Accessing `pgClient` in hooks for `explain` data

Summary

We’re using the embeddable Apollo Explorer in place of GraphiQL (for a number of reasons that are off topic) and we’d like to add the explain data onto result data.

Additional context

We already know that we can use the postgraphile:http:end hook to mutate result data. However, we’re hitting a wall trying to find the right hook or right method to access pgClient._explainResults. We’re starting postgraphile with allowExplain: true fwiw.

I’ve tried to trace through contexts to figure out where postgraphile might be exposing that, but I’m coming up empty. (It’s a bit dizzying trying to follow what is sent where and how contexts are constructed, without being intimately familiar with it).

Any pointers to solve this one?

[shellscape]

awesome. got it. here’s the last bit, which must be placed before app.use(postgraphile) :

  // koa
  app.use(async (ctx, next) => {
    // Note: must use lowercase here, as postgraphile looks for the lowercase variant
    ctx.request.headers['x-postgraphile-explain'] = 'on';
    await next();
  });

I haven’t figured out how to hack Apollo Explorer to inject explain into the results pane (it seems to be ignoring anything that’s not "data") but this will allow someone to log the explain at the very least:

  'postgraphile:http:result': (result, opts) => {
    const { explain } = result as any;

    if (explain) {
      explain.forEach(({ plan, query }: { plan: any; query: any }) => {
        log.info('Explain:\n\n  query:', query, '\n\n  plan:', plan);
      });
    }
  }

Thanks @benjie

[benjie]

Turns out it’s X-PostGraphile-Explain: on; set here in the GraphiQL fetcher: https://github.com/graphile/postgraphile/blob/a33712580ae3304d9f68cedaf4816a0661dfbad1/postgraphiql/src/components/PostGraphiQL.js#L376

Which results in explain being set to true on the withPostGraphileContext options: https://github.com/graphile/postgraphile/blob/ed6ecb4596088120969795a9ec30cb625ef93eae/src/postgraphile/http/createPostGraphileHttpRequestHandler.ts#L152

Which then uses the startExplain / stopExplain methods that we patch onto the Postgres client:

https://github.com/graphile/postgraphile/blob/263ba7477bc2133eebdf89d29acd0460e58501ec/src/postgraphile/withPostGraphileContext.ts#L481-L508

(Note that this will not work with schema-only usage because we don’t mod the postgres client with schema-only usage.)