Postgres: Extend schema with views¶

Table of contents

What are views?
Creating views
Tracking views
Use cases
- Example: Group by and then aggregate
- Example: Hide certain fields of a table

What are views?¶

Views can be used to expose the results of a custom query as a virtual table. Views are not persisted physically i.e. the query defining a view is executed whenever data is requested from the view.

Hasura GraphQL engine lets you expose views over the GraphQL API to allow querying them using both queries and subscriptions just like regular tables.

Creating views¶

Views can be created using SQL which can be run in the Hasura console:

Head to the Data -> SQL section of the Hasura console
Enter your create view SQL statement
Hit the Run button

Create a migration manually and add your create view SQL statement to the up.sql file. Also, add an SQL statement to the down.sql file that reverts the previous statement.
Apply the migration and metadata by running:
```
hasura migrate apply
```

You can add a view by using the run_sql metadata API:

POST /v1/query HTTP/1.1
Content-Type: application/json
X-Hasura-Role: admin

{
  "type": "run_sql",
  "args": {
    "sql": "<create view statement>"
  }
}

Tracking views¶

Views can be present in the underlying Postgres database without being exposed over the GraphQL API. In order to expose a view over the GraphQL API, it needs to be tracked.

While creating views from the Data -> SQL page, selecting the Track this checkbox will expose the new view over the GraphQL API right after creation.

You can track any existing views in your database from the Data -> Schema page:

To track the view and expose it over the GraphQL API, edit the tables.yaml file in the metadata directory as follows:

   - table:
       schema: public
       name: author
   - table:
       schema: public
       name: article
   - table:
       schema: public
       name: <name of view>

Apply the metadata by running:
```
hasura metadata apply
```

To track the view and expose it over the GraphQL API, make the following API call to the track_table metadata API:

POST /v1/query HTTP/1.1
Content-Type: application/json
X-Hasura-Role: admin

{
  "type": "track_table",
  "args": {
    "schema": "public",
    "name": "<name of view>"
  }
}

Use cases¶

Views are ideal solutions for retrieving some derived data based on some custom business logic. If your custom logic requires any user input, you should use custom SQL functions instead.

Let’s look at a few example use cases for views:

Example: Group by and then aggregate¶

Sometimes we might want to fetch some data derived by aggregating (avg, min, max, etc.) over a group of rows in a table.

Let’s say we want to fetch the average article rating for each author in the following schema:

author(id integer, name text, city text, email text, phone integer, address text)

article(id integer, title text, content text, rating integer, author_id integer)

A view that averages the rating of articles for each author can be created using the following SQL query:

CREATE VIEW author_average_rating AS
  SELECT author_id, avg(rating)
    FROM article
    GROUP BY author_id

Example: Hide certain fields of a table¶

Sometimes we might have some sensitive information in a table which we wouldn’t want to expose.

Let’s say, we want to expose the following author table without the fields email, phone and address:

author(id integer, name text, city text, email text, phone integer, address text)

A view that only exposes the non-sensitive fields of the author table can be created using the following SQL query:

CREATE VIEW author_public AS
  SELECT id, name, city
    FROM author

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Hasura GraphQL engine is open source. See license