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Data Blocks and Operating System Blocks

Oracle Data block consisting of three OS blocks

  1. Oracle data block: Can be based on one or more operating system blocks, but is the same for the entire database.
  2. Operating system blocks: The operating system block is the smallest unit of operating system I/O.

Index Segments

Index segments are objects designed to increase the speed of data accessed in a table. Using a library analogy, think about how a card catalog works.
Suppose we want to find where the book, "Professional Oracle" is located in the library. We can wander around the library and hope to stumble upon this book or we can go to the card catalog to find its location. The card catalog is an index of all the books available in the library including their locations and we can jump to a listing of library books starting with the letter "O" to quickly find our book. Once our book has been located with the card catalog, we can look up the unique decimal number of the book that provides a path to the physical location of the book we want.
Back to index segments, an index contains the value for one or more columns in a table. Furthermore, a ROWID is used to identify values located in any corresponding columns. What this means is that Oracle can access data directly from a table by first looking up the ROWID in an index. Indexes are not always the most efficient way for retrieving data and this particularly holds true for smaller tables that require less than 100 data blocks. Oracle 8i provides many types of index, of which the most commonly used are:
  1. B*-Tree,
  2. reverse key,
  3. bitmapped, and
  4. function-based.

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