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Topic: How search in an index if there is no primary key works?

Can  a question, but nevertheless.
There is a table on many millions records.
There there are not clustered indexes, and the primary key is not present.
How search in this case works?
For example, there is an index by date. I want to select records with date "20170831". What does SQL the Server?
In the execution plan I see that at first there is a search in an index (index seek), and then any RID Lookup. And did not understand that it.

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Re: How search in an index if there is no primary key works?

At sheet level of an index links (row locators) on the main table always lie.
If the main table cluster row locator it is a key cluster.
If the table - a heap, there RID, the real address of a line in a file

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Re: How search in an index if there is no primary key works?

Nonclustered Index Structures