Query complexity

#Overview

When working with GraphQL (GQL) queries, it's important to manage the complexity of your queries to ensure efficient and effective data retrieval.

In the context of GQL, query complexity refers to the computational resources needed to fulfill a query. The complexity of a query increases with the number of fields and the depth of the query.

• Scalar fields: Each scalar field in a query contributes one point to the query complexity.
• Relations / Unions: Relations multiply their complexity times the level of nesting in the query.

For example, if a query retrieves a list of posts and each post has multiple comments, the complexity of the query increases with each nested comment.

This guide will help you with the following:

• How to split up your GQL queries to manage complexity
• How to optimize union queries
• How to use the complexity tree JSON output to calculate the cost of your queries

#Splitting GQL queries

To manage query complexity, you can split your GQL queries into smaller, more manageable parts:

Remember that the goal is to reduce the complexity of your queries to ensure efficient and effective data retrieval. By limiting the depth of your queries, fetching only necessary fields, and using pagination, you can manage the complexity of your GQL queries effectively.

The following examples show you how you can split your GQL queries:

#Example 1: Limiting query depth

Instead of a deeply nested query like this:

{
  posts {
    id
    comments {
      id
      author
      replies {
        id
        text
        user {
          id
          name
        }
      }
    }
  }
}

You can split it into two separate queries:

#Example 2: Fetching only necessary fields

Intead of retrieving all fields, like this:

{
  post(where: { id: "..." }) {
    id
    title
    body
    author
    comments
  }
}

You can retrieve only the necessary fields, like this:

{
  post(where: { id: "..." }) {
    id
    title
  }
}

#Example 3: Using pagination

Hygraph supports various arguments for paginating content entries:

• first: Seek forwards from the start of the result set.
• last: Seek backwards from the end of the result set.
• skip: Skip result set by a given amount.
• before: Seek backwards before a specific ID.
• after: Seeks forwards after a specific ID.

The default result size of results returned by queries fetching multiple entries is 10. You can provide a maximum of 100 to the first, or last arguments.

You can use first, last, skip, before, and after arguments with any nested relations. In the following example, the posts model has comments:

{
  posts {
    id
    comments(first: 6, skip: 6) {
      id
      createdAt
    }
  }
}

#Union queries

Union types allow to setup relational fields that point to different model types, while this feature allows for very flexible modelling of content, it can also open the door to queries that might not perform as well and could use some optimizations. Below we document means to optimize querying for content that is backed by a Union relation.

Unions are typically queried like so:

{
  page(where: { id: "ckrks0ge0334m0b52onduq7r2" }) {
    id
    title
    blocks {
      __typename
      ... on Hero {
        title
        ctaLink
      }
      ... on Grid {
        title
        subtitle {
          markdown
        }
      }
      ... on Gallery {
        photos {
          url
          handle
        }
      }
    }
  }
}

As schemas evolve and Union relations expand to many models, querying unions this way can become problematic. Particularly when every single possible type is queried with this format within the same query.

#Optimizing union queries

In order to avoid performance impacts due to a large number of Union types in a relation, it is possible to change the way the content is queried so that it is done in a 2 step approach.

Below we will be using the same query from the previous section as an example:

1. Step 1: Find out which documents are in fact connected

   We will get the __typename and the id for all the connected documents in the union relation by using the Node interface like so:

   Query

   Response

2. Step 2: Query the connected types by id

   With the retrieved information we can construct queries dynamically to fetch the affected documents. Considering the response we received from the previous query in Step 1, we will now go over the response and generate another query that will in fact get only the connected documents by id:

   query heroBlocks {
     heros(where: { id_in: ["cks8t3o943h1l0d099v8xd072"] }) {
       title
       ctaLink
     }
   }
   
   query gridBlocks {
     grids(
       where: {
         id_in: ["cksj3dxww0o2r0c57savzceub", "cksrocxds3mwa0a07rdtj7qvx"]
       }
     ) {
       title
       subtitle {
         markdown
       }
     }
   }
   
   query galleryBlocks {
     galleries(where: { id_in: ["cks8t36i83iq70b6035caxp6n"] }) {
       photos {
         url
         handle
       }
     }
   }

   Copy

   Alternatively, you can combine these into a single query by using aliasing:

   query blocks {
     heroBlocks: heros(where: { id_in: ["cks8t3o943h1l0d099v8xd072"] }) {
       title
       ctaLink
     }
     gridBlocks: grids(
       where: {
         id_in: ["cksj3dxww0o2r0c57savzceub", "cksrocxds3mwa0a07rdtj7qvx"]
       }
     ) {
       title
       subtitle {
         markdown
       }
     }
     galleryBlocks: galleries(
       where: { id_in: ["cks8t36i83iq70b6035caxp6n"] }
     ) {
       photos {
         url
         handle
       }
     }
   }

   Copy

#Complexity tree JSON output

The complexity tree JSON output provides a detailed breakdown of the estimated and actual costs of your GraphQL query. This information can help you understand the computational resources required to fulfill your query and guide you in optimizing your queries for better performance.

#JSON Output

Here is a brief explanation of the keys in the JSON output:

• total_estimated_docs: The total number of documents estimated to be fetched by the query.
• total_actual_docs: The total number of documents actually fetched by the query.
• total_estimated_cost: The total estimated cost of the query. This includes the cost of fetching documents and any additional costs.
• total_actual_cost: The total actual cost of the query.
• complexityTree: A nested structure that breaks down the cost of each field in the query.

Each node in the complexityTree has the following keys:

• field_name: The name of the field in the query.
• xpath: The path to the field in the query.
• estimated_no_of_docs: The estimated number of documents fetched by this field.
• additional_cost: Any additional cost associated with this field.
• estimated_cost: The total estimated cost of this field (the sum of estimated_no_of_docs and additional_cost).
• actual_no_of_docs: The actual number of documents fetched by this field.
• actual_cost: The actual cost of this field.
• children: Any nested fields within this field. Each child is also a node with the same structure.

#JSON Output Example

Consider the following query and its related complexity tree JSON output:

This JSON output shows us that the total estimated cost of the query is 1116, which includes fetching 1110 documents and additional costs. However, since the query did not return any content for this example(there was no real content in the project), the actual costs and documents fetched are 0. Despite this, the query is still costly due to the nested structure, hence the high estimated cost.

The complexityTree provides a breakdown of the costs for each field in the query. For example, the posts field is estimated to fetch 10 documents with an additional cost of 2, resulting in an estimated cost of 12. Within the posts field, the comments field is estimated to fetch 100 documents with an additional cost of 2, resulting in an estimated cost of 102. The authors field within comments is estimated to fetch 1000 documents with an additional cost of 2, resulting in an estimated cost of 1002. This is because of the multiplication of nested fields that we mentioned before.