That could also very well be about CouchDB which implemented indexes/views as MapReduce functions.
Back in the day we had a CouchDB MapReduce view (on Cloudant) which took a full month to rebuild (while an angry customer was waiting). The I/O inefficiency was absolutely off the charts.
alt Hacker News
Back in the day we had a CouchDB MapReduce view (on Cloudant) which took a full month to rebuild (while an angry customer was waiting). The I/O inefficiency was absolutely off the charts.