EnsureRequirements Physical Preparation Rule

EnsureRequirements is a physical preparation rule that transforms physical operators (up the plan tree):

  1. Removes two adjacent ShuffleExchange physical operators if the child partitioning scheme guarantees the parent’s partitioning

  2. For other non-ShuffleExchange physical operators, ensures partition distribution and ordering (possibly adding new physical operators, e.g. BroadcastExchangeExec and ShuffleExchange for distribution or SortExec for sorting)

EnsureRequirements is a part of preparations batch of physical plan rules and is executed in executedPlan phase of a query execution.

EnsureRequirements takes a SQLConf when created.

createPartitioning Internal Method


defaultNumPreShufflePartitions Internal Method


Ensuring Partition Requirements (Distribution and Ordering) of Physical Operator — ensureDistributionAndOrdering Internal Method

ensureDistributionAndOrdering(operator: SparkPlan): SparkPlan

Internally, ensureDistributionAndOrdering takes the following from the input physical operator:

The number of requirements for partitions and their sort ordering has to match the number and the order of the child physical plans.

ensureDistributionAndOrdering matches the operator’s required partition requirements of children (requiredChildDistributions) to the children’s output partitioning and (in that order):

  1. If the child satisfies the requested distribution, the child is left unchanged

  2. For BroadcastDistribution, the child becomes the child of BroadcastExchangeExec unary operator for broadcasting joins

  3. Any other pair of child and distribution leads to ShuffleExchange unary physical operator (with proper partitioning for distribution and with spark.sql.shuffle.partitions number of partitions, i.e. 200 by default)

ShuffleExchange can appear in the physical plan when the children’s output partitioning cannot satisfy the physical operator’s required child distribution.

If the input operator has multiple children and specifies child output distributions, then the children’s output partitionings have to be compatible.

If the children’s output partitionings are not all compatible, then…​FIXME

ensureDistributionAndOrdering adds ExchangeCoordinator (only when adaptive query execution is enabled which is not by default).

At this point in ensureDistributionAndOrdering the required child distributions are already handled.

ensureDistributionAndOrdering matches the operator’s required sort ordering of children (requiredChildOrderings) to the children’s output partitioning and if the orderings do not match, SortExec unary physical operator is created as a new child.

ensureDistributionAndOrdering sets the new children for the input operator.

ensureDistributionAndOrdering is used exclusively when EnsureRequirements is executed (i.e. applied to a physical plan).

Adding ExchangeCoordinator (When Adaptive Query Execution Enabled) — withExchangeCoordinator Internal Method

  children: Seq[SparkPlan],
  requiredChildDistributions: Seq[Distribution]): Seq[SparkPlan]

withExchangeCoordinator adds ExchangeCoordinator to ShuffleExchange operators if adaptive query execution is enabled (per spark.sql.adaptive.enabled property) and partitioning scheme of the ShuffleExchanges support ExchangeCoordinator.

spark.sql.adaptive.enabled property is disabled by default.

Internally, withExchangeCoordinator checks if the input children operators support ExchangeCoordinator which is that either holds:

With adaptive query execution enabled (i.e. when spark.sql.adaptive.enabled flag is true) and the operator supports ExchangeCoordinator, withExchangeCoordinator creates a ExchangeCoordinator and:

Otherwise (when adaptive query execution is disabled or children do not support ExchangeCoordinator), withExchangeCoordinator returns the input children unchanged.

withExchangeCoordinator is used exclusively for enforcing partition requirements of a physical operator.

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