Connection scaling and GetSnapshotData (the PG14 dense-array snapshot rewrite)
Simple terms
You have heard that thousands of idle connections wreck snapshot performance for the whole cluster. Half of that is out of date. Every query starts by taking a snapshot of which transactions are still running. Before Postgres 14, building that snapshot got more expensive with every connection open, idle or not, so the folklore was earned. From 14 onward the cost tracks how many transactions are actually active. Idle connections are close to free for readers now. They still cost you, just not here.
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Production wisdom says 'idle-in-transaction connections destroy snapshot performance for the whole cluster' and 'thousands of idle connections make GetSnapshotData O(N) and crush you'. On a modern PostgreSQL (14+), which half of that is still true and which half is folklore? Walk me through what GetSnapshotData actually scans, what makes a snapshot cheap to take, and prove where the real wall is when you scale connections.
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