DEFAULT values and expressions in PostgreSQL - Step-by-Step Execution

Concept Flow - DEFAULT values and expressions

Start INSERT

↓

Check each column

↓

Value provided?

Yes→Use provided value

No↓

Default defined?

Yes→Use DEFAULT value/expression

No↓

Use NULL or error if NOT NULL

↓

Insert row with final values

↓

END

When inserting a row, for each column, if no value is given, PostgreSQL uses the DEFAULT value or expression if defined; otherwise NULL or error if NOT NULL.

Execution Sample

PostgreSQL

CREATE TABLE users (
  id SERIAL PRIMARY KEY,
  name TEXT DEFAULT 'Anonymous',
  created_at TIMESTAMP DEFAULT now()
);

INSERT INTO users (id) VALUES (1);

Creates a table with default values and inserts a row providing only id, so defaults fill other columns.

Execution Table

Step	Column	Value Provided?	Default Defined?	Value Used	Action
1	id	Yes (1)	Yes (serial)	1	Use provided value
2	name	No	Yes ('Anonymous')	'Anonymous'	Use default value
3	created_at	No	Yes (now())	current timestamp	Use default expression
4	Insert row	-	-	(1, 'Anonymous', current timestamp)	Row inserted with defaults
5	End	-	-	-	Insertion complete

💡 All columns processed; defaults used where no value provided; row inserted successfully.

Variable Tracker

Variable	Start	After Step 1	After Step 2	After Step 3	Final
id	NULL	1	1	1	1
name	NULL	NULL	'Anonymous'	'Anonymous'	'Anonymous'
created_at	NULL	NULL	NULL	current timestamp	current timestamp

Key Moments - 3 Insights

What happens if no value is provided and no DEFAULT is defined?

If the column allows NULL, NULL is used; otherwise, an error occurs. See execution_table row 2 where default exists, but if it didn't, NULL or error would happen.

How does PostgreSQL handle DEFAULT expressions like now()?

It evaluates the expression at insertion time to get the current value, as shown in execution_table row 3 where now() returns the current timestamp.

Can you override a DEFAULT value by providing a value explicitly?

Yes, if you provide a value, PostgreSQL uses it instead of the default, as in execution_table row 1 where id is provided explicitly.

Visual Quiz - 3 Questions

Look at the execution_table, what value is used for the 'name' column at step 2?

A 'Anonymous'

B NULL

C Error

D Empty string

💡 Hint

Check the 'Value Used' column in execution_table row 2.

At which step does PostgreSQL use the provided value instead of the default?

A Step 2

B Step 1

C Step 3

D Step 4

💡 Hint

Look at the 'Value Provided?' column in execution_table; only step 1 has a provided value.

If the 'created_at' column had no DEFAULT, what would happen at step 3?

A Use 'Anonymous' string

B Use current timestamp anyway

C Use NULL if allowed or error if NOT NULL

D Insert fails immediately at step 1

💡 Hint

Refer to key_moments about missing DEFAULT values and NULL handling.

Concept Snapshot

DEFAULT values and expressions in PostgreSQL:
- When inserting, if no value given, use DEFAULT if defined.
- DEFAULT can be a constant or expression (e.g., now()).
- If no DEFAULT and column allows NULL, NULL is used.
- If NOT NULL without DEFAULT and no value, insertion errors.
- Explicit values override DEFAULTs.

Full Transcript

This visual execution shows how PostgreSQL handles DEFAULT values and expressions during an INSERT. For each column, it checks if a value is provided. If yes, it uses that value. If no, it checks if a DEFAULT is defined. If so, it uses the default value or evaluates the default expression like now(). If no default exists, it uses NULL if allowed or raises an error if the column is NOT NULL. The example inserts a row into a users table with id provided, and name and created_at filled by defaults. The step-by-step table tracks each column's value choice and the final inserted row. Key moments clarify common confusions about missing defaults, expression evaluation, and overriding defaults. The quiz tests understanding of these steps.