optimization - 具有多个聚合的 SPARQL 查询超出内存限制

Question

我正在尝试使用 SPARQL 从三元组商店生成一些用户统计信息。请参阅下面的查询。如何改进？我在这里做坏事吗？为什么这会消耗这么多内存？ (见本文末尾的背景故事)

我更喜欢在三元组商店内进行聚合和连接。拆分查询意味着我必须在数据库之外“手动”加入结果，从而失去三重存储的效率和优化。无需无缘无故地重新发明轮子。

查询

SELECT
    ?person
    (COUNT(DISTINCT ?sent_email) AS ?sent_emails)
    (COUNT(DISTINCT ?received_email) AS ?received_emails)
    (COUNT(DISTINCT ?receivedInCC_email) AS ?receivedInCC_emails)
    (COUNT(DISTINCT ?revision) AS ?commits)

WHERE {
  ?person rdf:type foaf:Person.

  OPTIONAL {
    ?sent_email rdf:type email:Email.
    ?sent_email email:sender ?person.
  }

  OPTIONAL {
    ?received_email rdf:type email:Email.
    ?received_email email:recipient ?person.
  }

  OPTIONAL {
    ?receivedInCC_email rdf:type email:Email.
    ?receivedInCC_email email:ccRecipient ?person.
  }

  OPTIONAL {
    ?revision rdf:type vcs:VcsRevision.
    ?revision vcs:committedBy ?person.
  }
}
GROUP BY ?person
ORDER BY DESC(?commits)

背景

问题是我在 AllegroGraph 中收到错误“QUERY MEMORY LIMIT REACHED”(另请参阅我的相关 SO question)。由于存储库仅包含大约 20 万个三元组，这些三元组很容易放入一个(ntriples)输入文件中。 60MB，不知道怎么执行查询结果需要4GB以上的内存，大概高了两个数量级。

Answer 1

尝试在子查询中拆分计算，例如:

SELECT
    ?person
    (MAX(?sent_emails_) AS ?sent_emails_)
    (MAX(?received_emails_ AS ?received_emails_)
    (MAX(?receivedInCC_emails_ AS ?receivedInCC_emails_)
    (MAX(?commits_) AS ?commits)
WHERE {
  { 
   SELECT 
          ?person 
          (COUNT(DISTINCT ?sent_email) AS ?sent_emails_) 
          (0 AS ?received_emails_) 
          (0 AS ?commits_) 
   WHERE {
    ?sent_email rdf:type email:Email.
    ?sent_email email:sender ?person.
    ?person rdf:type foaf:Person.
   } GROUP BY ?person 
  } union {
     (similar pattern for the others)
     ....
  }
}
GROUP BY ?person
ORDER BY DESC(?commits)

目标是:

避免在结果集中生成大量需要处理聚合的行

避免使用 OPTIONAL{} 模式，这也会影响性能