example-running-total]] Running Total

The running total is the sum of all previous lines including the current one.

[source, scala]

val sales = Seq( (0, 0, 0, 5), (1, 0, 1, 3), (2, 0, 2, 1), (3, 1, 0, 2), (4, 2, 0, 8), (5, 2, 2, 8)) .toDF("id", "orderID", "prodID", "orderQty")

scala> sales.show +---+-------+------+--------+ | id|orderID|prodID|orderQty| +---+-------+------+--------+ | 0| 0| 0| 5| | 1| 0| 1| 3| | 2| 0| 2| 1| | 3| 1| 0| 2| | 4| 2| 0| 8| | 5| 2| 2| 8| +---+-------+------+--------+

val orderedByID = Window.orderBy('id)

val totalQty = sum('orderQty).over(orderedByID).as('running_total) val salesTotalQty = sales.select('*, totalQty).orderBy('id)

scala> salesTotalQty.show 16/04/10 23:01:52 WARN Window: No Partition Defined for Window operation! Moving all data to a single partition, this can cause serious performance degradation. +---+-------+------+--------+-------------+ | id|orderID|prodID|orderQty|running_total| +---+-------+------+--------+-------------+ | 0| 0| 0| 5| 5| | 1| 0| 1| 3| 8| | 2| 0| 2| 1| 9| | 3| 1| 0| 2| 11| | 4| 2| 0| 8| 19| | 5| 2| 2| 8| 27| +---+-------+------+--------+-------------+

val byOrderId = orderedByID.partitionBy('orderID) val totalQtyPerOrder = sum('orderQty).over(byOrderId).as('running_total_per_order) val salesTotalQtyPerOrder = sales.select('*, totalQtyPerOrder).orderBy('id)

scala> salesTotalQtyPerOrder.show +---+-------+------+--------+-----------------------+ | id|orderID|prodID|orderQty|running_total_per_order| +---+-------+------+--------+-----------------------+ | 0| 0| 0| 5| 5| | 1| 0| 1| 3| 8| | 2| 0| 2| 1| 9| | 3| 1| 0| 2| 2| | 4| 2| 0| 8| 8| | 5| 2| 2| 8| 16| +---+-------+------+--------+-----------------------+


==== [[example-rank]] Calculate rank of row

See <> for an elaborate example.

=== Interval data type for Date and Timestamp types

See https://issues.apache.org/jira/browse/SPARK-8943[[SPARK-8943] CalendarIntervalType for time intervals].

With the Interval data type, you could use intervals as values specified in <value> PRECEDING and <value> FOLLOWING for RANGE frame. It is specifically suited for time-series analysis with window functions.

==== Accessing values of earlier rows

FIXME What's the value of rows before current one?

==== [[example-moving-average]] Moving Average

==== [[example-cumulative-aggregates]] Cumulative Aggregates

Eg. cumulative sum

=== User-defined aggregate functions

See https://issues.apache.org/jira/browse/SPARK-3947[[SPARK-3947] Support Scala/Java UDAF].

With the window function support, you could use user-defined aggregate functions as window functions.

=== [[explain-windows]] "Explaining" Query Plans of Windows

import org.apache.spark.sql.expressions.Window
val byDepnameSalaryDesc = Window.partitionBy('depname).orderBy('salary desc)

scala> val rankByDepname = rank().over(byDepnameSalaryDesc)
rankByDepname: org.apache.spark.sql.Column = RANK() OVER (PARTITION BY depname ORDER BY salary DESC UnspecifiedFrame)

// empsalary defined at the top of the page
scala> empsalary.select('*, rankByDepname as 'rank).explain(extended = true)
== Parsed Logical Plan ==
'Project [*, rank() windowspecdefinition('depname, 'salary DESC, UnspecifiedFrame) AS rank#9]
+- LocalRelation [depName#5, empNo#6L, salary#7L]

