An index is a physical structure containing pointers to the data. Indices are created in an
existing table to locate rows more quickly and efficiently. It is possible to create an index on
one or more columns of a table, and each index is given a name. The users cannot see the
indexes; they are just used to speed up queries. Effective indexes are one of the best ways
to improve performance in a database application. A table scan happens when there is no
index available to help a query. In a table scan SQL Server examines every row in the table
to satisfy the query results. Table scans are sometimes unavoidable, but on large tables,
scans have a terrific impact on performance.
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