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Answer Posted / binyam
A covering index is a non clustered index built upon all the
columns required to satisfy a SQL query without going to the
base table. If a query encounters an index and does not need
to refer to the underlying data table at all, then the index
can be considered a covering index.
To understand a covering index it is very important first
understand non clustered index. a non clustered index does
not affect the order of the data in the table pages,because
the leaf pages of a non clustered index and the data pages
of the table are separate .A pointer is required to navigate
from an index row to the data row. so when a query requests
columns that are not part of the non clustered index chosen
by the optimizer , a lookup is required .The lookup fetches
the corresponding data row from the table by the following
row locator value from the index row, requiring a logical
read on the data page besides the logical read on the index
page. However, if all the columns required by the query are
available in the index itself , them access to the data page
not required then this known as a covering index.
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