VARIADIC parameters in PostgreSQL - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ query_result

intermediate

Output of a function using VARIADIC integer array

Consider the following PostgreSQL function that sums all integers passed as VARIADIC parameters:

CREATE OR REPLACE FUNCTION sum_variadic(VARIADIC nums integer[]) RETURNS integer AS $$
DECLARE
  total integer := 0;
  n integer;
BEGIN
  FOREACH n IN ARRAY nums LOOP
    total := total + n;
  END LOOP;
  RETURN total;
END;
$$ LANGUAGE plpgsql;

What is the output of the query SELECT sum_variadic(1, 2, 3, 4);?

PostgreSQL

CREATE OR REPLACE FUNCTION sum_variadic(VARIADIC nums integer[]) RETURNS integer AS $$
DECLARE
  total integer := 0;
  n integer;
BEGIN
  FOREACH n IN ARRAY nums LOOP
    total := total + n;
  END LOOP;
  RETURN total;
END;
$$ LANGUAGE plpgsql;

SELECT sum_variadic(1, 2, 3, 4);

A 10

B ARRAY[1,2,3,4]

C 1

D ERROR: function sum_variadic(integer, integer, integer, integer) does not exist

💡 Hint

VARIADIC parameters allow passing multiple arguments as an array inside the function.

The function sums all integers passed as VARIADIC parameters. The call passes 1, 2, 3, 4 which sum to 10.

📝 Syntax

intermediate

Identify the correct syntax to declare a VARIADIC parameter

Which of the following is the correct way to declare a VARIADIC parameter in a PostgreSQL function that accepts text arguments?

A CREATE FUNCTION example(args text VARIADIC) RETURNS void AS $$ BEGIN END; $$ LANGUAGE plpgsql;

B CREATE FUNCTION example(args VARIADIC text[]) RETURNS void AS $$ BEGIN END; $$ LANGUAGE plpgsql;

C CREATE FUNCTION example(VARIADIC args text[]) RETURNS void AS $$ BEGIN END; $$ LANGUAGE plpgsql;

D CREATE FUNCTION example(VARIADIC args text) RETURNS void AS $$ BEGIN END; $$ LANGUAGE plpgsql;

💡 Hint

VARIADIC must appear before the parameter name and type as an array.

The correct syntax is to write VARIADIC before the parameter name and specify the type as an array, e.g., VARIADIC args text[].

❓ optimization

advanced

Optimizing a function using VARIADIC parameters for concatenation

You have this function that concatenates text arguments passed as VARIADIC parameters:

CREATE OR REPLACE FUNCTION concat_variadic(VARIADIC texts text[]) RETURNS text AS $$
DECLARE
  result text := '';
  t text;
BEGIN
  FOREACH t IN ARRAY texts LOOP
    result := result || t;
  END LOOP;
  RETURN result;
END;
$$ LANGUAGE plpgsql;

Which option provides a more efficient way to concatenate all VARIADIC text arguments?

A Convert texts to JSON and then concatenate the JSON strings.

B Use a nested loop to concatenate each character of each string.

C Use string_agg(texts, '') inside the function.

D Use array_to_string(texts, '') to concatenate all elements at once.

💡 Hint

PostgreSQL has built-in functions to join array elements efficiently.

array_to_string(texts, '') concatenates all array elements efficiently without looping manually.

🧠 Conceptual

advanced

Understanding VARIADIC parameter behavior with arrays

Given this function:

CREATE OR REPLACE FUNCTION test_variadic(VARIADIC vals integer[]) RETURNS integer AS $$
BEGIN
  RETURN array_length(vals, 1);
END;
$$ LANGUAGE plpgsql;

What is the result of the query SELECT test_variadic(ARRAY[1,2,3]);?

PostgreSQL

CREATE OR REPLACE FUNCTION test_variadic(VARIADIC vals integer[]) RETURNS integer AS $$
BEGIN
  RETURN array_length(vals, 1);
END;
$$ LANGUAGE plpgsql;

SELECT test_variadic(ARRAY[1,2,3]);

A 1

B 3

C NULL

D ERROR: function test_variadic(integer[]) does not exist

💡 Hint

Passing an array to a VARIADIC parameter treats the whole array as one argument unless VARIADIC is specified in the call.

