TOP slows down query

Question

I have a pivot query on a table with millions of rows. Running the query normally, it runs in 2 seconds and returns 2983 rows. If I add TOP 1000 to the query it takes 10 secon

Answer 1

There is a specific order in which queries are processed.

A normal SQL query will be written as follows:

SELECT [...]
  FROM [table1]
  JOIN [table2]
    ON [condition]
 WHERE [...]
 GROUP BY [...]
HAVING [...]
 ORDER BY [...]

But the processing order is different:

FROM [table1]
    ON [condition]
  JOIN [table2]
 WHERE [...]
 GROUP BY [...]
HAVING [...]
SELECT [...]
 ORDER BY [...]

When using SELECT DISTINCT [...] or SELECT TOP [...] the processing order will be as follows:

FROM [table1]
    ON [condition]
  JOIN [table2]
 WHERE [...]
 GROUP BY [...]
HAVING [...]
SELECT [...] DISTINCT[...]
ORDER BY [...]
TOP [....]

Hence it's taking longer as your SELECT TOP 1000 is processed last.

Take a look at this link for further details: http://blogs.msdn.com/b/sqlqueryprocessing/