Oracle index is a database structure that optimizes query performance. It points to data by creating pointers to specific columns in a table. Common types include B-Tree index, hash index, and bitmap index. Indexes speed up queries by narrowing the search range, but you need to consider the query type, table size and data distribution to select the appropriate index. As the data is updated, the index needs to be maintained, and Oracle will automatically maintain most of the indexes, but the bitmap index may require manual reconstruction. Indexes have an impact on performance, including space overhead, update overhead, and affect query plans.
Oracle Index: User Guide
Index Overview
Oracle index is a database structure that optimizes query performance by creating pointers to specific columns or column groups in a table.
Type of index
Oracle supports multiple types of indexes, the most common ones are:
- B-Tree index: The most commonly used index type, suitable for most queries.
- Hash index: used for equal value comparison, which is more suitable for large tables than B-Tree index.
- Bitmap index: used for range query, which can improve the query speed of subsets of data.
Creation of indexes
To create an index, use the following syntax:
<code class="sql">CREATE INDEX ON ()</code>
For example:
<code class="sql">CREATE INDEX idx_employees_name ON employees (last_name, first_name)</code>
Use of indexes
Indexes improve performance by narrowing the search range during querying. When a query contains columns with an index, Oracle will use the index to quickly find matching data.
Select the correct index
Choosing the correct index is very important for optimizing query performance. Consider the following factors:
- Query type: B-Tree index is suitable for range query, while hash index is suitable for equal value comparison.
- Table size: Hash index is more efficient for large tables.
- Data distribution: Hash indexes can provide better performance if the data is distributed evenly.
Maintenance of indexes
As the data is updated, the index needs to be maintained to maintain its validity. Oracle automatically maintains most indexes, but for some types of indexes, such as bitmap indexes, it may require manual reconstruction.
Performance impact of indexes
Although indexes can improve query performance, they can also have some impact on database performance:
- Space overhead: The index requires additional storage space.
- Update overhead: When updating indexed tables, you need to update the index, which will reduce the performance of insertion and update operations.
- Query Plan: Oracle Query Optimizer will select the best query plan based on the index. Choosing the wrong index may cause query performance to degrade.
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