## 摘要

Dictionaries remain the most well studied class of data structures. A dictionary supports insertions, deletions, membership queries, and usually successor, predecessor, and extract-min. In a RAM, all such operations take O(logN) time on N elements. Dictionaries are often cross-referenced as follows. Con- sider a set of tuples fhai; bi; ci : : :ig. A database might in- clude more than one dictionary on such a set, for example, one indexed on the a's, another on the b's, and so on. Once again, in a RAM, inserting into a set of L cross-referenced dictionaries takes O(LlogN) time, as does deleting. The situation is more interesting in external memory. On a Disk Access Machine (DAM), B-trees achieve O(logB N) I/Os for insertions and deletions on a single dictionary and K-element range queries take optimal O(logB N + K=B) I/Os. These bounds are also achievable by a B-tree on cross-referenced dictionaries, with a slowdown of an L factor on insertion and deletions. In recent years, both the theory and practice of external- memory dictionaries has been revolutionized by write- optimization techniques. A dictionary is write optimized if it is close to a B-tree for query time while beating B- trees on insertions. The best (and optimal) dictionaries achieve a substantially improved insertion and deletion cost of, amortized I/Os on a single dictionary while maintaining optimal O(log1+B" N +K=B)- I/O range queries. Although write optimization still helps for insertions into cross-referenced dictionaries, its value for deletions would seem to be greatly reduced. A deletion into a cross- referenced dictionary only specifies a key a. It seems to be necessary to look up the associated values b; c : : : in order to delete them from the other dictionaries. This takes (logB N) I/Os, well above the per-dictionary write- optimization budget of O( So the total deletion cost is O(logB N + L In short, for deletions, write optimization offers an ad- vantage over B-trees in that L multiplies a lower order term, but when L = 2, write optimization seems to offer no asymp- totic advantage over B-trees. That is, no known query- optimal solution for pairs of cross-referenced dictionaries seem to beat B-trees for deletions. In this paper, we show a lower bound establishing that a pair of cross-referenced dictionaries that are optimal for range queries and that supports deletions cannot match the write optimization bound available to insert-only dictionar- ies. This result thus establishes a limit to the applicability of write-optimization techniques on which many new databases and file systems are based.

原文 | English |
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主出版物標題 | 28th Annual ACM-SIAM Symposium on Discrete Algorithms, SODA 2017 |

編輯 | Philip N. Klein |

發行者 | Association for Computing Machinery |

頁面 | 1523-1532 |

頁數 | 10 |

ISBN（電子） | 9781611974782 |

DOIs | |

出版狀態 | Published - 1 一月 2017 |

事件 | 28th Annual ACM-SIAM Symposium on Discrete Algorithms, SODA 2017 - Barcelona, Spain 持續時間: 16 一月 2017 → 19 一月 2017 |

### 出版系列

名字 | Proceedings of the Annual ACM-SIAM Symposium on Discrete Algorithms |
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### Conference

Conference | 28th Annual ACM-SIAM Symposium on Discrete Algorithms, SODA 2017 |
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國家 | Spain |

城市 | Barcelona |

期間 | 16/01/17 → 19/01/17 |