具有多個聚合的SPARQL查詢超出內存限制

Question

我正嘗試使用SPARQL從三重存儲中生成一些用戶統計信息。請參閱下面的查詢。這怎麼能改進？我在這裏做什麼壞事？爲什麼這消耗這麼多的記憶？ （請參閱本文結尾處的背景故事）

我更喜歡在三重商店內進行聚合和連接。拆分查詢意味着我必須在數據庫之外「手動」加入結果，從而失去了三重商店的效率和優化。沒有理由不需要重新發明輪子。

查詢

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

的問題是，我得到的錯誤 「已達到QUERY內存限制」 在AllegroGraph（請參閱我的相關SO question）。由於存儲庫只包含大約200k的三元組，容易放入ca的（ntriples）輸入文件中。 60 MB，我想知道如何執行查詢結果需要超過4 GB的RAM，大約高兩個數量級。

Answer 1

嘗試拆分計算子查詢，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) 

目標是：

• 避免行的需要是結果集數量龐大的一代進行聚合處理
• 避免使用可選的{}模式，這也會影響性能