DEFAULT values and expressions in PostgreSQL - Time & Space Complexity

Time Complexity: DEFAULT values and expressions

O(n)

Understanding Time Complexity

We want to understand how the time to insert data with DEFAULT values changes as we add more rows.

How does the database handle setting default values when many rows are inserted?

Scenario Under Consideration

Analyze the time complexity of this INSERT statement using DEFAULT values.

CREATE TABLE users (
  id SERIAL PRIMARY KEY,
  username TEXT NOT NULL,
  created_at TIMESTAMP DEFAULT now()
);

INSERT INTO users (username) VALUES ('alice'), ('bob'), ('carol');
    

This code creates a table with a default timestamp and inserts multiple rows without specifying the timestamp.

Identify Repeating Operations

Look for repeated actions during the insert process.

• Primary operation: For each row inserted, the database evaluates the DEFAULT expression (now()) to set the timestamp.
• How many times: Once per row inserted, so the number of evaluations grows with the number of rows.

How Execution Grows With Input

As you insert more rows, the database must run the DEFAULT expression for each one.

Input Size (n)	Approx. Operations
10	10 evaluations of now()
100	100 evaluations of now()
1000	1000 evaluations of now()

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

Final Time Complexity

Time Complexity: O(n)

This means the time to insert rows with DEFAULT values grows linearly with how many rows you add.

Common Mistake

[X] Wrong: "DEFAULT expressions run only once no matter how many rows are inserted."

[OK] Correct: Each row needs its own value, so the DEFAULT expression runs separately for every row inserted.

Interview Connect

Understanding how DEFAULT values affect insert performance shows you know how databases handle data behind the scenes.

Self-Check

"What if the DEFAULT expression was a constant value instead of a function call? How would the time complexity change?"

Practice

1. What does the DEFAULT keyword do in a PostgreSQL table column definition?

easy

A. Sets a value automatically when no value is provided during insert

B. Deletes the column if no value is provided

C. Requires the user to always provide a value

D. Creates an index on the column

Solution

1. Step 1: Understand the purpose of DEFAULT

   The DEFAULT keyword assigns a value automatically if no value is given during an insert operation.

2. Step 2: Compare options with this behavior

   Only Sets a value automatically when no value is provided during insert correctly describes this automatic assignment behavior.

3. Final Answer:

   Sets a value automatically when no value is provided during insert -> Option A

4. Quick Check:

   DEFAULT = automatic value assignment [OK]

Hint: DEFAULT fills missing values automatically [OK]

Common Mistakes:

• Thinking DEFAULT deletes or modifies columns
• Confusing DEFAULT with NOT NULL constraint
• Assuming DEFAULT requires manual input

2. Which of the following is the correct syntax to set a default value of 100 for a column named score in PostgreSQL?

easy

A. ALTER TABLE mytable ALTER COLUMN score SET DEFAULT 100;

B. ALTER TABLE mytable SET DEFAULT score = 100;

C. ALTER TABLE mytable CHANGE score DEFAULT 100;

D. ALTER TABLE mytable ADD DEFAULT score = 100;

Solution

1. Step 1: Recall ALTER TABLE syntax for setting default

   In PostgreSQL, to set a default value, use: ALTER TABLE table ALTER COLUMN column SET DEFAULT value;

2. Step 2: Match syntax with options

   Only ALTER TABLE mytable ALTER COLUMN score SET DEFAULT 100; matches the correct syntax exactly.

3. Final Answer:

   ALTER TABLE mytable ALTER COLUMN score SET DEFAULT 100; -> Option A

4. Quick Check:

   ALTER COLUMN ... SET DEFAULT = correct syntax [OK]

Hint: Use ALTER COLUMN ... SET DEFAULT to set defaults [OK]

Common Mistakes:

• Using SET DEFAULT before column name
• Using CHANGE instead of ALTER COLUMN
• Trying to ADD DEFAULT as a separate clause

3. Given the table definition:

CREATE TABLE orders (
  id SERIAL PRIMARY KEY,
  order_date DATE DEFAULT CURRENT_DATE,
  status TEXT DEFAULT 'pending'
);

What will be the result of this query?

