QMON Process is not Defragmenting Queue Table when tablespace is set up with ASSM

Database Version: Oracle DB 19.3

Problem Description

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We are using a standard Oracle AQ (non-sharded) to manage a high throughput message stream. When the test starts the enqueue and dequeue process keeps up with the incoming messages (3k transactions per second). Gradually the enqueue and in particular the dequeue rates drop. Looking at the segments associated with this Queue, I can see that the Table Segment has become fragmented, typically after 10min the Queue table holds 700 records but is in 2.5GB in size. The tablespace for the Queue table is set up as an ASSM (Auto Segment Space Management). I found the note