Reverse¶

Overview¶

The Reverse expression reverses the order of elements in an array or the order of characters in a string. It is a unary expression that operates on a single input and returns the reversed result while preserving the original data type.

Syntax¶

REVERSE(expr)

// DataFrame API
col("column_name").reverse()
reverse(col("column_name"))

Arguments¶

Argument	Type	Description
child	Expression	The input expression containing an array or string to be reversed

Return Type¶

Returns the same data type as the input expression:

For ArrayType: Returns an ArrayType with the same element type and nullability
For StringType: Returns a StringType

Supported Data Types¶

StringType with collation support (including trim collation support)
ArrayType of any element type

Algorithm¶

For strings, delegates to the UTF8String's reverse() method which reverses character order
For arrays, converts the ArrayData to an object array, applies Scala's reverse operation, and wraps in GenericArrayData
Preserves the original array's element nullability characteristics
Uses lazy evaluation pattern with doReverse function computed once based on data type

Partitioning Behavior¶

Preserves partitioning as it operates on individual rows without requiring data movement
Does not require shuffle operations
Can be executed in parallel across partitions

Edge Cases¶

Null handling: Expression is null-intolerant, meaning null inputs produce null outputs without evaluation
Empty arrays: Returns an empty array of the same type
Empty strings: Returns an empty string
Single element collections: Returns the same single element unchanged
Array nullability: Preserves the containsNull property of the original array type

Code Generation¶

Supports Tungsten code generation with optimized implementations:

String reversal: Generates direct call to UTF8String.reverse() method
Array reversal: Generates optimized loop code that iterates through elements in reverse order using index arithmetic
Falls back to interpreted mode (nullSafeEval) when code generation is disabled

Examples¶

-- String reversal
SELECT REVERSE('hello') AS result;
-- Returns: 'olleh'

-- Array reversal
SELECT REVERSE(ARRAY(1, 2, 3, 4)) AS result;
-- Returns: [4, 3, 2, 1]

-- With null values in array
SELECT REVERSE(ARRAY(1, null, 3)) AS result;
-- Returns: [3, null, 1]

// DataFrame API usage
import org.apache.spark.sql.functions.reverse

// String reversal
df.select(reverse(col("name"))).show()

// Array reversal
df.select(reverse(col("numbers_array"))).show()