Supermarkets existed before barcodes, but how did the checkout function? They are so ubiquitous now that I have a hard time visualizing how a busy supermarket checkout could calculate prices at a reasonable speed. Even if the price was directly on the product, how were price changes handled? I assume supermarkets had pretty advanced cash registers with useful functions before barcodes, but with the amount of products available in one store, was memorization crucial to the job? Was a supermarket cashier considered a professional job at some point in history?
Prior to barcodes, the prices were marked on stickers and put on the item itself. This did require cashiers to manually enter every price. When prices were changed, workers would manually redo all of those stickers; ideally, the change would be something like a three for one or two for one sale, so cashiers would only have to enter the price of one item while checking out three of them. Obviously it was a lot of work to physically change the prices on everything. Another way stores coped with this was to simply not stock as much as they do today on the shelves. If you put 10 cans on a shelf, for example, and the price changes, you have to change 10 cans. That's time-consuming but not as time-consuming as changing 20 cans. Stores could also put some items in the front of the display with prices, and then, behind them, put unopened packs of multiples, allowing store workers to restock easily when the front items were gone instead of having to go into the back.
Yes, workers often had to remember prices in ways they don't today.
Before supermarkets, every store would have its own policy for pricing, its own sales, and its own displays. Things were much less uniform. They would also do sales on consistent days, which meant that they could reliably schedule more labor if needed on those days.
Now, prior to even having the nice easy sticker machines, you were generally dealing with even smaller stores, sometimes general stores, but often local groceries run by local families, or local chains. With much less merchandise, they did not necessarily need to mark every single item with a price. Cashiers could simply remember it or refer to a list at the register, or, if they didn't have a register, at the counter where customers paid. In some stores, some of the goods would be behind the counter, and the clerk would pull them down for you upon request, cut length of fabrics upon request, measure out salt or sugar upon request, etc. This is time intensive, but people were also more self-reliant, especially in the times of country and general stores. They grew vegetables at home, they raised chicken, they slaughtered and cured their own meat, made cheese, they may have sewn their own clothes, and they bought staples in bulk if they didn't live near stores.