ASA SQL User's Guide
Query Optimization and Execution
Indexes
Types of index
Compressed B-tree indexes store a compressed form of each indexed value in the index's internal nodes. To do this, compressed B-tree indexes store the values using Patricia tries, an optimized form of a trie data structure that is augmented with a skip-count to compress its representation. As a result, compressed B-tree indexes offer substantial improvements over hash indexes when the overall data length is reasonably large. More significantly, the compaction algorithm efficiently handles index values that are identical (or nearly so), so common substrings within the indexed values have negligible impact on storage requirements and performance. Compressed B-tree indexes are chosen automatically if the sum of the declared width of the indexed columns is between 10 bytes and one-eighth of the database page size to a maximum of 256 bytes.