SQL Server Hash Joins versus Nested Loops

Question

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Quick note

So, as I was writing the problem below I found a way to fix it myself. I thought I'd still post the question because:

1. Someone might find it useful.
2. I don't understand too much why it works.

Anyway the fixed code (see answers).

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I originally wrote:

I've spent ages googling this and can find lots of related answers, but none that exactly match my question.

I run the code below against a SQL Server (10) database and it executes very fast. The execution plan it uses involves a hash join.

Then I run it again, but this time uncomment the first two lines (DECLARE and SET lines), and also delete the '+1' beside y.[in date] and uncomment the '+ @COUNTER'. Now the query takes ages (upon ages) to complete - the execution plan instead using nested loops. Note I'm still just adding one to the date, but using a variable instead of a constant.

The question is: can I make the query using @COUNTER use a hash join instead of a nested loop?

( A bit of background: What I'm trying to do is loosely match x.[in date] and y.[in date] so that they match if they're within a specified number of days of each other. The number of days for the query to use is populated from a field in another table. I tried using datediff() first with abs() and less than, but I'm pretty sure that's going to always use nested loops. (It does when I try it anyway!)

I tried doing everything referred to in various parameter sniffing articles, but they didn't change things. Anyway I'm not running this as a stored procedure. I'm guessing there's something to do with an index on the [in date] field. )

-- DECLARE @COUNTER INT
-- SET @COUNTER = 1

BEGIN

    SELECT
        x.[line id]
        , y.[line id]

    FROM
        lines1 AS x
        JOIN lines2 AS y ON (

            x.[in date] = y.[in date] + 1 -- + @COUNTER
            AND x.[country] = y.[country]

        )

    WHERE
        x.[country] = 'USA'

END

Answer 1

The question is: can I make the query using @COUNTER use a hash join instead of a nested loop?

Yes. You can use a join hint to force this:

INNER HASH JOIN