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QueryExecution — Structured Query Execution Pipeline

QueryExecution is the execution pipeline (workflow) of a structured query.

QueryExecution is made up of execution stages (phases).

Query Execution — From SQL through Dataset to RDD

QueryExecution is the result of executing a LogicalPlan in a SparkSession (and so you could create a Dataset from a logical operator or use the QueryExecution after executing a logical operator).

val plan: LogicalPlan = ...
val qe = new QueryExecution(sparkSession, plan)

Creating Instance

QueryExecution takes the following to be created:

QueryExecution is created when:

QueryPlanningTracker

QueryExecution can be given a QueryPlanningTracker when created.

Accessing QueryExecution

QueryExecution is part of Dataset using queryExecution attribute.

val ds: Dataset[Long] = ...
ds.queryExecution

Execution Pipeline Phases

Analyzed Logical Plan

Analyzed logical plan that has passed Logical Analyzer.

Tip

Beside analyzed, you can use Dataset.explain basic action (with extended flag enabled) or SQL's EXPLAIN EXTENDED to see the analyzed logical plan of a structured query.

Analyzed Logical Plan with Cached Data

Analyzed logical plan after CacheManager was requested to replace logical query segments with cached query plans.

withCachedData makes sure that the logical plan was analyzed and uses supported operations only.

Optimized Logical Plan

Logical plan after executing the logical query plan optimizer on the withCachedData logical plan.

Physical Plan

Physical plan (after SparkPlanner has planned the optimized logical plan).

sparkPlan is the first physical plan from the collection of all possible physical plans.

Note

It is guaranteed that Catalyst's QueryPlanner (which SparkPlanner extends) will always generate at least one physical plan.

Optimized Physical Plan

Optimized physical plan that is in the final optimized "shape" and therefore ready for execution, i.e. the physical sparkPlan with physical preparation rules applied.

RDD

toRdd: RDD[InternalRow]

Spark Core's execution graph of a distributed computation (RDD of internal binary rows) from the executedPlan after execution.

The RDD is the top-level RDD of the DAG of RDDs (that represent physical operators).

Note

toRdd is a "boundary" between two Spark modules: Spark SQL and Spark Core.

After you have executed toRdd (directly or not), you basically "leave" Spark SQL's Dataset world and "enter" Spark Core's RDD space.

toRdd triggers a structured query execution (i.e. physical planning, but not execution of the plan) using SparkPlan.execute that recursively triggers execution of every child physical operator in the physical plan tree.

Note

SparkSession.internalCreateDataFrame applies a schema to an RDD[InternalRow].

Note

Dataset.rdd gives the RDD[InternalRow] with internal binary rows deserialized to a concrete Scala type.

You can access the lazy attributes as follows:

val dataset: Dataset[Long] = ...
dataset.queryExecution.executedPlan

QueryExecution uses the Logical Query Optimizer and Tungsten for better structured query performance.

QueryExecution uses the input SparkSession to access the current SparkPlanner (through SessionState) when <>. It then computes a SparkPlan (a PhysicalPlan exactly) using the planner. It is available as the <sparkPlan attribute>>.

Note

A variant of QueryExecution that Spark Structured Streaming uses for query planning is IncrementalExecution.

Refer to IncrementalExecution — QueryExecution of Streaming Datasets in the Spark Structured Streaming online gitbook.

SparkPlanner

SparkPlanner

Text Representation With Statistics

stringWithStats: String

stringWithStats...FIXME

stringWithStats is used when ExplainCommand logical command is executed (with cost flag enabled).

Physical Query Optimizations

Physical Query Optimizations are Catalyst Rules for transforming physical operators (to be more efficient and optimized for execution). They are executed in the following order:

  1. InsertAdaptiveSparkPlan (if defined)
  2. CoalesceBucketsInJoin
  3. PlanDynamicPruningFilters
  4. PlanSubqueries
  5. RemoveRedundantProjects
  6. EnsureRequirements
  7. RemoveRedundantSorts
  8. DisableUnnecessaryBucketedScan
  9. ApplyColumnarRulesAndInsertTransitions
  10. CollapseCodegenStages
  11. ReuseExchange
  12. ReuseSubquery

preparations

preparations: Seq[Rule[SparkPlan]]

preparations creates an InsertAdaptiveSparkPlan (with a new AdaptiveExecutionContext) that is added to the other preparations rules.

preparations is used when:

preparations Internal Utility

preparations(
  sparkSession: SparkSession,
  adaptiveExecutionRule: Option[InsertAdaptiveSparkPlan] = None): Seq[Rule[SparkPlan]]

preparations is the Physical Query Optimizations.

preparations is used when:

prepareExecutedPlan for Physical Operators

prepareExecutedPlan(
  spark: SparkSession,
  plan: SparkPlan): SparkPlan

prepareExecutedPlan applies the preparations physical query optimization rules (with no InsertAdaptiveSparkPlan optimization) to the physical plan.

prepareExecutedPlan is used when:

prepareExecutedPlan for Logical Operators

prepareExecutedPlan(
  spark: SparkSession,
  plan: LogicalPlan): SparkPlan

prepareExecutedPlan is...FIXME

prepareExecutedPlan is used when PlanSubqueries physical optimization is executed.

