More space is used by data, improving the ability of multiple users to access the same block of
data in memory. Doubling the size of the Oracle blocks has little effect on the block header;
therefore, a smaller percentage of space is used to store block header information.
To set the block size, modify the DB_BLOCK_SIZE initialization parameter prior to creating a
new database.
Using Index-Organized Tables
An index-organized table (IOT) is an index in which an entire row is stored, rather than just the
key values for the row. Rather than storing a RowID for the row, the primary key for the row is
treated as the row??™s logical identifier. Rows in IOTs do not have RowIDs.
Within the IOT, the rows are stored sorted by their primary key values. Thus, any range query
that is based on the primary key may benefit because the rows are stored near each other (see the
section ???Tuning SQL??? earlier in this chapter for the steps involved in ordering the data within
normal tables). Additionally, any equivalence query based on the primary key may benefit
because the table??™s data is all stored in the index. In the traditional table/index combination, an
index-based access requires an index access followed by a table access. In an IOT, only the IOT
is accessed; there is no companion index.
However, the performance gains from a single index access in place of a normal index/table
combination access may be minimal??”any index-based access should be fast.
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