Boolean type behavior in PostgreSQL - Time & Space Complexity

Time Complexity: Boolean type behavior

O(n)

Understanding Time Complexity

We want to understand how the time it takes to work with Boolean values changes as we use more data.

Specifically, how does checking or using Boolean values in queries grow when the data grows?

Scenario Under Consideration

Analyze the time complexity of the following code snippet.

SELECT id, is_active
FROM users
WHERE is_active = TRUE;
    

This query selects user IDs where the Boolean column is_active is true.

Identify Repeating Operations

Identify the loops, recursion, array traversals that repeat.

• Primary operation: Scanning each row in the users table to check the is_active value.
• How many times: Once for every row in the table.

How Execution Grows With Input

As the number of users grows, the database checks more rows one by one.

Input Size (n)	Approx. Operations
10	10 checks of is_active
100	100 checks of is_active
1000	1000 checks of is_active

Pattern observation: The number of checks grows directly with the number of rows.

Final Time Complexity

Time Complexity: O(n)

This means the time to run the query grows in a straight line as the table gets bigger.

Common Mistake

[X] Wrong: "Checking a Boolean column is instant no matter how big the table is."

[OK] Correct: Even though Booleans are simple, the database still looks at each row, so more rows mean more work.

Interview Connect

Understanding how simple Boolean checks scale helps you explain query performance clearly and confidently.

Self-Check

"What if the is_active column had an index? How would the time complexity change?"

Practice

1. In PostgreSQL, which of the following is a valid boolean value stored in a boolean column?

easy

A. TRUE

B. 2

C. 'foo'

D. 'NULL'

Solution

1. Step 1: Understand boolean data type values

   PostgreSQL boolean columns accept TRUE, FALSE, or NULL (unknown).

2. Step 2: Identify valid boolean literals

   TRUE and FALSE are valid boolean literals; 2, 'foo', and 'NULL' are not valid boolean literals in PostgreSQL.

3. Final Answer:

   TRUE -> Option A

4. Quick Check:

   Boolean values = TRUE or FALSE [OK]

Hint: Boolean columns accept TRUE, FALSE, or NULL only [OK]

Common Mistakes:

• Using 1 or 0 as boolean values directly
• Using strings like 'yes' or 'no' as boolean
• Confusing NULL with a boolean TRUE or FALSE

2. Which of the following is the correct way to declare a boolean column named is_active in a PostgreSQL table?

easy

A. CREATE TABLE users (is_active BOOLEAN(1));

B. CREATE TABLE users (is_active BOOLEAN);

C. CREATE TABLE users (is_active BOOL(1));

D. CREATE TABLE users (is_active BOOLIAN);

Solution

1. Step 1: Check correct boolean type syntax

   PostgreSQL uses BOOLEAN or BOOL as valid types; BOOLEAN(1) is invalid.

2. Step 2: Identify syntax errors

   A uses invalid BOOLEAN(1) syntax; B has a typo 'BOOLIAN'; C uses invalid BOOL(1) syntax.

3. Final Answer:

   CREATE TABLE users (is_active BOOLEAN); -> Option B

4. Quick Check:

   BOOLEAN keyword is correct type [OK]

Hint: Use BOOLEAN or BOOL, no size or typo allowed [OK]

Common Mistakes:

• Typo in BOOLEAN keyword
• Adding size like BOOLEAN(1)
• Using unsupported types like BOOLIAN

3. Given the table settings with a boolean column enabled, what will this query return?

SELECT enabled FROM settings WHERE enabled = FALSE;

medium

A. All rows where enabled is TRUE

B. All rows where enabled is NULL

C. All rows where enabled is FALSE

D. Syntax error

Solution

1. Step 1: Understand the WHERE clause condition

   The query filters rows where the boolean column enabled equals FALSE.

2. Step 2: Determine which rows match

   Only rows with enabled = FALSE will be returned; TRUE or NULL rows are excluded.

3. Final Answer:

   All rows where enabled is FALSE -> Option C

4. Quick Check:

   WHERE enabled = FALSE returns FALSE rows [OK]

Hint: WHERE boolean_column = FALSE returns only FALSE rows [OK]

Common Mistakes:

• Assuming NULL rows are included
• Confusing TRUE and FALSE values
• Thinking it returns all rows

4. You wrote this query but it returns no rows:

SELECT * FROM users WHERE is_admin = TRUE;

The is_admin column is boolean but some rows have NULL. What is the likely issue?

medium

A. TRUE is not a valid boolean value in PostgreSQL

B. The query syntax is incorrect

C. The column should be text, not boolean

D. NULL values cause the condition to fail, so no rows with NULL appear

Solution

1. Step 1: Understand boolean NULL behavior in WHERE clause

   Comparing boolean column to TRUE excludes NULL rows because NULL means unknown, not TRUE.

2. Step 2: Explain why no rows returned

   Rows with NULL in is_admin do not satisfy is_admin = TRUE, so they are excluded.

3. Final Answer:

   NULL values cause the condition to fail, so no rows with NULL appear -> Option D

4. Quick Check:

   NULL in boolean = TRUE returns no rows [OK]

Hint: NULL in boolean columns excludes rows in equality checks [OK]

Common Mistakes:

• Thinking NULL equals TRUE
• Assuming TRUE is invalid boolean
• Believing syntax error caused no rows

5. You want to select all rows from tasks where the boolean column completed is either FALSE or NULL. Which query correctly does this?

hard

A. SELECT * FROM tasks WHERE completed = FALSE OR completed IS NULL;

B. SELECT * FROM tasks WHERE completed = FALSE AND completed IS NULL;

C. SELECT * FROM tasks WHERE NOT completed;

D. SELECT * FROM tasks WHERE completed != TRUE;

Solution

1. Step 1: Understand boolean NULL and FALSE filtering

   To get rows where completed is FALSE or NULL, use OR with completed = FALSE and completed IS NULL.

2. Step 2: Analyze each option

   SELECT * FROM tasks WHERE completed = FALSE AND completed IS NULL; uses AND, which cannot be both FALSE and NULL simultaneously. SELECT * FROM tasks WHERE NOT completed; excludes NULL rows. SELECT * FROM tasks WHERE completed != TRUE; excludes TRUE and NULL (since NULL != TRUE is unknown), includes only FALSE.

3. Final Answer:

   SELECT * FROM tasks WHERE completed = FALSE OR completed IS NULL; -> Option A

4. Quick Check:

   Use OR to include FALSE or NULL rows [OK]

Hint: Use OR with IS NULL to include unknown boolean values [OK]

Common Mistakes:

• Using AND instead of OR
• Ignoring NULL rows in boolean filters
• Using NOT completed excludes NULL rows