GENERATED columns (stored and virtual) in PostgreSQL - Step-by-Step Execution

Concept Flow - GENERATED columns (stored and virtual)

Create Table

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Define Columns

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Define Regular Columns

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Define GENERATED Column

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Specify Expression

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Choose STORED or VIRTUAL

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Insert Data

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GENERATED Column Value Computed

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Query Table

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See Computed Values

When creating a table, you define normal columns and GENERATED columns with expressions. The GENERATED column computes its value automatically, either stored physically or computed on the fly.

Execution Sample

PostgreSQL

CREATE TABLE products (
  price numeric,
  quantity int,
  total_cost numeric GENERATED ALWAYS AS (price * quantity) STORED
);

Creates a table with a stored generated column that calculates total_cost as price times quantity.

Execution Table

Step	Action	Input Data	Generated Column Computation	Resulting Row
1	Insert row	price=10, quantity=3	total_cost = 10 * 3 = 30	(10, 3, 30)
2	Insert row	price=5, quantity=7	total_cost = 5 * 7 = 35	(5, 7, 35)
3	Query table	N/A	Values retrieved from stored column	(10, 3, 30), (5, 7, 35)
4	Update price	price=6 for first row	total_cost recalculated = 6 * 3 = 18	(6, 3, 18)
5	Query table	N/A	Updated stored values shown	(6, 3, 18), (5, 7, 35)
6	Insert row with NULL	price=NULL, quantity=4	total_cost = NULL * 4 = NULL	(NULL, 4, NULL)
7	Query table	N/A	NULL shown for total_cost	(6, 3, 18), (5, 7, 35), (NULL, 4, NULL)
8	Attempt to insert invalid data	price='abc', quantity=2	Error: invalid input for numeric	Insert fails

💡 Execution stops after error on invalid data insert.

Variable Tracker

Variable	Start	After 1	After 2	After 4	After 6	Final
price	N/A	10	5	6	NULL	6
quantity	N/A	3	7	3	4	3
total_cost	N/A	30	35	18	NULL	18

Key Moments - 3 Insights

Why does total_cost update automatically when price changes?

Because total_cost is a GENERATED column defined as price * quantity, it recalculates whenever price or quantity changes, as shown in execution_table row 4.

What happens if one of the input columns is NULL?

The generated column result becomes NULL because any arithmetic with NULL yields NULL, as seen in execution_table row 6.

Why does inserting a non-numeric value for price cause an error?

Because price is numeric type, inserting 'abc' is invalid and the database rejects it, shown in execution_table row 8.

Visual Quiz - 3 Questions

Look at the execution_table, what is total_cost after the first insert?

A 13

B 30

C 3

D NULL

💡 Hint

Check execution_table row 1 under Generated Column Computation.

At which step does total_cost become NULL?

A Step 6

B Step 2

C Step 4

D Step 8

💡 Hint

Look at execution_table row 6 where price is NULL.

If total_cost was defined as VIRTUAL instead of STORED, what would change?

A Values would be stored physically in the table.

B You could insert values manually into total_cost.

C Values would be computed on query, not stored.

D The table would reject NULL values.

💡 Hint

Recall the difference between STORED and VIRTUAL generated columns.

Concept Snapshot

GENERATED columns compute values automatically from other columns.
Syntax: column_name data_type GENERATED ALWAYS AS (expression) STORED|VIRTUAL.
STORED saves computed values physically; VIRTUAL computes on query.
Useful for derived data like totals or concatenations.
Cannot insert or update generated columns directly.

Full Transcript

This lesson shows how GENERATED columns work in MySQL. When creating a table, you can define columns that automatically calculate their values from other columns using an expression. These columns can be STORED, meaning their values are saved in the table, or VIRTUAL, meaning their values are calculated when you query the table. For example, a total_cost column can be generated as price times quantity. When you insert or update rows, the database automatically calculates total_cost. If input columns are NULL, the generated column becomes NULL. Invalid data types cause errors. This visual trace shows step-by-step how data is inserted, how generated columns compute values, and how updates affect them.