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spark-sql-streaming-Dataset-explain.adoc

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explain Operator — Explaining Query Plan

explain(): Unit // (1)
explain(extended: Boolean): Unit

  1. Calls explain with extended flag disabled

explain prints the logical and (with extended flag enabled) physical plans to the console.

val records = spark.
  readStream.
  format("rate").
  load
scala> records.explain
== Physical Plan ==
StreamingRelation rate, [timestamp#0, value#1L]

scala> records.explain(extended = true)
== Parsed Logical Plan ==
StreamingRelation DataSource(org.apache.spark.sql.SparkSession@4071aa13,rate,List(),None,List(),None,Map(),None), rate, [timestamp#0, value#1L]

== Analyzed Logical Plan ==
timestamp: timestamp, value: bigint
StreamingRelation DataSource(org.apache.spark.sql.SparkSession@4071aa13,rate,List(),None,List(),None,Map(),None), rate, [timestamp#0, value#1L]

== Optimized Logical Plan ==
StreamingRelation DataSource(org.apache.spark.sql.SparkSession@4071aa13,rate,List(),None,List(),None,Map(),None), rate, [timestamp#0, value#1L]

== Physical Plan ==
StreamingRelation rate, [timestamp#0, value#1L]

Internally, explain creates a ExplainCommand runnable command with the logical plan and extended flag.

explain then executes the plan with ExplainCommand runnable command and collects the results that are printed out to the standard output.

Note

explain uses SparkSession to access the current SessionState to execute the plan.

import org.apache.spark.sql.execution.command.ExplainCommand
val explain = ExplainCommand(...)
spark.sessionState.executePlan(explain)

For streaming Datasets, ExplainCommand command simply creates a IncrementalExecution for the SparkSession and the logical plan.

Note
 For the purpose of explain, IncrementalExecution is created with the output mode Append, checkpoint location <unknown>, run id a random number, current batch id 0 and offset metadata empty. They do not really matter when explaining the load-part of a streaming query.