Seems to me this long thread could be summed up to a simple advice ... if you're running out of memory, check whether you really need to keep all the information in memory at all times or whether you could rather extract just a small subset of the data of each "object" and keep just that.

Let me give you an example. Suppose you are looking for duplicate files in a directory structure? Will you keep all the contents of all those files in memory? Of course not! You'll compute a hash and store just the hash (and maybe the file size) in memory and compare that. Is that rocket science? Do you need to talk about the O() notation? In your case it's actually even simpler, you do not compute some value out of the whole objects, you just extract a few of the attributes.

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