== Analyzed Logical Plan ==
depName: string, empNo: bigint, salary: bigint, rank: int
Project [depName#5, empNo#6L, salary#7L, rank#9]
+- Project [depName#5, empNo#6L, salary#7L, rank#9, rank#9]
   +- Window [rank(salary#7L) windowspecdefinition(depname#5, salary#7L DESC, ROWS BETWEEN UNBOUNDED PRECEDING AND CURRENT ROW) AS rank#9], [depname#5], [salary#7L DESC]
      +- Project [depName#5, empNo#6L, salary#7L]
         +- LocalRelation [depName#5, empNo#6L, salary#7L]

== Optimized Logical Plan ==
Window [rank(salary#7L) windowspecdefinition(depname#5, salary#7L DESC, ROWS BETWEEN UNBOUNDED PRECEDING AND CURRENT ROW) AS rank#9], [depname#5], [salary#7L DESC]
+- LocalRelation [depName#5, empNo#6L, salary#7L]

== Physical Plan ==
Window [rank(salary#7L) windowspecdefinition(depname#5, salary#7L DESC, ROWS BETWEEN UNBOUNDED PRECEDING AND CURRENT ROW) AS rank#9], [depname#5], [salary#7L DESC]
+- *Sort [depname#5 ASC, salary#7L DESC], false, 0
   +- Exchange hashpartitioning(depname#5, 200)
      +- LocalTableScan [depName#5, empNo#6L, salary#7L]

=== [[lag]] lag Window Function

[source, scala]

lag(e: Column, offset: Int): Column lag(columnName: String, offset: Int): Column lag(columnName: String, offset: Int, defaultValue: Any): Column lag(e: Column, offset: Int, defaultValue: Any): Column


lag returns the value in e / columnName column that is offset records before the current record. lag returns null value if the number of records in a window partition is less than offset or defaultValue.

[source, scala]

val buckets = spark.range(9).withColumn("bucket", 'id % 3) // Make duplicates val dataset = buckets.union(buckets)

import org.apache.spark.sql.expressions.Window val windowSpec = Window.partitionBy('bucket).orderBy('id) scala> dataset.withColumn("lag", lag('id, 1) over windowSpec).show +---+------+----+ | id|bucket| lag| +---+------+----+ | 0| 0|null| | 3| 0| 0| | 6| 0| 3| | 1| 1|null| | 4| 1| 1| | 7| 1| 4| | 2| 2|null| | 5| 2| 2| | 8| 2| 5| +---+------+----+

scala> dataset.withColumn("lag", lag('id, 2, "") over windowSpec).show +---+------+----+ | id|bucket| lag| +---+------+----+ | 0| 0|null| | 3| 0|null| | 6| 0| 0| | 1| 1|null| | 4| 1|null| | 7| 1| 1| | 2| 2|null| | 5| 2|null| | 8| 2| 2| +---+------+----+


CAUTION: FIXME It looks like lag with a default value has a bug -- the default value's not used at all.

=== [[lead]] lead Window Function

[source, scala]

lead(columnName: String, offset: Int): Column lead(e: Column, offset: Int): Column lead(columnName: String, offset: Int, defaultValue: Any): Column lead(e: Column, offset: Int, defaultValue: Any): Column


lead returns the value that is offset records after the current records, and defaultValue if there is less than offset records after the current record. lag returns null value if the number of records in a window partition is less than offset or defaultValue.

[source, scala]

val buckets = spark.range(9).withColumn("bucket", 'id % 3) // Make duplicates val dataset = buckets.union(buckets)

import org.apache.spark.sql.expressions.Window val windowSpec = Window.partitionBy('bucket).orderBy('id) scala> dataset.withColumn("lead", lead('id, 1) over windowSpec).show +---+------+----+ | id|bucket|lead| +---+------+----+ | 0| 0| 0| | 0| 0| 3| | 3| 0| 3| | 3| 0| 6| | 6| 0| 6| | 6| 0|null| | 1| 1| 1| | 1| 1| 4| | 4| 1| 4| | 4| 1| 7| | 7| 1| 7| | 7| 1|null| | 2| 2| 2| | 2| 2| 5| | 5| 2| 5| | 5| 2| 8| | 8| 2| 8| | 8| 2|null| +---+------+----+

