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TypeCoercionRule Contract — Type Coercion Rules

TypeCoercionRule is the contract of logical rules to coerce and propagate types in logical plans.

package org.apache.spark.sql.catalyst.analysis

trait TypeCoercionRule extends Rule[LogicalPlan] {
  // only required methods that have no implementation
  // the others follow
  def coerceTypes(plan: LogicalPlan): LogicalPlan
}

Table 1. (Subset of) TypeCoercionRule Contract
Method	Description
`coerceTypes`	Coerce types in a logical plan Used exclusively when `TypeCoercionRule` is executed

TypeCoercionRule is simply a Catalyst rule for transforming logical plans, i.e. Rule[LogicalPlan].

Table 2. TypeCoercionRules
TypeCoercionRule	Description
CaseWhenCoercion
ConcatCoercion
DecimalPrecision
Division
EltCoercion
FunctionArgumentConversion
IfCoercion
ImplicitTypeCasts
InConversion
PromoteStrings
StackCoercion
WindowFrameCoercion

Executing Rule — `apply` Method

apply(plan: LogicalPlan): LogicalPlan

Note	`apply` is part of the Rule Contract to execute (apply) a rule on a TreeNode (e.g. LogicalPlan).

apply coerceTypes in the input LogicalPlan and returns the following:

The input LogicalPlan itself if matches the coerced plan
The plan after propagateTypes on the coerced plan

`propagateTypes` Internal Method

propagateTypes(plan: LogicalPlan): LogicalPlan

propagateTypes…FIXME

Note	`propagateTypes` is used exclusively when `TypeCoercionRule` is executed.

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