Because Lucene wasn't good at near-realtime in 2009 or so, Twitter's original (acquired via summize) search was written in mysql. It might have even been a row

Because Lucene wasn't good at near-realtime in 2009 or so, Twitter's original (acquired via summize) search was written in mysql. It might have even been a row for every token, not quite sure. IIRC, when we moved to a highly-customized-Lucene-based system in 2011, we dropped the server count on the cluster from around 400 nodes to around 15.