When I do dbcc show_statistics ('Reports_Documents', PK_Reports_Documents) I get the following result for Report ID 18698:

For this query:

SELECT * 
FROM Reports_Documents 
WHERE ReportID = 18698 option (recompile)

I get a query plan that makes a Clustered Index Seek on PK_Reports_Documents as expected.

But what baffles me is the incorrect value for Estimated Number of Rows:

According to this:

When the sample query WHERE clause value is equal to a histogram RANGE_HI_KEY value, SQL Server will use the EQ_ROWS column in the histogram to determine the number of rows that are equal to

This is also the way I would expect it to be, however it seems not to be the case in real life. I also tried some other RANGE_HI_KEY values that were present in the histogram provided by show_statistics and experienced the same. This issue in my case seems to cause some queries to use very unoptimal execution plans resulting in an execution time of a few minutes whereas I can get it to run in 1 sec with a query hint.

All in all: Can someone explain me why is EQ_ROWS from the histogram not being used for the Estimated Number of Rows and where does the incorrect estimate come from?

A bit more (possibly helpful) info:

• Auto create statistics is on and all the statistics are up to date.
• The table being queried has about 80 million rows.
• PK_Reports_Documents is a combination PK consisting of ReportID INT and DocumentID CHAR(8)