Snapshot Standby in OCI RAC: When Listener Registration Breaks Quietly

 It started like a routine DR testing request.

Application team wanted a safe environment on the standby side to run validations without impacting primary. The standby database was part of a 2-node Oracle RAC setup running on Oracle Cloud Infrastructure Database Base Service. By default, the standby was in mount mode, so the usual debate began - switchover or snapshot standby.

Switchover was immediately ruled out. Too heavy for a simple test cycle, too risky for timing constraints. Snapshot standby looked like the obvious choice.

Why Oracle Execution Plans Suddenly Change

 A query that normally runs in under a second suddenly starts taking 20 seconds after midnight maintenance. CPU spikes. Application teams report slowness. Storage looks healthy. Nothing obvious changed in the SQL itself.

In many Oracle production incidents, the real culprit is not the query. It is the execution plan.

Oracle's optimizer constantly evaluates how SQL should execute based on statistics, metadata, bind values, memory conditions, and optimizer behaviour. The same SQL can behave very differently depending on what Oracle believes is the cheapest execution path at parse time.