# Indexes

## What is a Secondary Index?

A secondary index can be added to any column or set of columns to convert lookup queries involving those columns into indexed lookups. Indexed lookups can be accomplished in constant time (ie. O(1)).

Secondary indexes are stored as separate data structures on disk or in memory. Thus, the use of secondary indexes uses more storage and increases insert and update time.

Secondary indexes are called "secondary" to distinguish them from [primary keys](/concepts/sql/primary-key.md) which also provide indexed lookups.

## How to use Secondary Indexes

Add secondary indexes to columns that are often accessed with a where clause that you would like to return fast. For these columns you will need to tolerate slightly reduced insert and update performance. Additionally, you should have the disk space available to store the indexes.

You create indexes using the `CREATE INDEX` SQL statement.

## Difference between Postgres Secondary Indexes and Doltgres Secondary Indexes

Functionally, Doltgres and Postgres indexes are equivalent.

## Interaction with Doltgres Version Control

Doltgres indexes are versioned along with the core table they reference. Practically, this means querying a historical version is as fast as querying the current version because the index is intact for the historical version.

Doltgres will merge indexes as part of a Doltgres merge. This can be used to offload index creation to a branch or offline clone.

## Example

```
create index index1 on complex(c1);
```


---

# Agent Instructions: Querying This Documentation

If you need additional information that is not directly available in this page, you can query the documentation dynamically by asking a question.

Perform an HTTP GET request on the current page URL with the `ask` query parameter:

```
GET https://docs.doltgres.com/concepts/sql/indexes.md?ask=<question>
```

The question should be specific, self-contained, and written in natural language.
The response will contain a direct answer to the question and relevant excerpts and sources from the documentation.

Use this mechanism when the answer is not explicitly present in the current page, you need clarification or additional context, or you want to retrieve related documentation sections.