有效計算XML(extensible markup language)查詢就是要能快速地找到所有符合支脈片斷的地方，也就是XML文件樹狀結構的子圖。第一種方法是將查詢片斷視為查詢樹，而且對於查詢樹裡的每個節點賦予一個指標(cursor)和堆壘(stack)，指標是為了能循序處理，而堆疊是為了存放部份滿足的XML元素。即使事先對元素建立的索引能有效地跳過不滿足的地方，但這個方法只對查詢片斷只含有祖孫(ancestor-descendant)關係的邊(edge)有效。另一種方法是先將XML資料和查詢先一對一地轉換成字串。接著利用字串索引做子字串的比對，也能達到同等於跳躍的效果。這種方法在支脈查詢包含有順序的父子(parent-child)關係的邊和節點有選擇性判斷時執行效果較好。本篇論文提出的方法是將第一種方法的堆疊改用雙向佇列(Queue)暫存索引樹的內部內容，以能夠處理階層式的搜尋。我們的設計允許佇列間交互的檢查以去滿足支脈配對。在索引樹內部階層提早過濾可以加快處理的速度。對於樹葉內容有查詢元素完整的資訊，我們的設計可以去除任何子圖是違反父子關係的邊和在查詢節點裡選擇性的判斷。實驗的結果也證明我們的方法是優於第二種方法。

Efficient evaluation of XML queries entails fast finding all the occurrences of a twig pattern, a subgraph of tree-structured XML documents. One approach models the pattern as a query tree directly, and associates a cursor and a stack to each tree node for coordinated sequential access and partial match tally of all the XML elements. Even if pre-built indices on the elements can facilitate skipping mismatches, this approach works well only when the pattern purely involves ancestor-descendent edges. Another strategy first transforms XML data and query trees one-to-one into sequences. Then, string indices are used for subsequence matching to achieve equivalent skips in transform domain. This strategy performs better when the twig contains ordered parent-child edges and nodes with their own selection predicates. This thesis proposes to replace the stacks used in the first approach by simple pipes (double-ended queues) to queue too inner entries of index trees accessed by level-order traversal. Our design permits the cross-verification among queued entries subject to the twig pattern. Early filtration at the inner level of index trees is thus achievable to further expedite processing. Joint with the leaf entries having full specifications of query elements, our design can also suppress any subgraph that violates parent-child edges and selection predicates at query tree nodes. Intensive experimental results also demonstrate such superiority over the second strategy.

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