<p>Failed preconditions aren't an issue with delete if not used. With splitting, yes.</p>

<p>As for impact, it's an additional join against way_nodes to backfill and at that point it's basically not different than a <code>/full</code> call in SQL required, with obviously less bandwidth.</p>

<p>To better understand the queries, it's probably easiest to look at cgimap, which has clearer queries.</p>

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