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 SPARQL query string to SPARQL algebra happens in the SPARQL Parse bus. The @comunica/actor-sparql-parse-algebra 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 install SPARQLAlgebra.js globally, and convert a query as follows:

    $ npm install -g sparqlalgebrajs
    $ sparqlalgebrajs -q "SELECT ?x ?y ?z WHERE { ?x ?y ?z }"
    
    {
      "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"
        }
      ]
    }

    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

    Using, you can also apply the reverse transformation from algebra to SPARQL query string:

    $ 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 }