scala> dataset.withColumn("lead", lead('id, 2, "") over windowSpec).show +---+------+----+ | id|bucket|lead| +---+------+----+ | 0| 0| 3| | 0| 0| 3| | 3| 0| 6| | 3| 0| 6| | 6| 0|null| | 6| 0|null| | 1| 1| 4| | 1| 1| 4| | 4| 1| 7| | 4| 1| 7| | 7| 1|null| | 7| 1|null| | 2| 2| 5| | 2| 2| 5| | 5| 2| 8| | 5| 2| 8| | 8| 2|null| | 8| 2|null| +---+------+----+


CAUTION: FIXME It looks like lead with a default value has a bug -- the default value's not used at all.

=== [[cume_dist]] Cumulative Distribution of Records Across Window Partitions -- cume_dist Window Function

[source, scala]

cume_dist(): Column

cume_dist computes the cumulative distribution of the records in window partitions. This is equivalent to SQL's CUME_DIST function.

[source, scala]

val buckets = spark.range(9).withColumn("bucket", 'id % 3) // Make duplicates val dataset = buckets.union(buckets)

import org.apache.spark.sql.expressions.Window val windowSpec = Window.partitionBy('bucket).orderBy('id) scala> dataset.withColumn("cume_dist", cume_dist over windowSpec).show +---+------+------------------+ | id|bucket| cume_dist| +---+------+------------------+ | 0| 0|0.3333333333333333| | 3| 0|0.6666666666666666| | 6| 0| 1.0| | 1| 1|0.3333333333333333| | 4| 1|0.6666666666666666| | 7| 1| 1.0| | 2| 2|0.3333333333333333| | 5| 2|0.6666666666666666| | 8| 2| 1.0| +---+------+------------------+


=== [[row_number]] Sequential numbering per window partition -- row_number Window Function

[source, scala]

row_number(): Column

row_number returns a sequential number starting at 1 within a window partition.

[source, scala]

val buckets = spark.range(9).withColumn("bucket", 'id % 3) // Make duplicates val dataset = buckets.union(buckets)

import org.apache.spark.sql.expressions.Window val windowSpec = Window.partitionBy('bucket).orderBy('id) scala> dataset.withColumn("row_number", row_number() over windowSpec).show +---+------+----------+ | id|bucket|row_number| +---+------+----------+ | 0| 0| 1| | 0| 0| 2| | 3| 0| 3| | 3| 0| 4| | 6| 0| 5| | 6| 0| 6| | 1| 1| 1| | 1| 1| 2| | 4| 1| 3| | 4| 1| 4| | 7| 1| 5| | 7| 1| 6| | 2| 2| 1| | 2| 2| 2| | 5| 2| 3| | 5| 2| 4| | 8| 2| 5| | 8| 2| 6| +---+------+----------+


=== [[ntile]] ntile Window Function

[source, scala]

ntile(n: Int): Column

ntile computes the ntile group id (from 1 to n inclusive) in an ordered window partition.

[source, scala]

val dataset = spark.range(7).select('*, 'id % 3 as "bucket")

import org.apache.spark.sql.expressions.Window val byBuckets = Window.partitionBy('bucket).orderBy('id) scala> dataset.select('*, ntile(3) over byBuckets as "ntile").show +---+------+-----+ | id|bucket|ntile| +---+------+-----+ | 0| 0| 1| | 3| 0| 2| | 6| 0| 3| | 1| 1| 1| | 4| 1| 2| | 2| 2| 1| | 5| 2| 2| +---+------+-----+


CAUTION: FIXME How is ntile different from rank? What about performance?

=== [[rank]][[dense_rank]][[percent_rank]] Ranking Records per Window Partition -- rank Window Function

[source, scala]

rank(): Column dense_rank(): Column percent_rank(): Column


rank functions assign the sequential rank of each distinct value per window partition. They are equivalent to RANK, DENSE_RANK and PERCENT_RANK functions in the good ol' SQL.

