-> Application team advised that Read-Only queries are executing indefinitely on secondary database that is part of Always On Availability group.
-> I tried below query to check for blocking and could see that a system session is blocking the select queries,
select * from sysprocesses where blocked<>0
-> I tried a simple select query as below on the secondary server and had the same symptoms where in the select query was running indefinitely,
select top 1 * from [Table]
-> My session was 73 and I tried below query specifically to check what it is waiting on,
select spid,blocked,waittype, waittime,lastwaittype,dbid,waitresource from sysprocesses where blocked<>0 go select spid,blocked,waittype, waittime,lastwaittype,dbid,waitresource from sysprocesses where spid=73
-> Select queries are blocked by Session 84. Checked what is session id 84,
-> Seems like a system session. Tried below query,
select session_id, wait_type,db_name(database_id),command from sys.dm_exec_requests where session_id = 84
-> Session ID 84 is a system thread that is leveraged for REDO operation on secondary also known as “DB STARTUP”.
-> Checked on the Primary replica to see if there are any huge write operations on object [Table].
-> I could see an alter index command on the same object [Table] on primary replica.
-> The redo thread on Secondary replica acquires schema modification (Sch-M) locks on the secondary database to make any DDL changes that are performed on primary replica. On the secondary replica, the read-only queries acquire schema stability (Sch-S) locks to process the query. In our case the select queries were not able to acquire schema stability (Sch-S) locks as the REDO thread has already acquired schema modification (Sch-M) locks and this is the reason why we were experiencing blocking.
-> In the meantime, Alter Index command completed on the primary replica and we were able to execute select queries without any issues after that.
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