Question

Within a distributed system utilizing Multi-Version Concurrency Control (MVCC), what is the function of the &#x27;garbage collection&#x27; process for expired versions?

Accepted Answer

In Multi-Version Concurrency Control (MVCC), the system maintains multiple physical versions of a data row to allow concurrent transactions to read consistent snapshots of the database without blocking. When a row is updated or deleted, the old version remains in storage to satisfy ongoing transactions that started before the change. Garbage collection is the background process responsible for identifying and physically removing these obsolete row versions that are no longer needed by any active transaction. A version becomes expired when its transaction ID is older than the oldest active transaction in the system, meaning no future or current operation can ever reference that specific data. By reclaiming the storage space occupied by these expired versions, garbage collection prevents the database from growing indefinitely, which would otherwise lead to storage exhaustion and performance degradation. Without this process, the system would suffer from version bloat, forcing the database engine to scan through an ever-increasing number of irrelevant records during query execution. For example, if a row is updated ten times, MVCC creates ten versions; once all transactions that could possibly see the first nine versions are finished, garbage collection deletes those nine entries so only the latest, relevant state occupies physical space.

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Within a distributed system utilizing Multi-Version Concurrency Control (MVCC), what is the function of the 'garbage collection' process for expired versions?