I’ve grown up reading Tom Clancy and probably most of you have at least seen Red October, so this book caught my eye when browsing used books for a recent trip. It’s a fairly human look at what’s involved in sailing on a Trident missile submarine…
I tend to write my queries using LEFT JOIN. Why? Because logically I see it in my head like this:
Give me all the rows from this table and only those rows that match from the other table.
But, wouldn’t this logic work just as well:
Give me only the rows in this table that match the rows from this other table where I’m selecting all of them.
I know. If I worked on it some more I could make that a better sentence, but I’m pretty sure the logic is still sound. Only matching rows from one data set, all the rows from another data set. In short, RIGHT JOIN.
I read recently that we ought to be making everything into a LEFT JOIN because it performs better. I suspect someone had a bad day, wrote the JOIN criteria poorly, and a new bit of cargo cult wisdom was discovered. Because, well, let’s look at the queries:
SELECT * FROM Sales.SalesOrderHeader AS soh LEFT JOIN Sales.SalesOrderDetail AS sod ON sod.SalesOrderID = soh.SalesOrderID; SELECT * FROM Sales.SalesOrderDetail AS sod RIGHT JOIN Sales.SalesOrderHeader AS soh ON soh.SalesOrderID = sod.SalesOrderID;
Yeah, I know. Not exactly the height of query optimization and elegance. But, the point is made pretty easily. When these queries run, both return 121317 rows. They have almost identical performance on my system at 2.8 seconds and 2.7 seconds with identical reads and CPU. One run. You can run it a million times if you want. I’m not going to and here’s why:
Those execution plans are the same. Just because they look the same? How do I know that for sure? Because they have the same query_plan_hash value: 0x857588F9A4394676.
Now, if we changed the logic so that SalesOrderDetail was on the “all” side of the query, of course that would change things, but, that changes to logic, not just the JOIN. The query optimizer is able to deal with what is basically just syntactic sugar. No. LEFT JOIN is not better than RIGHT JOIN. It’s just different logic that is dealt with by the optimizer.
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