tailieunhanh - Oracle 8 Database Administration volume 2 instruction guide phần 4

Tham khảo tài liệu 'oracle 8 database administration volume 2 instruction guide phần 4', công nghệ thông tin, cơ sở dữ liệu phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | Retrieving Information About Clusters Note If you need to use a join on three or more data dictionary views it may be preferable to use temporary tables based on the views and then use the tables in a join. Oracle8 Database Administration 15-23 Lesson 15 Using Clusters and Index-Organized Tables Index-Organized Tables Index-Organized Tables Indexed access on table ROWID Accessing index-organized table i 1Non-key columns I Key column Row header 15-15 Copyright Oracle Corporation 1998. All rights reserved. ORACLE Storage Structure An index-organized table keeps all data for a table within a B-tree structure. The B-tree index which is based on the primary key of the table is organized like an index. The leaf blocks in this structure contain the nonkey columns instead of the ROWID as the second element of the index leaf entry. Therefore the use of an index-organized table eliminates the need to have two separate segments the table and the index. Accessing an Index-Organized Table Index access to a regular table requires that one or more index blocks are read to retrieve the ROWID and the I O on the table based on the ROWID. In contrast reading an index-organized table requires only index block reads because the whole row is available in the leaf node. An index-organized table can be accessed using either the primary key or a combination of columns that constitute the leading part of the primary key. Changes to the table data such as adding new rows updating rows or deleting rows result only in updating the index. 15-24 Oracle8 Database Administration Index-Organized Tables Index-Organized Tables Compared with Regular Tables Regular Table Index-Organized Table Unique identifier ROWID Identified by primary key ROWID implicit Supports several indexes No ROWID No secondary indexes FTS returns rows in no specific order Full index scans return rows in PK order Unique constraints allowed No support for unique constraints Distribution replication and partitioning supported .

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