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ProcArray

Short Description

A process is waiting to access the shared per-process data structures (typically, to get a snapshot or report a session's transaction ID).

Detailed Description​

In PostgreSQL, the ProcArray is a critical data structure used internally by the database to track information about active backend processes (transactions). It is used for managing and coordinating concurrency control and visibility, particularly for features like:

  1. Transaction Visibility:
    1. Determines which transactions are active, committed, or rolled back to help establish what data is visible to each transaction. This is crucial for PostgreSQL’s MVCC (Multi-Version Concurrency Control) mechanism.
  2. Vacuum and Cleanup:
    1. The vacuum process uses the ProcArray to identify which tuples can be cleaned up (i.e., deleted) based on the active transactions.
  3. Deadlock Detection:
    1. Tracks transactions to help detect and resolve deadlocks.
  4. Snapshot Management:
    1. Enables the creation of consistent snapshots of the database by tracking transaction states.

The ProcArray is maintained in shared memory and is accessed by all PostgreSQL backend processes. While it is essential for PostgreSQL’s operation, its efficiency is critical because excessive contention or access to this structure can affect performance in high-concurrency environments.

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