Passing ARRAY[1,2,3] as a single argument means vals is an array with one element (the array itself), so length is 1.

🔧 Debug

expert

Diagnose the error when calling a VARIADIC function

You have this function:

CREATE OR REPLACE FUNCTION multiply_variadic(VARIADIC nums integer[]) RETURNS integer AS $$
DECLARE
  product integer := 1;
  n integer;
BEGIN
  FOREACH n IN ARRAY nums LOOP
    product := product * n;
  END LOOP;
  RETURN product;
END;
$$ LANGUAGE plpgsql;

When you run SELECT multiply_variadic(ARRAY[2,3,4]); you get an error:

ERROR: function multiply_variadic(integer[]) does not exist

What is the cause of this error?

A The function does not accept any arguments, so passing an array causes an error.

B The function expects separate integer arguments, but an integer array was passed without VARIADIC keyword.

C The function expects a single integer, but an array was passed.

D The function is missing a RETURNS clause causing the error.

💡 Hint

Check how VARIADIC functions are called with arrays.

To pass an array to a VARIADIC parameter, you must use the VARIADIC keyword in the call: SELECT multiply_variadic(VARIADIC ARRAY[2,3,4]);

Practice

1. What does the VARIADIC keyword do in a PostgreSQL function?

easy

A. Specifies that the function is a trigger

B. Defines a function that returns multiple result sets

C. Allows the function to accept a variable number of arguments as an array

D. Indicates the function is immutable

Solution

1. Step 1: Understand the role of VARIADIC

   VARIADIC allows a function to accept any number of arguments as a single array parameter.

2. Step 2: Compare with other options

   Options B, C, and D describe unrelated function features.

3. Final Answer:

   Allows the function to accept a variable number of arguments as an array -> Option C

4. Quick Check:

   VARIADIC = variable arguments as array [OK]

Hint: VARIADIC means flexible argument count packed as array [OK]

Common Mistakes:

• Thinking VARIADIC returns multiple result sets
• Confusing VARIADIC with triggers
• Assuming VARIADIC changes function volatility

2. Which of the following is the correct syntax to declare a VARIADIC parameter in a PostgreSQL function?

easy

A. CREATE FUNCTION example(VARIADIC nums int[]) RETURNS int AS $$ ... $$ LANGUAGE sql;

B. CREATE FUNCTION example(nums VARIADIC int) RETURNS int AS $$ ... $$ LANGUAGE sql;

C. CREATE FUNCTION example(nums int VARIADIC) RETURNS int AS $$ ... $$ LANGUAGE sql;

D. CREATE FUNCTION example(nums int[]) VARIADIC RETURNS int AS $$ ... $$ LANGUAGE sql;

Solution

1. Step 1: Identify correct VARIADIC syntax

   VARIADIC must precede the parameter name and the parameter type must be an array type, e.g., VARIADIC nums int[].

2. Step 2: Check each option

   CREATE FUNCTION example(VARIADIC nums int[]) RETURNS int AS $$ ... $$ LANGUAGE sql; correctly places VARIADIC before the parameter name and uses an array type. Options A, B, and C misuse the position or syntax of VARIADIC.

3. Final Answer:

   CREATE FUNCTION example(VARIADIC nums int[]) RETURNS int AS $$ ... $$ LANGUAGE sql; -> Option A

4. Quick Check:

   VARIADIC before param name + array type = correct syntax [OK]

Hint: VARIADIC goes before param name and uses array type [OK]

Common Mistakes:

• Placing VARIADIC after parameter name
• Using non-array type with VARIADIC
• Putting VARIADIC after RETURNS keyword

3. Given the function:

CREATE FUNCTION sum_all(VARIADIC nums int[]) RETURNS int AS $$
  SELECT SUM(n) FROM unnest(nums) AS n;
$$ LANGUAGE sql;

What is the result of SELECT sum_all(2, 4, 6);?

medium

A. ARRAY[2,4,6]

B. 12

C. Error: wrong number of arguments

D. NULL

Solution

1. Step 1: Understand VARIADIC input

   The function accepts any number of integers as separate arguments, packed into an array named nums.

