lancedb/docs/web/feature-engineering/add-computed-column.mdx

---
title: "Create a computed column"
sidebarTitle: "Create Computed Column"
description: "Declare a column from a SQL expression. Nothing is evaluated until you refresh it."
icon: table-columns
keywords: ["computed columns", "add_columns", "transforms", "schema"]
availability:
  embedded: available
  enterprise: available
---

import {
    PySchemaAddSetup,
    TsSchemaAddSetup,
    RsSchemaAddSetup,
    PyAddColumnsCalculated,
    TsAddColumnsCalculated,
    RsAddColumnsCalculated,
} from '/snippets/tables.mdx';

Lance can write a new column without rewriting the columns already in the
table. That matters most on a table that also holds large images, video or
embeddings, where rewriting everything just to add one small derived field
would be wasteful. See [Lance format](/basics/lance) for the storage model
this relies on.

A computed column is how LanceDB formalizes that capability for feature
engineering specifically: instead of writing the derived values immediately,
it is declared with an expression and starts with no values. Declaring it is
cheap on any table size, because nothing is evaluated until
[a refresh](/feature-engineering/refresh-column) asks for it.

<Note>
In the 0.41 SDK release, computed-column declaration and refresh in LanceDB Embedded require a
source build with the `experimental-functions` Cargo feature. Published Python
and TypeScript packages leave it disabled. LanceDB Enterprise operations
do not require this feature. Ordinary eager `add_columns` transforms remain available.
</Note>

## Add a computed column {#declare}

```python
import lancedb

db = lancedb.connect("./.lancedb")
table = db.create_table("computed_demo", [{"x": 1}, {"x": 2}])

table.add_columns(computed={"doubled": "x * 2"})
# AddColumnsResult(version=2)
```

That commits a new table version with the column present and empty. It is an
ordinary table version, so the usual [versioning](/management/table#versioning) applies,
including that you can travel back to before the column existed.

## Compute now or on refresh {#computed-vs-transforms}

`add_columns` takes either, and the difference matters:

- **SQL expressions in `transforms`** are evaluated *now*, for every row, as
  part of the call. Good for a small table, or a column you want populated
  immediately.
  {/* Draft for the Functions release:
  A `FunctionApplication` passed through `transforms` instead
  declares empty Function-bound columns, which a refresh fills; that needs a
  deployment with Functions enabled, and the embedded library refuses it. */}
- **`computed`** stores the expression and evaluates nothing. The column is
  committed empty and filled later by
  [`refresh_column`](/feature-engineering/refresh-column).

`computed` and `transforms` cannot be combined in one call. Reach for `computed` when the table is
large enough that you would rather control when the work happens.

Pass `auto_refresh_enabled` and `auto_refresh_interval_seconds` alongside
`computed` to have the server keep the column refreshed instead of calling
[`refresh_column`](/feature-engineering/refresh-column) yourself:

```python
table.add_columns(
    computed={"doubled": "x * 2"},
    auto_refresh_enabled=True,
    auto_refresh_interval_seconds=300,
)
```

A local table stores these settings but does not act on them -- automatic
scheduling runs on LanceDB Cloud or Enterprise. See
[Automatic refresh](/feature-engineering/refresh-column#automatic) for the
full behavior, including how to change the setting later.

An eager `transforms` call looks like this, starting from a small product
table:

<CodeGroup>
    <CodeBlock filename="Python" language="Python" icon="python">
    {PySchemaAddSetup}
    </CodeBlock>

    <CodeBlock filename="TypeScript" language="TypeScript" icon="square-js">
    {TsSchemaAddSetup}
    </CodeBlock>

    <CodeBlock filename="Rust" language="Rust" icon="rust">
    {RsSchemaAddSetup}
    </CodeBlock>
</CodeGroup>

<CodeGroup>
    <CodeBlock filename="Python" language="Python" icon="python">
    {PyAddColumnsCalculated}
    </CodeBlock>

    <CodeBlock filename="TypeScript" language="TypeScript" icon="square-js">
    {TsAddColumnsCalculated}
    </CodeBlock>

    <CodeBlock filename="Rust" language="Rust" icon="rust">
    {RsAddColumnsCalculated}
    </CodeBlock>
</CodeGroup>

For feature engineering on large existing tables, group related derived
features into one `add_columns` call instead of running many separate
writes: this creates one new table version for the schema change and
computes every new column from the existing rows in it, rather than growing
the table's version history with many small updates.

