问题
Today I learned about a thing in SQL Server called INNER LOOP JOIN.
What does this mean? (Google is not helping .. or should I say ... the blog posts about it are a bit .. technical and are blowing my mind).
Also, what are some common scenarios that would be a good idea to use an INNER LOOP JOIN over a standard INNER JOIN?
回答1:
LOOP | HASH | MERGE are Join hints, specifying that the join in the query should use looping, hashing, or merging. Using LOOP |HASH | MERGE JOIN enforces a particular join between two tables. LOOP cannot be specified together with RIGHT or FULL as a join type.
You should always use INNER JOIN. Let the query optimizer decide whether it wants to do a LOOP, MERGE, or HASH join. In almost all cases the optimizer will make a better judgement. Which one will be used and when can be found from my presentation http://sqlbits.com/Sessions/Event4/Understanding_Graphical_Execution_Plans.
回答2:
What you are referring to is a join hint. Like other hints, join hints should only be specified as a last resort as most of the time SQL server would pick correct algorithm. A good article to explain some of it is this.
来源：https://stackoverflow.com/questions/6529053/when-should-i-use-an-inner-loop-join-instead-of-an-inner-join