does the order of joins in a big search query affect the response time in sqlserver

Question

I have a query that has joins between 15 tables or even more. I need to optimize the response time. I created some index columns, changed some conditions from NOT IN to NOT EXISTS, but I found myself wondering about this.

Does the order of these joins affect the response time?

Answer 1

The order of JOIN s definitely does affect performance, as well as the type of join. INNER JOIN s, generally, will yield quicker results than RIGHT or LEFT OUTER JOIN s due to the selectivity of the join.

SQL Server also tries its best to optimize every query, but at 15 joins it may have a hard time. Consider breaking the statement up into smaller junks of fewer joins. A strategy to resolve things like this that I've used in the past is to create a temporary table to store the results, then INSERT into it and UPDATE it accordingly through several different statements, with the 15 tables being spread out into the appropriate insert/update spots.

Answer 2

The order of the joins definitely matters. The question is, can you change the order by rewriting the SQL? The query optimizer doesn't necessarily care what order you write the joins in, depending on the type of join. The query optimizer does its best to find the most efficient execution plan based on the SQL you've written. However, it is no where close to perfect. If you notice after looking at the execution plan that it could be done more efficiently another way you can trick it into doing it your way and see if it helps.

One way to trick it is to use temp tables to pare down the result set before joining to large tables. This will allow less records to be selected which will reduce I/O.

Another way, as demonstrated by Adam Machanic , is to use a top in the select clause with an order by.