What Is a Prefix-Compressed Indexes in Oracle Database?
Stores repeated leading key prefixes once within index leaf blocks and stores suffixes separately, reducing duplicate key bytes.
How the Prefix-Compressed Indexes Algorithm Works
Stores repeated leading key prefixes once within index leaf blocks and stores suffixes separately, reducing duplicate key bytes.
When to Use Prefix-Compressed Indexes
Composite B-tree keys with repeated leading prefixes and suitable distributions.
How to Create a Oracle Database Prefix-Compressed Indexes
Review data quality, existing structures, feature support, build time, and deployment requirements before creating an index on a production table.
CREATE INDEX orders_status_customer_ix ON orders (status, customer_id) COMPRESS 1;Prefix-Compressed Indexes Query Example
This example demonstrates the matching query shape. Check the execution plan to confirm that the engine can use the index.
SELECT customer_id FROM orders WHERE status = 'OPEN';Queries Supported by Prefix-Compressed Indexes
Composite B-tree keys with repeated leading prefixes and suitable distributions. A similar-looking predicate may not use the same access path; operators, expressions, key order, and data type all matter.
Read Performance, Storage, and Write Costs
Benefit depends on prefix repetition and chosen prefix length; compression adds CPU work. Indexes reduce some search work but require storage and add maintenance to writes.
Prefix-Compressed Indexes Limitations and Common Mistakes
Compression is a storage trade-off and does not compress every index block equally.
Do not assume that an index being present means the optimizer chose it or that the query became selective.
How to Measure and Tune Prefix-Compressed Indexes
Measure segment size and CPU with the real key distribution; choose a prefix based on repeated values.
Compare representative plans and timings before and after, including concurrent writes and realistic data volumes.
Prefix-Compressed Indexes Questions
Will the database always use this index?
No. The optimizer weighs selectivity, estimated costs, statistics, cache state, and the amount of data returned.
What should I verify before production?
Confirm version support, query compatibility, write overhead, build/rollback procedures, and index behavior during backup and recovery.
Official Oracle Database Index Documentation
Check Oracle Database Concepts: Indexes and Index-Organized Tables for release-specific requirements and supported operators.