Determine which query prevents applying WALs on standby

[Andrey Zhidenkov <andrey.zhidenkov@xxxxxxxxx>]

Is there a way to detect which query (or queries) on hot standby holds
streaming replication? Looked into the sources and found out that
function ResolveRecoveryConflictWithVirtualXIDs do that job but have
no idea how to do that with SQL.

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With best regards, Andrey Zhidenkov.