lancedb/docs/web/feature-engineering/data-evolution.mdx

---
title: "Data evolution"
sidebarTitle: "Data Evolution"
description: "Derive new columns from data already in the table, without rewriting what is already there."
icon: wand-magic-sparkles
keywords: ["data evolution", "derived columns", "add_columns", "zero copy", "feature engineering"]
---

import {
    PySchemaAddSetup,
    TsSchemaAddSetup as TsSchemaAddSetup,
    RsSchemaAddSetup as RsSchemaAddSetup,
    PyAddColumnsCalculated as AddColumnsCalculated,
    TsAddColumnsCalculated as TsAddColumnsCalculated,
    RsAddColumnsCalculated as RsAddColumnsCalculated,
    PyAddFeatureColumnsSql as AddFeatureColumnsSql,
    TsAddFeatureColumnsSql as TsAddFeatureColumnsSql,
    RsAddFeatureColumnsSql as RsAddFeatureColumnsSql,
} from '/snippets/tables.mdx';

Data evolution is adding **values** derived from what a table already holds.
[Schema evolution](/management/table#schema) changes the table's shape; this fills the
shape with something new.

The reason it is cheap is the format. Adding a derived column writes only that
column -- the existing data, which is the expensive part on a multimodal table,
is never rewritten. See [why that works](/basics/lance#evolution).

<Note>
In SQL this is [`ALTER TABLE ... ADD COLUMN ... AS`](/sql/ddl/table#computed-column).
</Note>

## Set up the example table {#set-up-the-example-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>

## Add derived columns {#add-derived-columns}

You can add new columns that are derived from existing data using SQL expressions.
For feature engineering on large existing tables, group related derived features into
one `add_columns` operation instead of running many separate writes. This creates one
new table version for the schema change and computes the new columns from the existing
rows, which avoids growing the table's version history with many small updates.

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

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

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

The same call can add multiple derived columns at once. For example, if you are
building several lightweight features from existing product fields, pass all of the
new column expressions together:

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

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

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

<Info>
LanceDB `add_columns` does not currently accept Python callables, batch UDFs, or
PyArrow `RecordBatch` iterators for populating new columns. 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>

## When SQL is not enough {#beyond-sql}

The expressions above are evaluated once, as part of the call. Two other shapes
exist when that does not fit:

- A [computed column](/feature-engineering/add-computed-column) declares the
  expression and leaves the column empty, so you choose when the work happens.
  That is the right default on a large table.
- A [function](/management/function) fills a column with Python, for values SQL
  cannot express -- an embedding, a model call, an image decode.

## What to read next {#next}

<CardGroup cols={2}>
  <Card title="Add a computed column" icon="table-columns" href="/feature-engineering/add-computed-column">
    Declare now, fill later.
  </Card>
  <Card title="Schema evolution" icon="boxes-stacked" href="/management/table#schema">
    Change the table's shape instead of its contents.
  </Card>
</CardGroup>