Executing queries against the replica

ERROR: canceling statement due to conflict with recovery

Detail: User query might have needed to see row versions that must be removed

The preferred solution is to allow the replica to pause applying the received WAL logs while a query executes:

# /etc/postgresql/10/main/postgresql.conf on a slave
max_standby_archive_delay = 900s
max_standby_streaming_delay = 900s

This is a cumulative delay - if multiple queries a running, then the last one might get terminated after just a second because the total time limit has been reached.