Multilevel indexes, also known as hierarchical indexes, are index structures used in relational database management systems (RDBMS) to efficiently access data. They organize the index in multiple levels, allowing for faster data retrieval by reducing the number of disk accesses required.

Here's an overview of how multilevel indexes work in an RDBMS:

1. Structure:

- Multilevel indexes consist of multiple levels or layers, where each level represents a subset of the index.
- The topmost level, also known as the root level, provides an entry point to access the lower levels.
- Each level, except the leaf level, contains index entries that point to the next level.
- The leaf level contains the actual index entries that point to the data records.

2. Index Hierarchy:

- The levels in a multilevel index form a hierarchical structure, where the root level is at the top and the leaf level is at the bottom.
- The root level contains a small number of index entries that partition the data into ranges or divisions.
- Each entry in the root level points to a specific range of values or a lower-level index.
- The lower-level indexes, in turn, further divide the data into more specific ranges or divisions.
- This hierarchical structure allows for efficient traversal of the index to locate the desired data.

3. Index Traversal:

- To search for a specific value or key, the search starts at the root level.
- The search key is compared with the index entries at each level to determine the appropriate branch or lower-level index.
- The search proceeds to the corresponding lower level, and the process continues until reaching the leaf level.
- At the leaf level, the index entry directly points to the data record(s) associated with the search key.

4. Performance Benefits:

- Multilevel indexes reduce the number of disk accesses required for data retrieval compared to single-level indexes.
- By dividing the index into levels, the search space is effectively partitioned, allowing for faster search operations.
- The number of levels in a multilevel index depends on the data size, index structure, and system configuration.
- With fewer levels to traverse, the time required to locate the desired data is significantly reduced.

Multilevel indexes offer improved query performance by minimizing disk accesses and efficiently narrowing down the search space. However, they require additional storage space to maintain the index structure. The choice of the number of levels and the design of the index hierarchy depend on factors such as the size of the database, the access patterns, and the performance requirements of the application.

It's worth noting that multilevel indexes are just one of several index structures available in RDBMS, and the selection of the appropriate index structure depends on the specific requirements and characteristics of the database application.

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