2. Step 2: Calculate the sum

   Input arguments 2, 4, 6 become array [2,4,6]. The function sums these values: 2 + 4 + 6 = 12.

3. Final Answer:

   12 -> Option B

4. Quick Check:

   Sum of 2,4,6 = 12 [OK]

Hint: VARIADIC packs inputs as array; sum them inside function [OK]

Common Mistakes:

• Expecting an array output instead of sum
• Thinking VARIADIC requires explicit array input
• Assuming function errors on multiple arguments

4. Consider this function definition:

CREATE FUNCTION concat_texts(VARIADIC texts text) RETURNS text AS $$
  SELECT string_agg(t, ',') FROM unnest(texts) AS t;
$$ LANGUAGE sql;

Which error will occur if you call SELECT concat_texts('hello', 'world');?

medium

A. Error: VARIADIC parameter must be declared as an array type

B. No error, returns 'hello,world'

C. Error: missing FROM-clause entry for table 'texts'

D. Error: function does not exist

Solution

1. Step 1: Check VARIADIC parameter type

   The parameter texts is declared as text, not text[]. VARIADIC requires an array type.

2. Step 2: Identify error cause

   PostgreSQL will raise a syntax error because VARIADIC must be followed by an array type, e.g., VARIADIC texts text[].

3. Final Answer:

   Error: VARIADIC parameter must be declared as an array type -> Option A

4. Quick Check:

   VARIADIC needs array type parameter [OK]

Hint: VARIADIC param must be array type, else syntax error [OK]

Common Mistakes:

• Declaring VARIADIC param as scalar type
• Expecting no error with wrong type
• Confusing error with missing FROM clause

5. You want to create a function that accepts a fixed integer and then any number of text arguments variadically. Which of these function declarations is correct?

hard

A. CREATE FUNCTION example(fixed int, VARIADIC texts text) RETURNS text AS $$ ... $$ LANGUAGE sql;

B. CREATE FUNCTION example(VARIADIC texts text[], fixed int) RETURNS text AS $$ ... $$ LANGUAGE sql;

C. CREATE FUNCTION example(fixed int, texts VARIADIC text[]) RETURNS text AS $$ ... $$ LANGUAGE sql;

D. CREATE FUNCTION example(fixed int, VARIADIC texts text[]) RETURNS text AS $$ ... $$ LANGUAGE sql;

Solution

1. Step 1: Recall VARIADIC position rule

   VARIADIC parameter must be the last parameter in the function declaration.

2. Step 2: Check parameter types and order

   CREATE FUNCTION example(fixed int, VARIADIC texts text[]) RETURNS text AS $$ ... $$ LANGUAGE sql; places fixed int first and VARIADIC texts text[] last, which is correct. CREATE FUNCTION example(VARIADIC texts text[], fixed int) RETURNS text AS $$ ... $$ LANGUAGE sql; places VARIADIC first, which is invalid. CREATE FUNCTION example(fixed int, texts VARIADIC text[]) RETURNS text AS $$ ... $$ LANGUAGE sql; misplaces VARIADIC keyword. CREATE FUNCTION example(fixed int, VARIADIC texts text) RETURNS text AS $$ ... $$ LANGUAGE sql; uses non-array type with VARIADIC.

3. Final Answer:

   CREATE FUNCTION example(fixed int, VARIADIC texts text[]) RETURNS text AS $$ ... $$ LANGUAGE sql; -> Option D

4. Quick Check:

   VARIADIC last and array type = correct [OK]

Hint: VARIADIC param must be last and array type [OK]

Common Mistakes:

• Placing VARIADIC parameter before fixed parameters
• Using scalar type with VARIADIC
• Misplacing VARIADIC keyword after parameter name