IO:ReorderBufferWrite

Waiting for a write during reorder buffer management. The ReorderBufferWrite wait event occurs when a PostgreSQL walsender process is waiting to write decoded transaction data from memory to a temporary file on disk. This event is specific to Logical Decoding.

What is the Reorder Buffer?

When logical replication is active, PostgreSQL doesn't send WAL records to the subscriber as they appear in the log. Instead, it must "reorder" them so that all changes for a specific transaction are grouped together and sent only after that transaction commits. To do this, the walsender process maintains a Reorder Buffer in memory.
  1. It reads WAL records.
  2. It assembles the changes into a per-transaction list in memory.
  3. When a transaction commits, it streams the whole list to the subscriber.