Hi, i am not aware if whether or not Discourse uses sphinx for its search function http://sphinxsearch.com but it is very powerful and light. I know on my forum which uses Vbulletin, i had to integrate the sphynix search which works really well than the Mysql default search which comes with VB =) b/c we have around 5 million posts and 1.7+ million members. Vb Mysql default search was not the right approach for big to growing communities.
Does Discourse uses it for their search function? If not, i would really recommend it and if there is any plans for it?