JA:Open Data License/Indexing - Guideline
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If OSM data is indexed, for example by a web search engine, that data is clearly in a database derived from OSM. Web searches performed on this index would constitute Public Use. Does that mean the search index itself must be a Derivative Database and be released?
Web indexing in this manner probably constitutes "Fair Use", given that the pages are publicly available (not blocked by robots.txt, for example) and aren't being included in a database similar to OSM's.
Web pages are produced works, so indexing them doesn't create a Derivative Database. However, there's a fine line between indexing web pages and scraping them to reverse-engineer the database, so there need to be some guidelines restricting the use of produced works in this manner.