What is Project node in execution query plan? - apache-spark

Question

What is the meaning of Project node in Sparks execution plan?
I have a plan containing the following:
+- Project [dm_country#population#6a1ad864-235f-4761-9a6d-0ca2a2b40686#834, dm_country#population#country#839, population#17 AS dm_country#population#population#844]
+- Project [dm_country#population#6a1ad864-235f-4761-9a6d-0ca2a2b40686#834, country#12 AS dm_country#population#country#839, population#17]
+- Project [6a1ad864-235f-4761-9a6d-0ca2a2b40686#22 AS dm_country#population#6a1ad864-235f-4761-9a6d-0ca2a2b40686#834, country#12, population#17]
+- RepartitionByExpression [country#12], 1000
+- Union
:- Project [ind#7 AS 6a1ad864-235f-4761-9a6d-0ca2a2b40686#22, country#12, population#17]
: +- Project [ind#7, country#12, population#2 AS population#17]
: +- Project [ind#7, country#1 AS country#12, population#2]
: +- Project [ind#0 AS ind#7, country#1, population#2]
: +- Relation[ind#0,country#1,population#2] JDBCRelation(schema_dbadmin.t_350) [numPartitions=100]
+- LogicalRDD [ind#45, country#46, population#47]

Answer 1

NOTE: Since the plan uses RepartitionByExpression node it must be a logical query plan.
Project node in a logical query plan stands for Project unary logical operator and is created whenever you use some kind of projection explicitly or implicitly.
Quoting Wikipedia's Projection (relational algebra):
In practical terms, it can be roughly thought of as picking a subset of all available columns.
A Project node can appear in a logical query plan explicitly for the following:
Dataset operators, i.e. joinWith, select, unionByName
KeyValueGroupedDataset operators, i.e. keys, mapValues
SQL's SELECT queries
A Project node can also appear analysis and optimization phases.
In Spark SQL, the Dataset API gives the high-level operators, e.g. select, filter or groupBy, that ultimately build a Catalyst logical plan of a structured query. In other words, this simple-looking Dataset.select operator is just to create a LogicalPlan with Project node.
val query = spark.range(4).select("id")
scala> println(query.queryExecution.logical)
'Project [unresolvedalias('id, None)]
+- Range (0, 4, step=1, splits=Some(8))
(You could have used query.explain(extended = true) for the above but that would have given you all the 4 plans which may have hidden the point)
You could have a look at the code of Dataset.select operator.
def select(cols: Column*): DataFrame = withPlan {
Project(cols.map(_.named), logicalPlan)
}
This simple-looking select operator is a mere wrapper around Catalyst operators to build a Catalyst tree of logical operators that gives a logical plan.
NOTE What's nice about Spark SQL's Catalyst is that it uses this recursive LogicalPlan abstraction that represents an logical operator or a tree of logical operator.
NOTE The same applies to the good ol' SQL where after being parsed the SQL text is transformed to a AST of logical operators. See the example below.
Project can come and go since projection is for the number of columns in the output and may or may not appear in your plans and queries.
Catalyst DSL
You can use Spark SQL's Catalyst DSL (in org.apache.spark.sql.catalyst.dsl package object) for constructing Catalyst data structures using Scala implicit conversions. That could be very useful if you are into Spark testing.
scala> spark.version
res0: String = 2.3.0-SNAPSHOT
import org.apache.spark.sql.catalyst.dsl.plans._ // <-- gives table and select
import org.apache.spark.sql.catalyst.dsl.expressions.star
val plan = table("a").select(star())
scala> println(plan.numberedTreeString)
00 'Project [*]
01 +- 'UnresolvedRelation `a`
Good ol' SQL
scala> spark.range(4).createOrReplaceTempView("nums")
scala> spark.sql("SHOW TABLES").show
+--------+---------+-----------+
|database|tableName|isTemporary|
+--------+---------+-----------+
| | nums| true|
+--------+---------+-----------+
scala> spark.sql("SELECT * FROM nums").explain
== Physical Plan ==
*Range (0, 4, step=1, splits=8)
scala> spark.sql("SELECT * FROM nums").explain(true)
== Parsed Logical Plan ==
'Project [*]
+- 'UnresolvedRelation `nums`
== Analyzed Logical Plan ==
id: bigint
Project [id#40L]
+- SubqueryAlias nums
+- Range (0, 4, step=1, splits=Some(8))
== Optimized Logical Plan ==
Range (0, 4, step=1, splits=Some(8))
== Physical Plan ==
*Range (0, 4, step=1, splits=8)