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CodegenFallback — Catalyst Expressions with Fallback Code Generation Mode

CodegenFallback is the <> of <> that do not support a Java code generation and want to <> (aka fallback mode).

CodegenFallback is used when CollapseCodegenStages physical optimization is executed (and enforce whole-stage codegen requirements for Catalyst expressions).

[[contract]] [source, scala]

package org.apache.spark.sql.catalyst.expressions.codegen

trait CodegenFallback extends Expression { // No properties (vals and methods) that have no implementation }

[[implementations]] .(Some Examples of) CodegenFallbacks [cols="1,2",options="header",width="100%"] |=== | CodegenFallback | Description

| CurrentDate | [[CurrentDate]]

| CurrentTimestamp | [[CurrentTimestamp]]

| Cube | [[Cube]]

| <> | [[JsonToStructs]]

| Rollup | [[Rollup]]

| StructsToJson | [[StructsToJson]] |===

.Example -- CurrentTimestamp Expression with nullable flag disabled [source, scala]

import org.apache.spark.sql.catalyst.expressions.CurrentTimestamp val currTimestamp = CurrentTimestamp()

import org.apache.spark.sql.catalyst.expressions.codegen.CodegenFallback assert(currTimestamp.isInstanceOf[CodegenFallback], "CurrentTimestamp should be a CodegenFallback")

assert(currTimestamp.nullable == false, "CurrentTimestamp should not be nullable")

import org.apache.spark.sql.catalyst.expressions.codegen.{CodegenContext, ExprCode} val ctx = new CodegenContext // doGenCode is used when Expression.genCode is executed val ExprCode(code, _, _) = currTimestamp.genCode(ctx)

scala> println(code)

Object obj_0 = ((Expression) references[0]).eval(null); long value_0 = (Long) obj_0;

=== [[doGenCode]] Generating Java Source Code (ExprCode) For Code-Generated Expression Evaluation -- doGenCode Method

[source, scala]

doGenCode(ctx: CodegenContext, ev: ExprCode): ExprCode

NOTE: doGenCode is part of <> to generate a Java source code (ExprCode) for code-generated expression evaluation.

doGenCode requests the input CodegenContext to add itself to the references.

doGenCode walks down the expression tree to find <> expressions and for every Nondeterministic expression does the following:

. Requests the input CodegenContext to add it to the references

. Requests the input CodegenContext to addPartitionInitializationStatement that is a Java code block as follows: + [source, scala]

((Nondeterministic) references[[childIndex]]) .initialize(partitionIndex);

In the end, doGenCode generates a plain Java source code block that is one of the following code blocks per the <> flag. doGenCode copies the input ExprCode with the code block added (as the code property).

.doGenCode Code Block for nullable flag enabled [source, scala]

[placeHolder] Object [objectTerm] = ((Expression) references[[idx]]).eval([input]); boolean [isNull] = [objectTerm] == null; [javaType] [value] = [defaultValue]; if (![isNull]) { [value] = ([boxedType]) [objectTerm]; }

.doGenCode Code Block for nullable flag disabled [source, scala]

[placeHolder] Object [objectTerm] = ((Expression) references[[idx]]).eval([input]); [javaType] [value] = ([boxedType]) [objectTerm];