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  1. Blazegraph (by SYSTAP)
  2. BLZG-1141

Strategies for VALUES+BIND queries

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    Details

    • Type: Bug
    • Status: Accepted
    • Priority: Highest
    • Resolution: Unresolved
    • Affects Version/s: BIGDATA_RELEASE_1_4_0
    • Fix Version/s: None
    • Component/s: Query Plan Generator
    • Labels:
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      Description

      There are still some issues related to BIND and VALUES queries.

      1.) Currently BIND expressions are always evaluated at the end of JOIN groups. This is suboptimal from a performance perspective.

      2.) There are issues related to the pushing of variables bound in BIND/VALUES clauses into subqueries. See for example ASTSubGroupJoinVarOptimizer

      3.) The computation of definiteVars for bind expressions could be improved: currently, variables bound in BIND expressions are never considered definite (as BIND in general may fail), but trivial cases such as BIND (?x as <http://constant/uri>) might be considered "safe".

      4.) The following should give the empty result, but does not. Probably, the outer VALUES clause is somehow pushed inside:

      SELECT *
      WHERE {

      { BIND ( "x" as $X ) FILTER( BOUND($Y)) }

      }
      VALUES ?Y { "y" }

      This leads to the more general question of when it is allowed to push down variables into subqueries (cf. the remark about SPARQL 1.1 semantics in ASTSubGroupJoinVarOptimizer). We should come up with a general framework to make qualified decisions about pushing in variables.

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              Assignee:
              michaelschmidt michaelschmidt
              Reporter:
              michaelschmidt michaelschmidt
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                Created:
                Updated: