Local grocers here (UK) have security guards (not armed!) I once saw one chasing a shoplifter; the shoplifter ran faster. But most of the time they're just standing around, diddling their mobile device.

I don't think their purpose is to deter shoplifters; it's really to deter assaults on staff. Shoplifting is just part of the cost of doing business as a grocers.

I once saw a couple of armed cops in my local grocers; UK cops are generally not armed, but these guys were carrying big old pistols as they wandered through the aisles looking for sandwiches. I'm not keen on sharing supermarket aisles with gunmen, even if they're in uniform; so I complained. Armed cops should disarm (e.g. leave their guns in the cop-car) if all they're doing is shopping for lunch. I was treated with scorn.