Variable declaration and assignment in PostgreSQL - Time & Space Complexity

Time Complexity: Variable declaration and assignment

O(n)

Understanding Time Complexity

Let's explore how the time needed to run code changes when we declare and assign variables in PostgreSQL.

We want to know how the work grows as we add more variables or assignments.

Scenario Under Consideration

Analyze the time complexity of the following code snippet.

DO $$
DECLARE
  counter INTEGER := 0;
  total INTEGER := 100;
BEGIN
  counter := total;
END $$;
    

This code declares two variables and assigns a value to one of them inside a block.

Identify Repeating Operations

Identify the loops, recursion, array traversals that repeat.

• Primary operation: Variable declaration and assignment
• How many times: Each happens once in this code

How Execution Grows With Input

When you add more variables or assignments, the work grows in a simple way.

Input Size (number of variables)	Approx. Operations
10	10 declarations and assignments
100	100 declarations and assignments
1000	1000 declarations and assignments

Pattern observation: The work grows directly with the number of variables.

Final Time Complexity

Time Complexity: O(n)

This means the time grows in a straight line as you add more variables or assignments.

Common Mistake

[X] Wrong: "Declaring variables takes no time or is instant no matter how many."

[OK] Correct: Each declaration and assignment takes some time, so more variables mean more work.

Interview Connect

Understanding how simple operations like variable assignments scale helps you reason about bigger database tasks confidently.

Self-Check

"What if we declared variables inside a loop that runs n times? How would the time complexity change?"

Practice

1. What is the correct way to declare a variable named counter of type integer in a PL/pgSQL block?

easy

A. DECLARE counter integer;

B. counter integer;

C. DECLARE counter integer = 0;

D. counter := integer;

Solution

1. Step 1: Understand variable declaration syntax

   In PL/pgSQL, variables are declared inside a DECLARE section without repeating the DECLARE keyword for each variable. The correct syntax is: variable_name data_type;

2. Step 2: Identify the correct declaration

   The line counter integer; correctly declares the variable inside the DECLARE block. Including the DECLARE keyword before each variable is incorrect.

3. Final Answer:

   counter integer; -> Option B

4. Quick Check:

   Declare variables inside DECLARE block without repeating DECLARE [OK]

Hint: Declare variables inside DECLARE block with type only [OK]

Common Mistakes:

• Omitting DECLARE keyword
• Assigning value during declaration without :=
• Using := in declaration line

2. Which of the following is the correct syntax to assign the value 10 to a variable count after it has been declared in PL/pgSQL?

easy

A. count = 10;

B. count == 10;

C. SET count = 10;

D. count := 10;

Solution

1. Step 1: Recall assignment operator in PL/pgSQL

   PL/pgSQL uses := to assign values to variables, not = or ==.

2. Step 2: Identify correct assignment syntax

   count := 10; uses count := 10; which is the correct way to assign a value.

3. Final Answer:

   count := 10; -> Option D

4. Quick Check:

   Use := for assignment in PL/pgSQL [OK]

Hint: Use := to assign values to variables [OK]

Common Mistakes:

• Using = instead of :=
• Using SET keyword incorrectly
• Using == like in other languages

3. Consider the following PL/pgSQL block:

DECLARE
  total integer := 5;
BEGIN
  total := total + 3;
  RAISE NOTICE '%', total;
END;

What will be the output when this block runs?

medium

A. 8

B. 5

C. 3

D. Error: variable not initialized

Solution

1. Step 1: Analyze initial value assignment

   The variable total is declared and initialized to 5.

2. Step 2: Calculate the new value after addition

   The statement total := total + 3; adds 3 to 5, resulting in 8.

3. Final Answer:

   8 -> Option A

4. Quick Check:

   5 + 3 = 8 [OK]

Hint: Add assigned values step-by-step to find final result [OK]

Common Mistakes:

• Ignoring initial value and assuming zero
• Confusing assignment operator with equality
• Expecting error due to missing BEGIN

4. Identify the error in the following PL/pgSQL code snippet:

DECLARE
  name text;
BEGIN
  name = 'Alice';
END;

medium

A. Incorrect assignment operator used

B. Missing DECLARE keyword

C. Variable name not declared

D. Missing semicolon after BEGIN

Solution

1. Step 1: Check variable declaration

   The variable name is declared correctly with type text.

2. Step 2: Check assignment syntax

   The assignment uses = which is incorrect in PL/pgSQL; it should use :=.

3. Final Answer:

   Incorrect assignment operator used -> Option A

4. Quick Check:

   Use := for assignment, not = [OK]

Hint: Use := for assignment, not = [OK]

Common Mistakes:

• Using = instead of :=
• Forgetting semicolon after assignment
• Confusing declaration and assignment syntax

5. You want to declare two variables a and b as integers, assign a the value 4, and then assign b the value of a multiplied by 3. Which of the following PL/pgSQL code snippets correctly does this?

hard

A. DECLARE
b integer := a * 3;
a integer := 4;
BEGIN
NULL;
END;

B. DECLARE
a integer;
b integer;
BEGIN
a = 4;
b = a * 3;
END;

C. DECLARE
a integer;
b integer;
BEGIN
a := 4;
b := a * 3;
END;

D. DECLARE
a integer := 4;
b integer;
BEGIN
b = a * 3;
END;

Solution

1. Step 1: Check variable declaration and initialization

   DECLARE a integer; b integer; BEGIN a := 4; b := a * 3; END; declares both variables without initial values, then assigns values inside the BEGIN block using correct assignment operator :=.

2. Step 2: Verify assignment and calculation

   It assigns a := 4; and then b := a * 3;, which correctly sets b to 12.

3. Final Answer:

   correctly declares and assigns variables with := -> Option C

4. Quick Check:

   Declare first, assign with := inside BEGIN [OK]

Hint: Declare variables first, assign values inside BEGIN with := [OK]

Common Mistakes:

• Using = instead of :=
• Assigning values during declaration with expressions
• Missing BEGIN block for assignments