> ## Documentation Index
> Fetch the complete documentation index at: https://docs.sqd.dev/llms.txt
> Use this file to discover all available pages before exploring further.

# OpenReader GraphQL Intro

> Introduction to GraphQL queries served by OpenReader, the Squid SDK GraphQL server — entity queries, filters, sorting, and pagination basics.

<Info>
  At the moment, [Squid SDK GraphQL server](/en/sdk/squid-sdk/reference/openreader-server) can only be used with squids that use Postgres as their target database.
</Info>

GraphQL is an API query language, and a server-side runtime for executing queries using a custom type system. Head over to the [official documentation website](https://graphql.org/learn/) for more info.

A GraphQL API served by the [GraphQL server](/en/sdk/squid-sdk/reference/openreader-server) has two components:

1. Core API is defined by the [schema file](/en/sdk/squid-sdk/reference/schema-file).
2. Extensions added via [custom resolvers](/en/sdk/squid-sdk/reference/openreader-server/configuration/custom-resolvers).

In this section we cover the core GraphQL API, with short explanations on how to perform GraphQL queries, how to paginate and sort results. This functionality is supported via [OpenReader](https://github.com/subsquid/squid-sdk/tree/master/graphql/openreader), SQD's own implementation of [OpenCRUD](https://www.opencrud.org).