Performance Tuning

One way to make a query run faster is to reduce the number of calculations that need to be performed. Some of the high-level things that will affect the number of calculations a given query will make include: Table size Joins Aggregations Query runtime is also dependent on some things that you can’t really control related to the database itself: Other users running queries concurrently on the database Database software and optimization (e.g., Postgres is optimized differently than Redshift)
The second thing you can do is to make joins less complicated, that is, reduce the number of rows that need to be evaluated. It is better to reduce table sizes before joining them. This can be done by creating subqueries and joining them to an outer query. Aggregating before joining will improve query speed; however, be sure that what you are doing is logically consistent. Accuracy is more important than run speed.
Adding the command EXPLAIN at the beginning of any query allows you to get a sense of how long it will take your query to run. This will output a Query Plan which outlines the execution order of the query. The query plan will attach a cost to the query and the higher the cost, the longer the runtime. EXPLAIN is most useful to identify and modify those steps that are expensive. Do this then run EXPLAIN again to see if the speed/cost has improved.