---
title: "Scope curation to what changed"
sidebarTitle: "Changes"
description: "Process only the rows added since your last curation run, instead of rescanning the whole table."
icon: clock-rotate-left
keywords: ["changes", "incremental", "cdc", "version", "append-only"]
availability:
  embedded: unavailable
  enterprise: available
---

A curation task you run repeatedly -- chunking, deduplication, profiling --
does not need to rescan rows it already processed. `CHANGES` reads only the
rows appended to a table since an earlier
[version](/management/table#versioning), so a recurring job can scope itself
to what is new.

## Example data {#example-data}

`curation_batches` gets a batch of new documents appended periodically, with
no updates or deletes between appends:

```sql
CREATE TABLE curation_batches (doc_id BIGINT, source VARCHAR, text VARCHAR);
INSERT INTO curation_batches VALUES
    (1, 'web', 'A cat on a chair'),
    (2, 'web', 'A dog in a field');
-- note the version after this insert, for example 5
INSERT INTO curation_batches VALUES
    (3, 'books', 'A bird in a tree'),
    (4, 'books', 'A cat by a window');
-- the table is now at a later version, for example 7
```

## Read only what is new {#read-new}

`CHANGES (INFORMATION => APPEND_ONLY) AT (VERSION => 5)` reads the rows
appended after version 5 -- documents 3 and 4 -- without rescanning 1 and 2:

```sql
SELECT * FROM curation_batches CHANGES (INFORMATION => APPEND_ONLY) AT (VERSION => 5)
```

Record the table's version after each curation run, then start the next
run's `CHANGES` from it. See [Versioning](/management/table#versioning) for
the APIs that read a table's current version.

## Chain it into a curation task {#chain}

`CHANGES` is a relation, like any other table reference, so it composes with
the rest of this page: chunk, dedupe, or join only the new rows, instead of
the whole table. Materializing the result keeps a running, append-only table
of work already done:

```sql
INSERT INTO curated_batches
SELECT doc_id, source, text
FROM curation_batches CHANGES (INFORMATION => APPEND_ONLY) AT (VERSION => 5)
WHERE source = 'books'
```

A [chunker Function](/curation/chunking) or a [`dedupe`](/sql/dql/dedupe)
call works the same way: put the `CHANGES` relation where the source table
would otherwise go, and only the new rows flow through it.

## Strictly append-only {#strict}

The clause is named for what it requires, not just what it does. If anything
other than a pure append happened to the table between the starting version
and the current one -- an update, a delete, a merge -- the query is refused
rather than silently returning an incomplete or wrong result:

```sql
-- refused if any row in curation_batches was updated or deleted after version 5
SELECT * FROM curation_batches CHANGES (INFORMATION => APPEND_ONLY) AT (VERSION => 5)
```

When that happens, either re-run from a later starting version that is
itself after the non-append write, or read the table in full instead of
incrementally for that run. See
[Changes since a version](/sql/dql/changes#strict) for the complete set of
refused forms.

## What to read next {#next}

<CardGroup cols={2}>
  <Card title="CHANGES" icon="clock-rotate-left" href="/sql/dql/changes">
    The full SQL syntax, placement rules, and unsupported forms.
  </Card>
  <Card title="Table" icon="table" href="/management/table#versioning">
    How table versions work, and how to find one to start from.
  </Card>
</CardGroup>