VARIADIC parameters in PostgreSQL - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does the VARIADIC keyword do in PostgreSQL functions?

VARIADIC allows a function to accept a variable number of arguments as an array, letting you pass many values without specifying each one separately.

beginner

How do you declare a VARIADIC parameter in a PostgreSQL function?

You add the keyword VARIADIC before the last parameter's type, for example: VARIADIC integer[]. This means the function can take many integers as separate arguments.

intermediate

Can a PostgreSQL function have more than one VARIADIC parameter?

No, a function can have only one VARIADIC parameter, and it must be the last parameter in the function's argument list.

beginner

How do you call a function with a VARIADIC parameter?

You can pass multiple arguments directly, or pass an array with the VARIADIC keyword before it. For example: SELECT func(1, 2, 3) or SELECT func(VARIADIC ARRAY[1,2,3]).

intermediate

Why use VARIADIC parameters instead of arrays in PostgreSQL functions?

VARIADIC parameters let you write cleaner calls by passing multiple values directly without manually creating an array, making the function easier to use.

What is the purpose of the VARIADIC keyword in PostgreSQL functions?

A To declare a function as recursive

B To optimize query performance

C To specify the return type of a function

D To allow a function to accept a variable number of arguments

VARIADIC lets a function accept many arguments as an array, enabling flexible input.

How many VARIADIC parameters can a PostgreSQL function have?

A None, VARIADIC is not allowed in PostgreSQL

B Multiple, anywhere in the parameter list

C One, and it must be the last parameter

D Two, but only if they are arrays

Only one VARIADIC parameter is allowed and it must be last.

Which of these is a valid way to call a function with a VARIADIC parameter?

A SELECT func(1, 2, 3);

B SELECT func(ARRAY[1, 2, 3]);

C SELECT func(VARIADIC 1, 2, 3);

D SELECT func(1; 2; 3);

You can pass multiple arguments directly without ARRAY keyword.

What type must a VARIADIC parameter be declared as?

A An array type, like integer[]

B A scalar type, like integer

C A composite type

D Any type except arrays

VARIADIC parameters must be declared as arrays.

Why might you prefer VARIADIC parameters over passing an array directly?

A Because VARIADIC improves query speed

B Because VARIADIC lets you pass multiple values without creating an array manually

C Because VARIADIC parameters can be anywhere in the parameter list

D Because VARIADIC parameters can return multiple values

VARIADIC simplifies function calls by allowing multiple separate arguments.

Explain how VARIADIC parameters work in PostgreSQL functions and how to use them in function calls.

Think about how you can pass many values without making an array yourself.

Describe the rules and limitations of using VARIADIC parameters in PostgreSQL functions.

Focus on placement, count, and type of VARIADIC parameters.

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