使用count distinct在postgres中查询速度慢

My goal is to create a query that will return a count of unique customers who have purchased in a 365 day window. I created the query below in postgres and and the resulting queries are very slow. My table is 812,024 rows of just order dates and customer id's. When I remove the distinct statement, I can get the query to return a result in approx 60 seconds, with it, I have yet to finish. I created an index on (order_date, id). I am a complete newb to SQL, this is really the first time i have ever done anything with it, and after trying to find a solution to this problem all day, I could not find anything that I could get to work, even though I have seen a lot about the slow performace of distinct. My goal is to create a query that will return a