Algebra

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Like most query engines, instead of internally working directly with a SPARQL query string, Comunica works with an algebraic representation of a SPARQL query, corresponding to the SPARQL 1.1 algebra. This SPARQL algebra makes it easier for operating on SPARQL operators in a consistent manner, and for applying transformations during query optimization.

Query Operation Actors

All actors on the Query Operation bus correspond to exactly one SPARQL algebra operator type. For example, @comunica/actor-query-operation-construct handles algebra operations with type 'construct'.

SPARQLAlgebra.js

Converting a query string to SPARQL algebra happens in the SPARQL Parse bus. The @comunica/actor-query-parse-sparql actor on this bus makes use of the SPARQLAlgebra.js package.

Examples on how the conversion between SPARQL query string and SPARQL algebra happens can be found in the tests: https://github.com/joachimvh/SPARQLAlgebra.js/tree/master/test

Converting a SPARQL query into algebra

If you want to quickly check what the algebra of a given SPARQL query string looks like, you can make use of Comunica's explain functionality as follows:

$ comunica-sparql https://fragments.dbpedia.org/2016-04/en -q 'SELECT * { ?s ?p ?o }' --explain parsed

{
  "type": "project",
  "input": {
    "type": "bgp",
    "patterns": [
      {
        "termType": "Quad",
        "value": "",
        "subject": {
          "termType": "Variable",
          "value": "s"
        },
        "predicate": {
          "termType": "Variable",
          "value": "p"
        },
        "object": {
          "termType": "Variable",
          "value": "o"
        },
        "graph": {
          "termType": "DefaultGraph",
          "value": ""
        },
        "type": "pattern"
      }
    ]
  },
  "variables": [
    {
      "termType": "Variable",
      "value": "s"
    },
    {
      "termType": "Variable",
      "value": "p"
    },
    {
      "termType": "Variable",
      "value": "o"
    }
  ]
}

This tool is therefore useful if you want to implement support for a SPARQL operator, but you need to find out to what algebra operation this corresponds.

Converting algebra into a SPARQL query

You can also apply the reverse transformation from algebra to SPARQL query string, for which you will need to globally install SPARQLAlgebra.js:

$ npm install -g sparqlalgebrajs
$ sparqlalgebrajs -q -r '
{
  "type": "project",
  "input": {
    "type": "bgp",
    "patterns": [
      {
        "type": "pattern",
        "termType": "Quad",
        "value": "",
        "subject": {
          "termType": "Variable",
          "value": "x"
        },
        "predicate": {
          "termType": "Variable",
          "value": "y"
        },
        "object": {
          "termType": "Variable",
          "value": "z"
        },
        "graph": {
          "termType": "DefaultGraph",
          "value": ""
        }
      }
    ]
  },
  "variables": [
    {
      "termType": "Variable",
      "value": "x"
    },
    {
      "termType": "Variable",
      "value": "y"
    },
    {
      "termType": "Variable",
      "value": "z"
    }
  ]
}
'

SELECT ?x ?y ?z WHERE { ?x ?y ?z }