Quads not being emitted as early as possible in construct query

[jeswr]

Issue type:

• 🐌 Performance issue

---

Description:

If I run the following query with the standard @comunica/query-sparql config, then 1104 results and only the link https://w3id.org/dpv#Purpose is fetched.

import { QueryEngine } from "@comunica/query-sparql";

const fetchFn = globalThis.fetch;

// @ts-ignore
globalThis.fetch = (...args) => {
    console.log(args[0]);
    return fetchFn(...args);
}

async function run() {
    const engine = new QueryEngine();
    const res = await engine.queryQuads('CONSTRUCT { ?s ?p ?o } WHERE { ?s a <https://w3id.org/dpv#Purpose> ; ?p ?o . }', { sources: ['https://w3id.org/dpv#Purpose'], lenient: true });

    let i = 0;
    res.on('data', (quad) => {
        console.log(i += 1);
    });
}

run();

On the other hand when @comunica/query-sparql-link-traversal is used, the links are indefinitely fetched whilst no results are emitted. Note that this is not an issue if the query contains only a single triple in the BGP pattern such as CONSTRUCT WHERE { ?s a <https://w3id.org/dpv#Purpose> . }

I would hazard a guess that the problem is associated with the inner join performing some kind of nested loop which requires the inner stream to terminate before it can iterate over outer stream. As a result I would suggest ensuring the some kind of symmetric hash join is always used in link traversal scenarios.

[rubensworks]

I just tried this in the web client (which should run the latest version), and things are non-blocking there: https://comunica.github.io/comunica-feature-link-traversal-web-clients/builds/default/#datasources=https%3A%2F%2Fw3id.org%2Fdpv%23Purpose&query=CONSTRUCT%20%7B%20%3Fs%20%3Fp%20%3Fo%20%7D%20WHERE%20%7B%20%3Fs%20a%20%3Chttps%3A%2F%2Fw3id.org%2Fdpv%23Purpose%3E%20%3B%20%3Fp%20%3Fo%20.%20%7D

(Also for the Solid config: #datasources=https%3A%2F%2Fw3id.org%2Fdpv%23Purpose&query=CONSTRUCT { %3Fs %3Fp %3Fo } WHERE { %3Fs a <https%3A%2F%2Fw3id.org%2Fdpv%23Purpose> %3B %3Fp %3Fo . } )

Can you double-check you're running on the latest version?

[jeswr]

I'm encountering this on the latest 0.4.1 version. I've put together a minimal repro here