<Info>
`add_columns` does not accept Python callables, batch UDFs, or PyArrow
`RecordBatch` iterators for populating new columns, whether through
`transforms` or `computed`. New column values must be defined with SQL
expressions, or added as NULL columns from an Arrow field or schema. If your
transformation cannot be expressed in SQL, compute the values outside
`add_columns` before writing them back through another workflow.
</Info>

## The expression {#expressions}

The expression is SQL, evaluated over the row. It can reference any column in
the table, including another computed column:

```python
table.add_columns(computed={
    "power_score": "cast(x as double) / 2.0",
    "is_strong":   "power_score > 0.5",
})
```

Declaration order is dependency order: a computed column may reference one
declared earlier in the same mapping, but not one declared after it.

The functions available are the ones the
[SQL dialect](/sql/functions) documents.

A literal in the expression has to fit the column it meets: `x / 2.0` is
refused when `x` is an integer column, so write `cast(x as double) / 2.0`.

<Warning>
In LanceDB Enterprise, keep the expression to top-level columns. A column
refresh can fail on an expression that reads a field inside a struct, such as
`stats.magic + stats.strength`, which the embedded library computes. Try a new
expression on a small table first.
</Warning>

## Change columns used by an expression {#input-constraints}

<Warning>
While a computed column's declaration references another column, that column
cannot be renamed, retyped, or dropped. Drop the computed column first
if you need to change one of its inputs.
</Warning>

This is worth knowing before deriving a column from something you expect to
reshape later. Dropping the computed column releases the constraint.

## Filling with Python instead of SQL {#functions}

<Badge color="red">Enterprise</Badge>

When the value cannot be expressed in SQL (an embedding, a model call, an
image decode), [Geneva](/geneva), the feature engineering package for LanceDB
Enterprise, fills the column with a Python function instead.

{/*
Draft for the Functions release. Replaces the paragraph above:

When the value cannot be expressed in SQL -- an embedding, a model call, an
image decode -- the column is filled by a registered
[function](/management/function) instead of an expression. Declaring it looks
the same; what differs is what fills it. It needs a deployment with Functions
enabled.

The call passed to `add_columns` is the tell: a SQL expression in `computed`
is a string, keyed by the column name --

<CodeGroup>
```python Python icon="python"
table.add_columns(computed={"caption_length": "length(caption)"})
```
</CodeGroup>

-- while a Function application is the registered version itself, called with
its parameters bound to source columns through `col(...)`, keyed the same way
but passed positionally rather than through `computed`:

<CodeGroup>
```python Python icon="python"
table.add_columns({"caption": caption_version(jpeg=col("jpeg"))})
```
</CodeGroup>

Both accept the same `auto_refresh_enabled` and `auto_refresh_interval_seconds`
options, and both refresh and toggle the same way; see
[Refresh a computed column](/feature-engineering/refresh-column#automatic).
*/}

## Related guides {#next}

<CardGroup cols={2}>
  <Card title="Refresh a computed column" icon="arrows-rotate" href="/feature-engineering/refresh-column">
    Fill the rows that lack values.
  </Card>
  <Card title="Geneva" icon="layer-group" href="/geneva">
    Fill a column with Python on LanceDB Enterprise.
  </Card>
  {/*
  Draft for the Functions release. Replaces the Geneva card above:
  <Card title="Functions" icon="code" href="/management/function">
    Fill a column with Python.
  </Card>
  */}
</CardGroup>