Mis juhtub Xmin ja Xmax-iga uuendamisel?

In PostgreSQL, during an UPDATE operation, not only is the row modified, but in fact a new version of the row is created (MVCC — multi-version concurrency control).

• Xmin is the transaction ID that created (inserted) the current version of the row.
• Xmax is the transaction ID that marked the row as deleted (for example, during an UPDATE or DELETE).

During an UPDATE:

1. The old version of the row receives in the Xmax field the ID of the current transaction performing the UPDATE — this means that this version is now considered deleted for new transactions.
2. A new version of the row is created with a new Xmin — the ID of the current transaction, and an empty Xmax.

Thus, an UPDATE is essentially a DELETE of the old version + an INSERT of the new version, which ensures transaction isolation and data consistency.