Select Query Running slowly for a particular table

In my table, I have more than 300 records in a table.

If, for example, I run

select top 200 * from 开发者_Python百科tablename

it runs fast. But if I run

select top 301 * from tablename

It's executing for a long time.....

Also, I can't run the following query at all:

select * from tablename

...it is too slow...

I want to the delete the records after 301 in that table.

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How wide is your table? What's the table definition look like?

If you have an identity column you could just run this:

delete from dbo.Table where TableID>301