Comparing Storage Mechanisms: Oracle’s PCTFREE, PCTUSED, and Row Migration vs. PostgreSQL’s MVCC and Tuple Management

The terms “PCTFREE“, “PCTUSED“, and “Row Migration” are closely associated with the Oracle RDBMS. However, while PostgreSQL operates differently, there are analogous concepts and behaviors worth discussing. Let’s delve into these terms as they relate to Oracle and then look at how PostgreSQL manages similar scenarios.

1. Oracle’s PCTFREE, PCTUSED, and Row Migration:

  • PCTFREE: In Oracle, PCTFREE is a parameter that determines the percentage of a block to be reserved (kept free) for future updates to rows that are already stored in that block. This helps in minimizing row migrations.
  • PCTUSED: This parameter sets the minimum percentage of a block that can be used before the block becomes available for inserting new rows. Once a block’s used space goes below PCTUSED, it’s added to the list of blocks available 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  • Vacuuming: To manage the outdated tuples and reclaim space, PostgreSQL has a VACUUM process.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 TOASTable 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.

About Shiv Iyer 452 Articles
Open Source Database Systems Engineer with a deep understanding of Optimizer Internals, Performance Engineering, Scalability and Data SRE. Shiv currently is the Founder, Investor, Board Member and CEO of multiple Database Systems Infrastructure Operations companies in the Transaction Processing Computing and ColumnStores ecosystem. He is also a frequent speaker in open source software conferences globally.