INSERT INTO orders (status) VALUES ('shipped') RETURNING *;

medium

A. A new row with default status 'pending' and NULL order_date

B. A new row with NULL order_date and status 'shipped'

C. A new row with current date as order_date and status 'shipped'

D. Error because order_date is missing

Solution

1. Step 1: Understand DEFAULT behavior on insert

   Since order_date has DEFAULT CURRENT_DATE, if not provided, it uses today's date automatically.

2. Step 2: Analyze the insert statement

   The insert provides status = 'shipped', so it overrides the default 'pending'. The order_date is omitted, so default applies.

3. Final Answer:

   A new row with current date as order_date and status 'shipped' -> Option C

4. Quick Check:

   DEFAULT fills missing columns, provided columns override [OK]

Hint: Defaults fill missing columns, provided values override [OK]

Common Mistakes:

• Assuming missing columns become NULL instead of default
• Expecting error when default exists
• Confusing default value with NOT NULL constraint

4. Consider this table:

CREATE TABLE users (
  id SERIAL PRIMARY KEY,
  username TEXT NOT NULL,
  created_at TIMESTAMP DEFAULT NOW()
);

Which of the following INSERT statements will cause an error?

medium

A. INSERT INTO users (id, username, created_at) VALUES (DEFAULT, 'alice', DEFAULT);

B. INSERT INTO users (username, created_at) VALUES ('bob', DEFAULT);

C. INSERT INTO users (username) VALUES ('alice');

D. INSERT INTO users (created_at) VALUES (NOW());

Solution

1. Step 1: Check NOT NULL column requirements

   The username column is NOT NULL and must be provided in inserts.

2. Step 2: Analyze each insert

   INSERT INTO users (created_at) VALUES (NOW()); inserts only created_at but omits username, causing a NOT NULL violation error.

3. Final Answer:

   INSERT INTO users (created_at) VALUES (NOW()); -> Option D

4. Quick Check:

   NOT NULL columns must be provided [OK]

Hint: NOT NULL columns require values, even with defaults [OK]

Common Mistakes:

• Assuming DEFAULT VALUES inserts NULL for NOT NULL columns
• Thinking DEFAULT can replace NOT NULL columns
• Ignoring NOT NULL constraints

5. You want to create a table products where the created_at column defaults to the current timestamp, and the price column defaults to 9.99 if no price is given. Which of the following CREATE TABLE statements correctly implements this?

hard

A. CREATE TABLE products (
id SERIAL PRIMARY KEY,
name TEXT NOT NULL,
price NUMERIC DEFAULT 9.99,
created_at TIMESTAMP DEFAULT CURRENT_TIMESTAMP()
);

B. CREATE TABLE products (
id SERIAL PRIMARY KEY,
name TEXT NOT NULL,
price NUMERIC DEFAULT 9.99,
created_at TIMESTAMP DEFAULT NOW()
);

C. CREATE TABLE products (
id SERIAL PRIMARY KEY,
name TEXT NOT NULL,
price NUMERIC DEFAULT '9.99',
created_at TIMESTAMP DEFAULT NOW
);

D. CREATE TABLE products (
id SERIAL PRIMARY KEY,
name TEXT NOT NULL,
price NUMERIC DEFAULT 9.99,
created_at TIMESTAMP DEFAULT CURRENT_DATE
);

Solution

1. Step 1: Check default for price column

   Price should default to numeric 9.99 without quotes (numeric literal).

2. Step 2: Check default for created_at column

   Use NOW() function call for current timestamp, not CURRENT_TIMESTAMP or CURRENT_DATE (date only).

3. Step 3: Validate syntax correctness

   CREATE TABLE products ( id SERIAL PRIMARY KEY, name TEXT NOT NULL, price NUMERIC DEFAULT 9.99, created_at TIMESTAMP DEFAULT NOW() ); uses price NUMERIC DEFAULT 9.99 and created_at TIMESTAMP DEFAULT NOW(), which is correct syntax and behavior.

4. Final Answer:

   CREATE TABLE products ( id SERIAL PRIMARY KEY, name TEXT NOT NULL, price NUMERIC DEFAULT 9.99, created_at TIMESTAMP DEFAULT NOW() ); -> Option B

5. Quick Check:

   Use NOW() for timestamp, numeric literals without quotes [OK]

Hint: Use NOW() for timestamp defaults, numeric literals without quotes [OK]

Common Mistakes:

• Using quotes around numeric default values
• Using CURRENT_DATE instead of CURRENT_TIMESTAMP for timestamps
• Omitting parentheses in NOW() function