Slow SQL query, not sure how to optimize

Question

So I have to deal with a database that has no indexes (not my design, it frustrates the hell out of me). I'm running a query that takes approximately three seconds to return, and I need it to be faster.

Here are the relevant tables and columns:

gs_pass_data          au_entry            ground_station
  -gs_pass_data_id      -au_id              -ground_station_id
  -start_time           -gs_pass_data_id    -ground_station_name
  -end_time             -comments
  -ground_station_id

And my query is:

SELECT DISTINCT gs_pass_data_id,start_time,end_time,
  ground_station_name FROM gs_pass_data 
  JOIN ground_station
  ON gs_pass_data.ground_station_id =
  ground_station.ground_station_id 
  JOIN au_entry ON au_entry.gs_pass_data_id =
  gs_pass_data.gs_pass_data_id
WHERE (start_time BETWEEN @prevTime AND @nextTime) 
  AND comments = 'AU is identified.'
  ORDER BY start_time

I've tried using EXISTS instead of DISTINCT with no improvements. I've read everything I can about SQL optimization but I cannot seem to get this query down to a reasonable time (reasonable being < 0.5 seconds). Any ideas would be greatly appreciated.

Answer 1

The query can also be written without the distinct and with a group by instead. It'll probably make no difference at all though. Standard advice is the same as everyone else's. Add indexes, drop 'order by` so +1 to @Marc B

SELECT gs_pass_data_id,start_time,end_time,ground_station_name 
  FROM gs_pass_data 
  JOIN ground_station
    ON gs_pass_data.ground_station_id = ground_station.ground_station_id 
  JOIN au_entry 
    ON au_entry.gs_pass_data_id = gs_pass_data.gs_pass_data_id
 WHERE (start_time BETWEEN @prevTime AND @nextTime) 
   AND comments = 'AU is identified.'
 GROUP BY gs_pass_data_id,start_time,end_time,ground_station_name 
 ORDER BY start_time