如果第一组内连接为空，则性能差异

Question

I have to use 5 tables of my database to obtain data and I wrote a SQL query like this:

SELECT *
FROM A 
INNER JOIN B ON A.id_b = B.id_b
INNER JOIN C ON B.id_c = C.id_c
INNER JOIN D ON D.id_d = C.id_d
INNER JOIN E ON E.id_e = D.id_e
WHERE A.column1 = somevalue

The columns I select doesn't matter for my explanation, but I require some columns of all tables to do the operation.

I'd like to know: If set A is empty according to the WHERE clause requirements, will it progress on the successive inner joins?

Answer 1

Yes and no and maybe. In all likelihood, the optimizer is going to choose an execution plan that starts with A , because you have filtering conditions in the WHERE clause.

Then, the subsequent JOIN s are going to be really fast, because the SQL engine doesn't have to do much work to JOIN an empty set to anything else.

That said, there is no guarantee the the optimizer will start with the first table. So, this is really a happenstance, but a reasonable expectation given your filtering conditions. Also, the subsequent JOIN s will be in the execution plan, but they will be fast, because each one will have one set that is empty.

Answer 2

You can try the following methods to optimize your query performance, however the actual result depends on many factors, and you have to test them to see whether they really work for your scenario or not, or which one works best:

• Use stored procedure and parametrized somevalue instead of raw query;
• Make sure there is an index on A.column1 and rebuild it regularly (clustered index is better if possible)
• Use "with (Forceseek)" on table a; (assuming that the index on column1 is already created)
• Use "OPTION (FAST 1)";
• Store the result of table A to a temporary table first, then use the temporary table for the following script. (This method should be the most effective one in most cases and table A is guaranteed to be executed first.)

Scripts will be like this:

SELECT * 
INTO #A 
FROM A
WHERE A.column1 = somevalue

SELECT * 
FROM #A 
INNER JOIN B ON A.id_b = B.id_b
INNER JOIN C ON B.id_c = C.id_c
INNER JOIN D ON D.id_d = C.id_d
INNER JOIN E ON E.id_e = D.id_e