Bây giờ, kể từ khi tách nút gốc cũ, chúng ta cần phải tạo ra một nút gốc mới để giữ mục phân biệt hai trang chỉ số phân chia. Các cây sau khi hoàn thành việc chèn * 8 nhập được thể hiện trong hình 9,14. Một biến thể của các thuật toán chèn cố gắng để mục phân phối lại của một nút N với anh chị em trước khi chia tách nút, điều này cải thiện suất phòng bình quân. | Evaluation of Relational Operators 353 have the same value in the join attribute there is a repeated pattern of access on the inner relation we can maximize the repetition by sorting the outer relation on the join attributes. POINTS TO REVIEW Queries are composed of a few basic operators whose implementation impacts performance. All queries need to retrieve tuples from one or more input relations. The alternative ways of retrieving tuples from a relation are called access paths. An index matches selection conditions in a query if the index can be used to only retrieve tuples that satisfy the selection conditions. The selectivity of an access path with respect to a query is the total number of pages retrieved using the access path for this query. Section Consider a simple selection query of the form ơR attr op vaiue R . If there is no index and the file is not sorted the only access path is a file scan. If there is no index but the file is sorted a binary search can find the first occurrence of a tuple in the query. If a B tree index matches the selection condition the selectivity depends on whether the index is clustered or unclustered and the number of result tuples. Hash indexes can be used only for equality selections. Section General selection conditions can be expressed in conjunctive normal form where each conjunct consists of one or more terms. Conjuncts that contain V are called disjunctive. A more complicated rule can be used to determine whether a general selection condition matches an index. There are several implementation options for general selections. Section The projection operation can be implemented by sorting and duplicate elimination during the sorting step. Another hash-based implementation first partitions the file according to a hash function on the output attributes. Two tuples that belong to different partitions are guaranteed not to be duplicates because they have different hash values. In a subsequent step each .
