Document best practice usage of pageContext

[hswolff]

Edit (by @m-allanson): original issue was solved - this is now a documentation issue. See #6467 (comment) and follow up comments.

Description

Recently upgraded my blog to Gatsby v2 and am seeing it take ~50 seconds longer to build.

Steps to reproduce

This is the PR that upgrades my PR to Gatsby v2: hswolff/blog#4

You can check out master and run install deps and then run yarn build on both branches to see the change in behavior.

[KyleAMathews]

Specifically updating his schema after createPages is taking nearly 2 minutes. Seems like it's hitting some sort of pathological case.

[KyleAMathews]

@pieh could you look into this?

[pieh]

Yeah, I'm on it

[pieh]

This is interesting:

===PROFILE===
├─ ROOT: 126363.297062ms | 100.00%
└─ generate schema: 106728.97009ms | 84.46%
   ├─ extractFieldExamples SitePage: 264.611494ms | 0.25%
   ├─ extractFieldExamples SitePlugin: 4.2849900000000005ms | 0.00%
   ├─ extractFieldExamples Site: 0.277943ms | 0.00%
   ├─ extractFieldExamples Directory: 1.577542ms | 0.00%
   ├─ extractFieldExamples File: 55.946252ms | 0.05%
   ├─ extractFieldExamples MarkdownRemark: 35.786373ms | 0.03%
   ├─ buildNodeTypes: 1343.161175ms | 1.26%
   ├─ buildNodeConnections: 217.83265400000002ms | 0.20%
   └─ new GraphQLSchema: 105165.92856ms | 98.54%

The slowdown is in this part:

gatsby/packages/gatsby/src/schema/index.js

Lines 20 to 25 in 28dec63

	const schema = new GraphQLSchema({
	query: new GraphQLObjectType({
	name: `RootQueryType`,
	fields: { ...connections, ...nodes },
	}),
	})

Will update when I get to the bottom of it

[pieh]

https://github.com/hswolff/blog/blob/4d9410166b7d0cc2da0e4ed89fc794ffce0107a0/gatsby-node.js#L152-L165

If you change it like that:

  edges.forEach(({ node }) => {
    if (node.frontmatter.tags) {
      node.frontmatter.tags.forEach(tag => {
        if (!tags[tag]) {
          tags[tag] = {
            name: tag,
            slug: createFullUrl(`tag/${tag}`),
-            nodes: [],
+            nodes___NODE: [],
          };
        }
-        tags[tag].nodes.push(node);
+        tags[tag].nodes___NODE.push(node.id);
      });
    }
  });

Before:

createNodeFields SitePage: 106747.933646ms | 98.17%

After:

createNodeFields SitePage: 1082.52279ms | 54.36%

Not sure why this is different from v1 to be honest ... there should be no difference in this regard

[pieh]

So this is interesting:

longer schema generation is because v2 have this fileAbsolutePath field (not there in v1) which we will do intensive processing to try to link to File node (95% of overall schema generation time is spent in FileType.shouldInfer). It also takes very long in general because tags is object with over 1000 fields and for each of those fields we do separate FileType.shouldInfer call

[pieh]

I've opened PR that fixes this.

But really full data shouldn't be passed by context to page - you should only pass minimal needed data ( like ids or slugs) to be used in page queries to get full data there.

[hswolff]

Exciting you found a fix! Should be a good thing to document to educate users - that there are ways you can misuse Gatsby resulting in unexpected performance/behavior.

[pieh]

I agree. TBH I thought we had this in docs, but apparently I was wrong.

Looking through docs I see those spots that could be good place for that information:

• https://www.gatsbyjs.org/docs/bound-action-creators/#createPage (adnotation to context param)
• https://www.gatsbyjs.org/docs/creating-and-modifying-pages/
• https://www.gatsbyjs.org/tutorial/part-seven/#creating-pages

Additionally - maybe we could try to display warning if passed context is really heavy - just not sure how to determine what's too heavy

[KyleAMathews]

We also might want to start removing context from the page nodes to prevent a schema for being created for them. It's of pretty limited utility I think.

[m-allanson]

@pieh's fix published in:

• gatsby@2.0.0-beta.46
• gatsby-source-filesystem@2.0.1-beta.6
• gatsby-transformer-toml@2.1.1-beta.4

I'm going to re-open this and mark it as a docs issue. See #6467 (comment) and the follow up comments.