How to select the first row of each group?
Solution 1:
Window functions:
Something like this should do the trick:
import org.apache.spark.sql.functions.{row_number, max, broadcast}
import org.apache.spark.sql.expressions.Window
val df = sc.parallelize(Seq(
(0,"cat26",30.9), (0,"cat13",22.1), (0,"cat95",19.6), (0,"cat105",1.3),
(1,"cat67",28.5), (1,"cat4",26.8), (1,"cat13",12.6), (1,"cat23",5.3),
(2,"cat56",39.6), (2,"cat40",29.7), (2,"cat187",27.9), (2,"cat68",9.8),
(3,"cat8",35.6))).toDF("Hour", "Category", "TotalValue")
val w = Window.partitionBy($"hour").orderBy($"TotalValue".desc)
val dfTop = df.withColumn("rn", row_number.over(w)).where($"rn" === 1).drop("rn")
dfTop.show
// +----+--------+----------+
// |Hour|Category|TotalValue|
// +----+--------+----------+
// | 0| cat26| 30.9|
// | 1| cat67| 28.5|
// | 2| cat56| 39.6|
// | 3| cat8| 35.6|
// +----+--------+----------+
This method will be inefficient in case of significant data skew. This problem is tracked by SPARK-34775 and might be resolved in the future (SPARK-37099).
Plain SQL aggregation followed by join:
Alternatively you can join with aggregated data frame:
val dfMax = df.groupBy($"hour".as("max_hour")).agg(max($"TotalValue").as("max_value"))
val dfTopByJoin = df.join(broadcast(dfMax),
($"hour" === $"max_hour") && ($"TotalValue" === $"max_value"))
.drop("max_hour")
.drop("max_value")
dfTopByJoin.show
// +----+--------+----------+
// |Hour|Category|TotalValue|
// +----+--------+----------+
// | 0| cat26| 30.9|
// | 1| cat67| 28.5|
// | 2| cat56| 39.6|
// | 3| cat8| 35.6|
// +----+--------+----------+
It will keep duplicate values (if there is more than one category per hour with the same total value). You can remove these as follows:
dfTopByJoin
.groupBy($"hour")
.agg(
first("category").alias("category"),
first("TotalValue").alias("TotalValue"))
Using ordering over structs:
Neat, although not very well tested, trick which doesn't require joins or window functions:
val dfTop = df.select($"Hour", struct($"TotalValue", $"Category").alias("vs"))
.groupBy($"hour")
.agg(max("vs").alias("vs"))
.select($"Hour", $"vs.Category", $"vs.TotalValue")
dfTop.show
// +----+--------+----------+
// |Hour|Category|TotalValue|
// +----+--------+----------+
// | 0| cat26| 30.9|
// | 1| cat67| 28.5|
// | 2| cat56| 39.6|
// | 3| cat8| 35.6|
// +----+--------+----------+
With DataSet API (Spark 1.6+, 2.0+):
Spark 1.6:
case class Record(Hour: Integer, Category: String, TotalValue: Double)
df.as[Record]
.groupBy($"hour")
.reduce((x, y) => if (x.TotalValue > y.TotalValue) x else y)
.show
// +---+--------------+
// | _1| _2|
// +---+--------------+
// |[0]|[0,cat26,30.9]|
// |[1]|[1,cat67,28.5]|
// |[2]|[2,cat56,39.6]|
// |[3]| [3,cat8,35.6]|
// +---+--------------+
Spark 2.0 or later:
df.as[Record]
.groupByKey(_.Hour)
.reduceGroups((x, y) => if (x.TotalValue > y.TotalValue) x else y)
The last two methods can leverage map side combine and don't require full shuffle so most of the time should exhibit a better performance compared to window functions and joins. These cane be also used with Structured Streaming in completed output mode.
Don't use:
df.orderBy(...).groupBy(...).agg(first(...), ...)
It may seem to work (especially in the local mode) but it is unreliable (see SPARK-16207, credits to Tzach Zohar for linking relevant JIRA issue, and SPARK-30335).
The same note applies to
df.orderBy(...).dropDuplicates(...)
which internally uses equivalent execution plan.
Solution 2:
For Spark 2.0.2 with grouping by multiple columns:
import org.apache.spark.sql.functions.row_number
import org.apache.spark.sql.expressions.Window
val w = Window.partitionBy($"col1", $"col2", $"col3").orderBy($"timestamp".desc)
val refined_df = df.withColumn("rn", row_number.over(w)).where($"rn" === 1).drop("rn")
Solution 3:
This is a exact same of zero323's answer but in SQL query way.
Assuming that dataframe is created and registered as
df.createOrReplaceTempView("table")
//+----+--------+----------+
//|Hour|Category|TotalValue|
//+----+--------+----------+
//|0 |cat26 |30.9 |
//|0 |cat13 |22.1 |
//|0 |cat95 |19.6 |
//|0 |cat105 |1.3 |
//|1 |cat67 |28.5 |
//|1 |cat4 |26.8 |
//|1 |cat13 |12.6 |
//|1 |cat23 |5.3 |
//|2 |cat56 |39.6 |
//|2 |cat40 |29.7 |
//|2 |cat187 |27.9 |
//|2 |cat68 |9.8 |
//|3 |cat8 |35.6 |
//+----+--------+----------+
Window function :
sqlContext.sql("select Hour, Category, TotalValue from (select *, row_number() OVER (PARTITION BY Hour ORDER BY TotalValue DESC) as rn FROM table) tmp where rn = 1").show(false)
//+----+--------+----------+
//|Hour|Category|TotalValue|
//+----+--------+----------+
//|1 |cat67 |28.5 |
//|3 |cat8 |35.6 |
//|2 |cat56 |39.6 |
//|0 |cat26 |30.9 |
//+----+--------+----------+
Plain SQL aggregation followed by join:
sqlContext.sql("select Hour, first(Category) as Category, first(TotalValue) as TotalValue from " +
"(select Hour, Category, TotalValue from table tmp1 " +
"join " +
"(select Hour as max_hour, max(TotalValue) as max_value from table group by Hour) tmp2 " +
"on " +
"tmp1.Hour = tmp2.max_hour and tmp1.TotalValue = tmp2.max_value) tmp3 " +
"group by tmp3.Hour")
.show(false)
//+----+--------+----------+
//|Hour|Category|TotalValue|
//+----+--------+----------+
//|1 |cat67 |28.5 |
//|3 |cat8 |35.6 |
//|2 |cat56 |39.6 |
//|0 |cat26 |30.9 |
//+----+--------+----------+
Using ordering over structs:
sqlContext.sql("select Hour, vs.Category, vs.TotalValue from (select Hour, max(struct(TotalValue, Category)) as vs from table group by Hour)").show(false)
//+----+--------+----------+
//|Hour|Category|TotalValue|
//+----+--------+----------+
//|1 |cat67 |28.5 |
//|3 |cat8 |35.6 |
//|2 |cat56 |39.6 |
//|0 |cat26 |30.9 |
//+----+--------+----------+
DataSets way and don't dos are same as in original answer