UnresolvedAttribute Leaf Expression

UnresolvedAttribute is a named Attribute leaf expression (i.e. it has a name) that represents a reference to an entity in a logical query plan.

UnresolvedAttribute is created when:

UnresolvedAttribute can never be resolved (and is replaced at analysis phase).


UnresolvedAttribute is resolved when Analyzer is executed by the following logical resolution rules:

Given UnresolvedAttribute can never be resolved it should not come as a surprise that it cannot be evaluated either (i.e. produce a value given an internal row). When requested to evaluate, UnresolvedAttribute simply reports a UnsupportedOperationException.

Cannot evaluate expression: [this]

UnresolvedAttribute takes name parts when created.

import org.apache.spark.sql.catalyst.analysis.UnresolvedAttribute

scala> val t1 = UnresolvedAttribute("t1")
t1: org.apache.spark.sql.catalyst.analysis.UnresolvedAttribute = 't1

scala> val t2 = UnresolvedAttribute("db1.t2")
t2: org.apache.spark.sql.catalyst.analysis.UnresolvedAttribute = 'db1.t2

scala> println(s"Number of name parts: ${t2.nameParts.length}")
Number of name parts: 2

scala> println(s"Name parts: ${t2.nameParts.mkString(",")}")
Name parts: db1,t2

UnresolvedAttribute can be created with a fully-qualified name with dots to separate name parts.

apply(name: String): UnresolvedAttribute

Use backticks (``) around names with dots (.) to disable them as separators.

The following is a two-part attribute name with a.b and c name parts.


UnresolvedAttribute can also be created without the dots with the special meaning.

import org.apache.spark.sql.catalyst.analysis.UnresolvedAttribute
val attr1 = UnresolvedAttribute.quoted("a.b.c")
scala> println(s"Number of name parts: ${attr1.nameParts.length}")
Number of name parts: 1

Catalyst DSL defines two Scala implicits to create an UnresolvedAttribute:

  • StringToAttributeConversionHelper is a Scala implicit class that converts $"colName" into an UnresolvedAttribute

  • symbolToUnresolvedAttribute is a Scala implicit method that converts 'colName into an UnresolvedAttribute

Both implicits are part of ExpressionConversions Scala trait of Catalyst DSL.

Import expressions object to get access to the expression conversions.

// Use `sbt console` with Spark libraries defined (in `build.sbt`)
import org.apache.spark.sql.catalyst.dsl.expressions._
scala> :type $"name"

import org.apache.spark.sql.catalyst.analysis.UnresolvedAttribute
val nameAttr: UnresolvedAttribute = 'name

scala> :type nameAttr

A UnresolvedAttribute can be replaced by (resolved) a NamedExpression using an analyzed logical plan (of the structured query the attribute is part of).

val analyzedPlan = Seq((0, "zero")).toDF("id", "name").queryExecution.analyzed

import org.apache.spark.sql.catalyst.analysis.UnresolvedAttribute
val nameAttr = UnresolvedAttribute("name")
val nameResolved = analyzedPlan.resolveQuoted(
  name = nameAttr.name,
  resolver = spark.sessionState.analyzer.resolver).getOrElse(nameAttr)

scala> println(nameResolved.numberedTreeString)
00 name#47: string

scala> :type nameResolved

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