SQL massive performance difference using SELECT TOP x even when x is much higher than selected rows

Question

I'm selecting some rows from a table valued function but have found an inexplicable massive performance difference by putting SELECT TOP in the query.

SELECT   col1, col2, col3 etc
FROM     dbo.some_table_function
WHERE    col1 = @parameter
--ORDER BY col1

is taking upwards of 5 or 6 mins to complete.

However

SELECT   TOP 6000 col1, col2, col3 etc
FROM     dbo.some_table_function
WHERE    col1 = @parameter
--ORDER BY col1

completes in about 4 or 5 seconds.

This wouldn't surprise me if the returned set of data were huge, but the particular query involved returns ~5000 rows out of 200,000.

So in both cases, the whole of the table is processed, as SQL Server continues to the end in search of 6000 rows which it will never get to. Why the massive difference then? Is this something to do with the way SQL Server allocates space in anticipation of the result set size (the TOP 6000 thereby giving it a low requirement which is more easily allocated in memory)? Has anyone else witnessed something like this?

Thanks

Solution 1

Table valued functions can have a non-linear execution time.

Let's consider function equivalent for this query:

SELECT  (
        SELECT  SUM(mi.value)
        FROM    mytable mi
        WHERE   mi.id <= mo.id
        )
FROM    mytable mo
ORDER BY
        mo.value

This query (that calculates the running SUM) is fast at the beginning and slow at the end, since on each row from mo it should sum all the preceding values which requires rewinding the rowsource.

Time taken to calculate SUM for each row increases as the row numbers increase.

If you make mytable large enough (say, 100,000 rows, as in your example) and run this query you will see that it takes considerable time.

However, if you apply TOP 5000 to this query you will see that it completes much faster than 1/20 of the time required for the full table.

Most probably, something similar happens in your case too.

To say something more definitely, I need to see the function definition.

Update:

SQL Server can push predicates into the function.

For instance, I just created this TVF:

CREATE FUNCTION fn_test()
RETURNS TABLE
AS
RETURN  (
        SELECT  *
        FROM    master
        );

These queries:

SELECT  *
FROM    fn_test()
WHERE   name = @name

SELECT  TOP 1000 *
FROM    fn_test()
WHERE   name = @name

yield different execution plans (the first one uses clustered scan, the second one uses an index seek with a TOP)

Solution 2

I had the same problem, a simple query joining five tables returning 1000 rows took two minutes to complete. When I added "TOP 10000" to it it completed in less than one second. It turned out that the clustered index on one of the tables was heavily fragmented.

After rebuilding the index the query now completes in less than a second.

Solution 3

Your TOP has no ORDER BY, so it's simply the same as SET ROWCOUNT 6000 first. An ORDER BY would require all rows to be evaluated first, and it's would take a lot longer.

If dbo.some_table_function is a inline table valued udf, then it's simply a macro that's expanded so it returns the first 6000 rows as mentioned in no particular order.

If the udf is multi valued, then it's a black box and will always pull in the full dataset before filtering. I don't think this is happening.

Not directly related, but another SO question on TVFs

Solution 4

I think Quassnois' suggestion seems very plausible. By adding TOP 6000 you are implicitly giving the optimizer a hint that a fairly small subset of the 200,000 rows are going to be returned. The optimizer then uses an index seek instead of an clustered index scan or table scan.

Another possible explanation could caching, as Jim davis suggests. This is fairly easy to rule out by running the queries again. Try running the one with TOP 6000 first.

Solution 5

You may be running into something as simple as caching here - perhaps (for whatever reason) the "TOP" query is cached? Using an index that the other isn't?

In any case the best way to quench your curiosity is to examine the full execution plan for both queries. You can do this right in SQL Management Console and it'll tell you EXACTLY what operations are being completed and how long each is predicted to take.

All SQL implementations are quirky in their own way - SQL Server's no exception. These kind of "whaaaaaa?!" moments are pretty common. ;^)

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Comments

[Ray]

'Fraid not in this case. The point of my query is that the same rows are returned regardless of whether the TOP clause it used or not (TOP 6000 being bigger than the result set). It therefore can't be to do with the calculation of those rows themselves.

[Ray]

Yeah - I'm having a look at the plan right now. Though I've altered the query for posting. In reality it's doing SELECT *. I can't see how using TOP would prompt an index use?

[Quassnoi]

@Arj: could you please post your function definition?

[ZygD]

@Quassnoi: the inline TVF is simply a macro.