[source, scala]

val dataset = spark.range(9).withColumn("bucket", 'id % 3)

import org.apache.spark.sql.expressions.Window val byBucket = Window.partitionBy('bucket).orderBy('id)

scala> dataset.withColumn("rank", rank over byBucket).show +---+------+----+ | id|bucket|rank| +---+------+----+ | 0| 0| 1| | 3| 0| 2| | 6| 0| 3| | 1| 1| 1| | 4| 1| 2| | 7| 1| 3| | 2| 2| 1| | 5| 2| 2| | 8| 2| 3| +---+------+----+

scala> dataset.withColumn("percent_rank", percent_rank over byBucket).show +---+------+------------+ | id|bucket|percent_rank| +---+------+------------+ | 0| 0| 0.0| | 3| 0| 0.5| | 6| 0| 1.0| | 1| 1| 0.0| | 4| 1| 0.5| | 7| 1| 1.0| | 2| 2| 0.0| | 5| 2| 0.5| | 8| 2| 1.0| +---+------+------------+


rank function assigns the same rank for duplicate rows with a gap in the sequence (similarly to Olympic medal places). dense_rank is like rank for duplicate rows but compacts the ranks and removes the gaps.

[source, scala]

// rank function with duplicates // Note the missing/sparse ranks, i.e. 2 and 4 scala> dataset.union(dataset).withColumn("rank", rank over byBucket).show +---+------+----+ | id|bucket|rank| +---+------+----+ | 0| 0| 1| | 0| 0| 1| | 3| 0| 3| | 3| 0| 3| | 6| 0| 5| | 6| 0| 5| | 1| 1| 1| | 1| 1| 1| | 4| 1| 3| | 4| 1| 3| | 7| 1| 5| | 7| 1| 5| | 2| 2| 1| | 2| 2| 1| | 5| 2| 3| | 5| 2| 3| | 8| 2| 5| | 8| 2| 5| +---+------+----+

// dense_rank function with duplicates // Note that the missing ranks are now filled in scala> dataset.union(dataset).withColumn("dense_rank", dense_rank over byBucket).show +---+------+----------+ | id|bucket|dense_rank| +---+------+----------+ | 0| 0| 1| | 0| 0| 1| | 3| 0| 2| | 3| 0| 2| | 6| 0| 3| | 6| 0| 3| | 1| 1| 1| | 1| 1| 1| | 4| 1| 2| | 4| 1| 2| | 7| 1| 3| | 7| 1| 3| | 2| 2| 1| | 2| 2| 1| | 5| 2| 2| | 5| 2| 2| | 8| 2| 3| | 8| 2| 3| +---+------+----------+

// percent_rank function with duplicates scala> dataset.union(dataset).withColumn("percent_rank", percent_rank over byBucket).show +---+------+------------+ | id|bucket|percent_rank| +---+------+------------+ | 0| 0| 0.0| | 0| 0| 0.0| | 3| 0| 0.4| | 3| 0| 0.4| | 6| 0| 0.8| | 6| 0| 0.8| | 1| 1| 0.0| | 1| 1| 0.0| | 4| 1| 0.4| | 4| 1| 0.4| | 7| 1| 0.8| | 7| 1| 0.8| | 2| 2| 0.0| | 2| 2| 0.0| | 5| 2| 0.4| | 5| 2| 0.4| | 8| 2| 0.8| | 8| 2| 0.8| +---+------+------------+


=== [[currentRow]] currentRow Window Function

[source, scala]

currentRow(): Column

currentRow...FIXME

=== [[unboundedFollowing]] unboundedFollowing Window Function

[source, scala]

unboundedFollowing(): Column

unboundedFollowing...FIXME

=== [[unboundedPreceding]] unboundedPreceding Window Function

[source, scala]

unboundedPreceding(): Column

unboundedPreceding...FIXME

=== [[i-want-more]] Further Reading and Watching


Last update: 2021-02-18
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