Performance Issues with Count(*) in SQL Server - sql-server

Question

I am having some performance issues with a query I am running in SQL Server 2008. I have the following query:
Query1:
SELECT GroupID, COUNT(*) AS TotalRows FROM Table1
INNER JOIN (
SELECT Column1 FROM Table2 WHERE GroupID = #GroupID
) AS Table2
ON Table2.Column1 = Table1.Column1
WHERE CONTAINS(Table1.*, #Word) GROUP BY GroupID
Table1 contains about 500,000 rows. Table2 contains about 50,000, but will eventually contain millions. Playing around with the query, I found that re-writing the query as follows will reduce the execution time of the query to under 1 second.
Query 2:
SELECT GroupID FROM Table1
INNER JOIN (
SELECT Column1 FROM Table2 WHERE GroupID = #GroupID
) AS Table2 ON Table2.Column1 = Table1.Column1
WHERE CONTAINS(Table1.*, #Word)
What I do not understand is it is a simple count query. If I execute the following query on Table 1, it returns in < 1 s:
Query 3:
SELECT Count(*) FROM Table1
This query returns around 500,000 as the result.
However, the Original query (Query 1) mentioned above only returns a count of 50,000 and takes 3s to execute even though simply removing the GROUP BY (Query 2) reduces the execution time to < 1s.
I do not believe this is an indexing issue as I already have indexes on the appropriate columns. Any help would be very appreciated.

Answer 1

Performing a simple COUNT(*) FROM table can do a much more efficient scan of the clustered index, since it doesn't have to care about any filtering, joining, grouping, etc. The queries that include full-text search predicates and mysterious subqueries have to do a lot more work. The count is not the most expensive part there - I bet they're still relatively slow if you leave the count out but leave the group by in, e.g.:
SELECT GroupID FROM Table1
INNER JOIN (
SELECT Column1 FROM Table2 WHERE GroupID = #GroupID
) AS Table2 ON Table2.Column1 = Table1.Column1
WHERE CONTAINS(Table1.*, #Word)
GROUP BY GroupID;
Looking at the provided actual execution plan in the free SQL Sentry Plan Explorer*, I see this:
And this:
Which lead me to believe you should:
Update the statistics on both Inventory and A001_Store_Inventory so that the optimizer can get a better rowcount estimate (which could lead to a better plan shape).
Ensure that Inventory.ItemNumber and A001_Store_Inventory.ItemNumber are the same data type to avoid an implicit conversion.
(*) disclaimer: I work for SQL Sentry.

Answer 2

You should have a look at the query plan to see what SQL Server is doing to retrieve the data you requested. Also, I think it would be better to rewrite your original query as follows:
SELECT
Table1.GroupID -- When you use JOINs, it's always better to specify Table (or Alias) names
,COUNT(Table1.GroupID) AS TotalRows
FROM
Table1
INNER JOIN
Table2 ON
(Table2.Column1 = Table1.Column1) AND
(Table2.GroupID = #GroupID)
WHERE
CONTAINS(Table1.*, #Word)
GROUP BY
Table1.GroupID
Also, keep in mind that a simple COUNT and a COUNT with a JOIN and GROUP BY are not the same thing. In one case, it's just a matter of going through an index and making a count, in the other there are other tables and grouping involved, which can be time consuming depending on several factors.