Oh, sorry I read your comment quickly and hadn't realized you were talking about memory solely.

But that's because memory usage isn't the relevant bottleneck here -- it's disk IO. It's reading the entire table from disk, or only the entire column(s) if applicable.

That's not something you ever want to do in production. Not even if it's only a single column. That will destroy performance on any reasonably large table.

If a full-table scan of all columuns takes 180 seconds, then a "small fraction" to scan a single column might take 10 seconds, but queries running on a production database need to be measured in small numbers of milliseconds.