Another reason to not use a DBMS for a search engine is that typical implementations of transaction-oriented SQL databases are a terrible fit for the performance requirements of a search engine. For example, search engines don't need concurrent writes or ACID transactions, or SQL-like query language; search engines want to optimize for large-scale updates, not small, random writes; typical DBMS index structures (btree) don't work well for search engine indices.Eric Brewer has an interesting paper that lays out an architecture for a search engine that is consistent with DBMS design principles, but differs significantly in the implementation details:
http://www.cs.berkeley.edu/~brewer/papers/SearchDB.pdf
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