The terms “PCTFREE“, “PCTUSED“, and “Row Migration” are closely associated with the Oracle RDBMS. However, PostgreSQL works differently. Although PostgreSQL lacks direct equivalents, it has similar concepts worth exploring. Let’s delve into these terms as they relate to Oracle, then compare how PostgreSQL manages similar scenarios.
1. Oracle’s PCTFREE, PCTUSED, and Row Migration:
- PCTFREE: Oracle uses PCTFREE to reserve a percentage of a block for future updates. This prevents row migrations by ensuring space for modified rows.
- PCTUSED: This parameter defines the minimum space a block must use before allowing new row inserts. When a block’s usage drops below PCTUSED, Oracle adds it to the free list for new inserts.
- Row Migration: In Oracle, if an updated row’s new size doesn’t fit in its original block (even after considering PCTFREE), it is moved to a new block, leaving behind a pointer in the original block. This phenomenon is called row migration and can affect performance due to increased I/O operations.
2. PostgreSQL and Analogous Behaviors:
Unlike Oracle, PostgreSQL does not use the concepts of PCTFREE and PCTUSED. However, it has its own mechanisms to handle data updates and storage:
- MVCC and Tuple Versions: PostgreSQL uses Multi-Version Concurrency Control (MVCC). When a row (tuple) is updated, PostgreSQL doesn’t overwrite the original but instead writes a new version of the tuple. The old version is marked as obsolete but remains in place until vacuumed.
- Tuple “Bloating”: Over time, as rows are updated, multiple versions of a tuple can exist concurrently, leading to what’s often termed as “bloat.” This can increase space usage and reduce query performance.
- Vacuuming: To manage the outdated tuples and reclaim space, PostgreSQL has a VACUUM process. The autovacuum daemon runs in the background and removes dead tuples, making space available for reuse. In some ways, this can be thought of as an ongoing space management and optimization process, a bit like how PCTFREE and PCTUSED work in Oracle, but with different mechanics.
- Tuple “Migration”: While PostgreSQL doesn’t have row migration in the same way Oracle does, there’s a related concept called “tuple chaining.” If a row is updated frequently and has TOAST-able columns (large values that get stored out-of-line), the out-of-line values might move around, creating a chain of pointers. This isn’t exactly the same as Oracle’s row migration but can have some similar performance implications.
Conclusion:
While PostgreSQL doesn’t have the same PCTFREE, PCTUSED, and row migration mechanisms as Oracle, it has its own unique ways of managing storage, updates, and space reclamation. Understanding these mechanisms is essential for database administrators migrating from Oracle to PostgreSQL or those looking to optimize PostgreSQL’s performance.
