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  1. Apache Jena
  2. JENA-89

Avoid a total sort for ORDER BY + LIMIT queries




      In case of SPARQL queries with ORDER BY + LIMIT, ARQ sorts the entire result set and then produces the first N, according to the specified LIMIT.
      As an alternative, discussed on jena-dev [1], we can use a PriorityQueue [2] (which is based on a priority heap) to avoid a sort operation.

      ARQ's algebra package contains already a OpTopN [3] operator. The OpExecutor [4] will need to use a new QueryIterTopN instead of QueryIterSort + QueryIterSlice. A new TransformOrderByLimit to be used by Optimize is also necessary.

      ORDER BY + LIMIT queries are typically used to construct the first page when results are paginated. Then the following query is ORDER BY + OFFSET + LIMIT. (Often users stop at the first page). Ideally, we could cache the ORDER BY and implement the OFFSET|LIMIT using results from the cache. However, the improvement described by this issue is limited to the ORDER BY + LIMIT case for which a priority heap is a good enough solution.

      Hopefully, this would improve the scalability of ORDER BY + LIMIT queries in case of small values specified on the LIMIT.

      [1] http://markmail.org/thread/5d2gtazkoxsa2ayv
      [2] http://download.oracle.com/javase/6/docs/api/java/util/PriorityQueue.html
      [3] https://svn.apache.org/repos/asf/incubator/jena/Jena2/ARQ/trunk/src/com/hp/hpl/jena/sparql/algebra/op/OpTopN.java
      [4] https://svn.apache.org/repos/asf/incubator/jena/Jena2/ARQ/trunk/src/com/hp/hpl/jena/sparql/engine/main/OpExecutor.java


        1. ARQ_JENA-89_r1156212.patch
          12 kB
          Paolo Castagna
        2. JENA-89-TopTests.patch
          33 kB
          Andy Seaborne
        3. JENA-89-Top-FixHeapCollation.patch
          6 kB
          Andy Seaborne

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              castagna Paolo Castagna
              castagna Paolo Castagna
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