How does computed column work?

The expression reads other columns. Inserting or editing a dependency recomputes the dependents of that row. Unchanged rows keep their cached values.

Orchestration

What is a computed column?

A computed column is a value produced by an expression or a model when the row or its dependencies change, then cached.

Updated 2026-09-29 · Part of What is a declarative pipeline?

How it works

The expression reads other columns.
Inserting or editing a dependency recomputes the dependents of that row.
Unchanged rows keep their cached values.

What it is not

It is not a trigger you maintain by hand, an edge function, or a batch job that rewrites the whole table.

computed column: this, and the thing it is confused with

computed column: this, and the thing it is confused with
	This	Not this
Written by	The engine, from an expression	Your application, every time
Cache	Per row	A full-table refresh
Failure	That row, visible	A job that retries the world

Where Pixeltable fits

In Pixeltable a computed column is an assignment on the class. Insert a row and the expression runs.

import pixeltable as pxt
from pixeltable.functions.openai import chat_completions

TableModel = pxt.model_base()

class Docs(TableModel, name='docs'):
    text: pxt.String

    summary = chat_completions(
        messages=[{'role': 'user', 'content': text}],
        model='gpt-4o-mini',
    ).choices[0].message.content

Questions

How does computed column work?: The expression reads other columns. Inserting or editing a dependency recomputes the dependents of that row. Unchanged rows keep their cached values.
What is computed column often confused with?: It is not a trigger you maintain by hand, an edge function, or a batch job that rewrites the whole table.