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B*-tree indexes
The B*-tree index is the default index used in Oracle. It gets its name from its resemblance
to an inverted tree, as shown in Figure 4-1.
The B*-tree index is composed of one or more levels of branch blocks and a single
level of leaf blocks. The branch blocks contain information about the range of values
contained in the next level of branch blocks. The number of branch levels between
the root and leaf blocks is called the depth of the index. The leaf blocks contain the
actual index values and the ROWID for the associated row.
Basic Data Structures | 93
The B*-tree index structure doesn??™t contain many blocks at the higher levels of
branch blocks, so it takes relatively few I/O operations to read quite far down the B*-
tree index structure. All leaf blocks are at the same depth in the index, so all
retrievals require essentially the same amount of I/O to get to the index entry, which
evens out the performance of the index.
Oracle allows youto create index organized tables (IOTs), in which the leaf blocks
store the entire row of data rather than only the ROWID that points to the associated
row.
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