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Note that the second query generates a table with compound/nested columns qty and price.
I don’t know why the second .Q.gc call takes significantly longer, given that the memory usage of the two queries are comparable (although the second takes slightly more). But I don’t think it’s relevant to fragmented memory; after all, the space of whole temporary result is released rather than part of it, as is the case from the .Q.gc doc. My suspect is that it just takes longer to garbage collection when it involves nested columns (aka lists of vectors) than simple columns (aka vectors).