Postgres: Computed fields¶

What are computed fields?¶

Computed fields are virtual values or objects that are dynamically computed and can be queried along with a table/view’s columns. Computed fields are computed when requested for via custom SQL functions (a.k.a. stored procedures) using other columns of the table/view and other custom inputs if needed.

Computed field types¶

Based on the SQL function’s return type, we can define two types of computed fields:

authors(id integer, first_name text, last_name text)

CREATE FUNCTION author_full_name(author_row authors)
RETURNS TEXT AS $$
  SELECT author_row.first_name || ' ' || author_row.last_name
$$ LANGUAGE sql STABLE;

query {
  authors {
    id
    first_name
    last_name
    full_name
  }
}

authors(id integer, first_name text, last_name text)

articles(id integer, title text, content text, author_id integer)

CREATE FUNCTION filter_author_articles(author_row authors, search text)
RETURNS SETOF articles AS $$
  SELECT *
  FROM articles
  WHERE
    ( title ilike ('%' || search || '%')
      OR content ilike ('%' || search || '%')
    ) AND author_id = author_row.id
$$ LANGUAGE sql STABLE;

query {
  authors {
    id
    first_name
    last_name
    filtered_articles(args: {search: "Hasura"}){
      id
      title
      content
    }
  }
}

Adding a computed field to a table/view¶

Console

CLI

API

Head to the Modify tab of the table/view and click on the Add button in the Computed fields section:

Supported from

• Console support for tables is available in v1.1.0 and above
• Console support for views is available in v1.3.0 and above

You can add a computed field in the tables.yaml file inside the metadata directory:

 - table:
     schema: public
     name: authors
   computed_fields:
   - name: full_name
     definition:
       function:
         schema: public
         name: author_full_name
       table_argument: null
     comment: ""

Apply the metadata by running:

hasura metadata apply

A computed field can be added to a table/view using the add_computed_field metadata API:

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

{
  "type": "add_computed_field",
  "args": {
    "table": {
      "name": "authors",
      "schema": "public"
    },
    "name": "full_name",
    "definition": {
      "function": {
        "name": "author_full_name",
        "schema": "public"
      },
      "table_argument": "author_row"
    }
  }
}

Computed fields permissions¶

Access control to computed fields depends on the type of computed field.

• For scalar computed fields, permissions are managed similar to the columns permissions of the table.
• For table computed fields, the permissions set on the return table are respected.

Accessing Hasura session variables in computed fields¶

It can be useful to have access to the session variable from the SQL function defining a computed field. For instance, suppose we want to record which users have liked which articles. We can do so using a table article_likes that specifies a many-to-many relationship between articles and users. In such a case it can be useful to know if the current user has liked a specific article, and this information can be exposed as a Boolean computed field on articles.

Create a function with an argument for session variables and add it with the add_computed_field API with the session_argument key set. The session argument is a JSON object where keys are session variable names (in lower case) and values are strings. Use the ->> JSON operator to fetch the value of a session variable as shown in the following example.

-- 'hasura_session' will be the session argument
CREATE OR REPLACE FUNCTION article_liked_by_user(article_row articles, hasura_session json)
RETURNS boolean AS $$
SELECT EXISTS (
    SELECT 1
    FROM article_likes A
    WHERE A.user_id = hasura_session ->> 'x-hasura-user-id' AND A.article_id = article_row.id
);
$$ LANGUAGE sql STABLE;

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

{
    "type":"add_computed_field",
    "args":{
        "table":{
            "name":"articles",
            "schema":"public"
        },
        "name":"liked_by_user",
        "definition":{
            "function":{
                "name":"article_liked_by_user",
                "schema":"public"
            },
            "table_argument":"article_row",
            "session_argument":"hasura_session"
        }
    }
}

query {
  articles(where: {id: {_eq: 3}}) {
    id
    liked_by_user
  }
}

Computed fields vs. Postgres generated columns¶

Postgres, from version 12, is introducing Generated Columns. The value of generated columns is also computed from other columns of a table. Postgres’ generated columns come with their own limitations. Hasura’s computed fields are defined via an SQL function, which allows users to define any complex business logic in a function. Generated columns will go together with computed fields where Hasura treats generated columns as normal Postgres columns.