Allow query explanation via HTTPRepository / REST API

[abrokenjester]

The query explain feature (see GH-2044) should be extended to allow use over HTTP. This will enable using it in the HTTPRepository as well as in any non-Java environment that communicates via the REST API.

[odysa]

@hmottestad @kenwenzel Hello, I am interested in this feature and would like to contribute.
If RDF4J maintainers still want this rest API, I am happy to discuss api design with you

[hmottestad]

That would be very welcome from our side.

https://rdf4j.org/documentation/reference/rest-api/

Here is the API docs that we have. Did you already have an idea for how to extend it to support query explanation?

[odysa]

Add POST Request /repositories/{repository_id}/explain

Request Body:

{
  "query": "SELECT * WHERE {?S ?O ?P}",
  "level": "Timed"
}

Response:

{
  "explain": ".........."
}

Core Logic:

try (final RepositoryConnection connection = RepositoryInterceptor.getRepositoryConnection()) {
    final Explanation explanation = connection.prepareQuery(query).explain(level);
}

Optionally, we may integrate explain into the Workbench UI to enhance user interaction.

[nguyenm100]

@hmottestad @kenwenzel would you prefer extending the sparql to do smth like

EXPLAIN [with limit] SPARQL

or try and do it through custom functions?

[kenwenzel]

Neptune uses an additional HTTP Parameter:
https://docs.aws.amazon.com/neptune/latest/userguide/sparql-explain-using.html

Maybe this is also something that we could do?

[odysa]

@kenwenzel Good idea.
If I understand correctly, we will use the existing SPARQL execution endpoints. If explain=mode is passed, it does query explanation, otherwise, execute this query.

[kenwenzel]

@odysa Yes, this is how I understand it, too.