Multi-stream: the N+1 lookup hiding inside a map step

Every row doing its own lookup against the other list is the N+1 problem wearing a stream. Counting the calls is the only way to see it - the code reads identically either way.

• Multi-stream: a record with a mutable component is a broken Map key
• Multi-stream: an unmodifiable view is not a snapshot of the feed
• Multi-stream: toMap's merge function runs only on a collision
• Multi-stream: nothing runs until the terminal operation
• Multi-stream: indexing the other side costs memory a sorted merge does not
• Multi-stream: UNORDERED is a promise you make, not one you receive

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Published 2026-09-20

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