Oracle Database / Sessions, Locks & Transactions
Find blocking sessions across RAC instances
A local session view can identify a blocker on another RAC instance without showing that session's full details. Preserve instance identity in every lookup.
Environment and inputs: Oracle Database 19c baseline · SQL*Plus-compatible SQL unless RMAN, DGMGRL, ADRCI or a host shell is specified. Check release, patch level, edition and container first.
Start with this diagnostic
Use the stated SQL, RMAN, DGMGRL or host shell and replace sample names with approved targets. DBA and V$ views need authorized catalog access in the correct container. Do not grant broad privileges for an example.
SELECT inst_id, sid, serial#, username, event, blocking_instance, blocking_session FROM gv$session WHERE blocking_session IS NOT NULL;Read the result
INST_ID identifies the waiting session's instance; BLOCKING_INSTANCE identifies the reported blocker instance. SID alone is not cluster-wide unique.
Resolve and verify
Look up the blocker using both instance and SID, verify its serial number and transaction, then coordinate with its owner. Compare contention across services.
Operational precautions
Do not issue a session action using a SID copied from another instance without verifying the correct target.