📚 About transaction identifier (transaction_id) in PostgreSQL.

In PostgreSQL, the transaction_id, or transaction identifier, is a crucial mechanism for ensuring data integrity and isolating transactions within the database. It is used to manage and track changes made by each transaction.

Every transaction in PostgreSQL is assigned a unique transaction identifier (XID), which marks all the data rows altered or created during that transaction. This identifier allows the system to determine which versions of rows should be visible to various transactions based on their start time.

PostgreSQL utilizes the MVCC (Multi-Version Concurrency Control) model to enable concurrent data access without blocking reads during writes. MVCC ensures that each transaction sees a consistent state of the database, excluding inconsistent changes made by other transactions. The transaction identifier plays a key role in this process by helping to determine which data versions are current for each transaction.

Once transactions are completed, the old versions of data that are no longer needed by any active or future transactions must be removed or reprocessed. PostgreSQL employs a process called “vacuuming” to clean up such obsolete data, which helps to free up space and maintain system performance.

Since transaction identifiers are limited in size (32 bits), they can exhaust their range and “wrap around” after reaching their maximum value (~4 billion transactions). PostgreSQL addresses this problem with a mechanism known as “wraparound protection,” which prevents the overflow of transaction identifiers through periodic cleaning and reuse of old identifiers.

Transaction identifiers are a fundamental aspect of transaction management and data integrity maintenance in PostgreSQL, ensuring the reliable operation of the database in multi-user and multitasking environments.

This query checks how long it has been since the PostgreSQL database underwent a VACUUM operation, which prevents the transaction counter from overflowing.

SELECT datname, age(datfrozenxid) 
FROM pg_database 
ORDER BY 2 DESC 
LIMIT 10;

To do a vacuum (regular, not FULL) and collect DB statistics on all the objects of all the DBs of the PostgreSQL DB instance, run the following command:

vacuumdb -h <db_host> -p <db_port> -U <db_user> -j 4 -z -v -a

To do a vacuum (regular, not FULL) and collect DB statistics on all the objects of some specific database of the PostgreSQL DB instance, run the following command:

vacuumdb -h <db_host> -p <db_port> -U <db_user> -j 4 -z -v <db_name>