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UnresolvedStar Expression

UnresolvedStar is a Star expression that represents a star (i.e. all) expression in a logical query plan.

UnresolvedStar is created when:

val q = spark.range(5).select("*")
val plan = q.queryExecution.logical
scala> println(plan.numberedTreeString)
00 'Project [*]
01 +- AnalysisBarrier
02       +- Range (0, 5, step=1, splits=Some(8))

import org.apache.spark.sql.catalyst.analysis.UnresolvedStar
val starExpr = plan.expressions.head.asInstanceOf[UnresolvedStar]

val namedExprs = starExpr.expand(input = q.queryExecution.analyzed, spark.sessionState.analyzer.resolver)
scala> println(namedExprs.head.numberedTreeString)
00 id#0: bigint

[[resolved]] UnresolvedStar can never be Expression.md#resolved[resolved], and is <> at analysis (when ResolveReferences logical resolution rule is executed).

NOTE: UnresolvedStar can only be used in Star.md#expand[Project, Aggregate or ScriptTransformation logical operators].

[[Unevaluable]][[eval]][[doGenCode]] Given UnresolvedStar can never be <> 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, UnresolvedStar simply reports a UnsupportedOperationException.

Cannot evaluate expression: [this]

[[creating-instance]] [[target]] When created, UnresolvedStar takes name parts that, once concatenated, is the target of the star expansion.

[source, scala]

import org.apache.spark.sql.catalyst.analysis.UnresolvedStar scala> val us = UnresolvedStar(None) us: org.apache.spark.sql.catalyst.analysis.UnresolvedStar = *

scala> val ab = UnresolvedStar(Some("a" :: "b" :: Nil)) ab: org.apache.spark.sql.catalyst.analysis.UnresolvedStar = List(a, b).*


[TIP]

Use star operator from Catalyst DSL's expressions to create an UnresolvedStar.

[source, scala]

import org.apache.spark.sql.catalyst.dsl.expressions._ val s = star() scala> :type s org.apache.spark.sql.catalyst.expressions.Expression

import org.apache.spark.sql.catalyst.analysis.UnresolvedStar assert(s.isInstanceOf[UnresolvedStar])

val s = star("a", "b") scala> println(s) WrappedArray(a, b).*


You could also use $"*" or '* to create an UnresolvedStar, but that requires sbt console (with Spark libraries defined in build.sbt) as the Catalyst DSL expressions implicits interfere with the Spark implicits to create columns.

[NOTE]

AstBuilder sql/AstBuilder.md#visitFunctionCall[replaces] count(*) (with no DISTINCT keyword) to count(1).

val q = sql("SELECT COUNT(*) FROM RANGE(1,2,3)")
scala> println(q.queryExecution.logical.numberedTreeString)
00 'Project [unresolvedalias('count(1), None)]
01 +- 'UnresolvedTableValuedFunction range, [1, 2, 3]

val q = sql("SELECT COUNT(DISTINCT *) FROM RANGE(1,2,3)")
scala> println(q.queryExecution.logical.numberedTreeString)
00 'Project [unresolvedalias('COUNT(*), None)]
01 +- 'UnresolvedTableValuedFunction RANGE, [1, 2, 3]

=== [[expand]] Star Expansion -- expand Method

[source, scala]

expand(input: LogicalPlan, resolver: Resolver): Seq[NamedExpression]

NOTE: expand is part of Star.md#expand[Star Contract] to...FIXME.

expand first expands to named expressions per <>:

  • For unspecified <>, expand gives the catalyst/QueryPlan.md#output[output] schema of the input logical query plan (that assumes that the star refers to a relation / table)

  • For <> with one element, expand gives the table (attribute) in the catalyst/QueryPlan.md#output[output] schema of the input logical query plan (using NamedExpression.md#qualifier[qualifiers]) if available

With no result earlier, expand then requests the input logical query plan to spark-sql-LogicalPlan.md#resolve[resolve] the <> name parts to a named expression.

For a named expression of StructType data type, expand creates an spark-sql-Expression-Alias.md#creating-instance[Alias] expression with a GetStructField unary expression (with the resolved named expression and the field index).

val q = Seq((0, "zero")).toDF("id", "name").select(struct("id", "name") as "s")
val analyzedPlan = q.queryExecution.analyzed

import org.apache.spark.sql.catalyst.analysis.UnresolvedStar
import org.apache.spark.sql.catalyst.dsl.expressions._
val s = star("s").asInstanceOf[UnresolvedStar]
val exprs = s.expand(input = analyzedPlan, spark.sessionState.analyzer.resolver)

// star("s") should expand to two Alias(GetStructField) expressions
// s is a struct of id and name in the query

import org.apache.spark.sql.catalyst.expressions.{Alias, GetStructField}
val getStructFields = exprs.collect { case Alias(g: GetStructField, _) => g }.map(_.sql)
scala> getStructFields.foreach(println)
`s`.`id`
`s`.`name`

expand reports a AnalysisException when:

  • The Expression.md#dataType[data type] of the named expression (when the input logical plan was requested to spark-sql-LogicalPlan.md#resolve[resolve] the <>) is not a StructType. +

    Can only star expand struct data types. Attribute: `[target]`
    

  • Earlier attempts gave no results +

    cannot resolve '[target].*' given input columns '[from]'
    

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