Re: Slow Count-Distinct Query

Depending on how often you need to run that query and how important it is to you, if you are willing to accept a performance hit on INSERT/UPDATE/DELETE of the "participants" table, you could create a summary table containing just the count of unique email addresses or the list of unique email addresses populated via trigger on INSERT/UPDATE/DELETE of the  participants table. Another option is try out the new Materialized views (http://www.postgresql.org/docs/current/static/sql-creatematerializedview.html) available in 9.3.