Index Space

Enhanced Definition

An Index Space is a physical storage structure within IBM Db2 for z/OS that holds the data pages for a single Db2 index. It is the underlying storage mechanism that allows Db2 to efficiently store and manage the index entries, which consist of key values and row identifiers (RIDs) pointing to data rows in a table space.

Key Characteristics

- Dedicated Storage: Each Db2 index (whether unique or non-unique, clustering or non-clustering) is physically stored in its own dedicated Index Space.
- Page-Oriented: Like table spaces, index spaces are composed of fixed-size pages (typically 4KB, 8KB, 16KB, or 32KB) that store index entries.
- Managed by Db2: Db2 manages the allocation, deallocation, and organization of pages within an Index Space, including the B-tree structure of the index.
- Partitioning: An Index Space can be non-partitioned (for non-partitioned indexes or partitioned indexes where the index is not partitioned) or partitioned (for partitioned indexes, where each partition of the index resides in a separate partition of the Index Space).
- Storage Parameters: Defined with storage parameters such as PRIQTY (primary space allocation), SECQTY (secondary space allocation), ERASE (erase data on delete), and COMPRESS (data compression).
- B-tree Structure: Internally, an Index Space stores the index entries in a B-tree structure, consisting of root, non-leaf, and leaf pages, facilitating rapid data lookup.

Use Cases

- Accelerating Data Retrieval: The primary use is to provide fast access paths to data rows in a table space by allowing Db2 to quickly locate rows based on key values, significantly improving query performance.
- Enforcing Uniqueness: Unique indexes, stored in their respective Index Spaces, are crucial for enforcing unique constraints (e.g., primary keys) on table columns, ensuring data integrity.
- Ordering Data: Clustering indexes, stored in their Index Spaces, help maintain the physical order of data rows in a table space, which can optimize range scans and sequential access.
- Supporting Foreign Keys: Indexes on foreign key columns, while not strictly required by Db2, are a best practice to improve the performance of referential integrity checks and cascading operations.
- Optimizing Joins: Indexes on join columns can drastically improve the performance of join operations between tables by providing efficient access to matching rows.

Related Concepts

An Index Space is intrinsically linked to a Db2 Index, serving as its physical manifestation. The index itself is a logical object defined on one or more columns of a Db2 Table, whose data resides in a Table Space. When a query accesses data via an index, Db2 reads pages from the Index Space into a Buffer Pool to find the Row ID (RID), then uses the RID to access the corresponding data page in the Table Space, also via a Buffer Pool. The metadata for Index Spaces, including their definitions and physical characteristics, is stored in the Db2 Catalog. Index Spaces can be managed by Storage Groups (STOGROUPs), which define the physical DASD volumes where the data sets for the Index Space will be allocated.

Best Practices:

Regular Reorganization (REORG): Periodically reorganize Index Spaces to eliminate fragmentation, reclaim free space, and rebuild the B-tree structure, which improves access efficiency and query performance.
Monitor Statistics: Regularly collect and analyze index statistics (RUNSTATS) to identify inefficient indexes, excessive fragmentation, or poor clustering, and use this data to guide REORG decisions or index redesign.
Appropriate FREEPAGE and PCTFREE: Set FREEPAGE and PCTFREE parameters carefully during index creation or REORG to allow for future inserts and updates without immediate page splitting, balancing update performance with storage utilization.
Strategic Placement: Place critical Index Spaces on faster storage devices (e.g., SSDs) to minimize I/O latency for frequently accessed indexes.
Compression (COMPRESS): Consider enabling compression for large, read-intensive Index Spaces to reduce storage consumption and potentially improve I/O performance, though it adds CPU overhead.
Backup and Recovery: Include Index Spaces in regular backup and recovery strategies, typically as part of the overall table space backup, to ensure data recoverability in case of failures.