Applying preparations Physical Query Optimization Rules to Physical Plan

prepareForExecution(
  preparations: Seq[Rule[SparkPlan]],
  plan: SparkPlan): SparkPlan

prepareForExecution takes physical preparation rules and executes them one by one with the given SparkPlan.

prepareForExecution is used when:

assertSupported Method

assertSupported(): Unit

assertSupported requests UnsupportedOperationChecker to checkForBatch when...FIXME

assertSupported is used when QueryExecution is requested for withCachedData logical plan.

Creating Analyzed Logical Plan and Checking Correctness

assertAnalyzed(): Unit

assertAnalyzed triggers initialization of analyzed (which is almost like executing it).

assertAnalyzed executes analyzed by accessing it and throwing the result away. Since analyzed is a lazy value in Scala, it will then get initialized for the first time and stays so forever.

assertAnalyzed then requests Analyzer to validate analysis of the logical plan (i.e. analyzed).

Note

assertAnalyzed uses SparkSession to access the current SessionState that it then uses to access the Analyzer.

In case of any AnalysisException, assertAnalyzed creates a new AnalysisException to make sure that it holds analyzed and reports it.

Building Text Representation with Cost Stats

toStringWithStats: String

toStringWithStats is a mere alias for completeString with appendStats flag enabled.

toStringWithStats is a custom toString with cost statistics.

val dataset = spark.range(20).limit(2)

// toStringWithStats in action - note Optimized Logical Plan section with Statistics
scala> dataset.queryExecution.toStringWithStats
res6: String =
== Parsed Logical Plan ==
GlobalLimit 2
+- LocalLimit 2
   +- Range (0, 20, step=1, splits=Some(8))

== Analyzed Logical Plan ==
id: bigint
GlobalLimit 2
+- LocalLimit 2
   +- Range (0, 20, step=1, splits=Some(8))

== Optimized Logical Plan ==
GlobalLimit 2, Statistics(sizeInBytes=32.0 B, rowCount=2, isBroadcastable=false)
+- LocalLimit 2, Statistics(sizeInBytes=160.0 B, isBroadcastable=false)
   +- Range (0, 20, step=1, splits=Some(8)), Statistics(sizeInBytes=160.0 B, isBroadcastable=false)

== Physical Plan ==
CollectLimit 2
+- *Range (0, 20, step=1, splits=Some(8))

toStringWithStats is used when ExplainCommand logical command is executed (with cost attribute enabled).

Extended Text Representation with Logical and Physical Plans

toString: String

toString is a mere alias for completeString with appendStats flag disabled.

Note

toString is on the "other" side of toStringWithStats which has appendStats flag enabled.

toString is part of Java's Object abstraction.

Simple (Basic) Text Representation

simpleString: String // (1)
simpleString(
  formatted: Boolean): String
  1. formatted is false

simpleString requests the optimized physical plan for the text representation (of all nodes in the query tree) with verbose flag turned off.

In the end, simpleString adds == Physical Plan == header to the text representation and redacts sensitive information.

simpleString is used when:

Demo

import org.apache.spark.sql.{functions => f}
val q = spark.range(10).withColumn("rand", f.rand())
val output = q.queryExecution.simpleString
scala> println(output)
== Physical Plan ==
*(1) Project [id#53L, rand(-5226178239369056152) AS rand#55]
+- *(1) Range (0, 10, step=1, splits=16)

explainString

explainString(
  mode: ExplainMode): String
explainString(
  mode: ExplainMode,
  maxFields: Int,
  append: String => Unit): Unit

explainString...FIXME

explainString is used when:

  • Dataset is requested to explain
  • SQLExecution utility is used to withNewExecutionId
  • AdaptiveSparkPlanExec leaf physical operator is requested to onUpdatePlan
  • ExplainCommand logical command is executed
  • debug utility is used to toFile

Redacting Sensitive Information

withRedaction(
  message: String): String

withRedaction takes the value of spark.sql.redaction.string.regex configuration property (as the regular expression to point at sensitive information) and requests Spark Core's Utils to redact sensitive information in the input message.

NOTE: Internally, Spark Core's Utils.redact uses Java's Regex.replaceAllIn to replace all matches of a pattern with a string.

NOTE: withRedaction is used when QueryExecution is requested for the <>, <> and <> text representations.

writePlans

writePlans(
   append: String => Unit,
   maxFields: Int): Unit

writePlans...FIXME

writePlans is used when...FIXME

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