Indexes in Relational Model
An index is an orderly arrangement used to logically access rows in a table. Indexes in the relational database environment work like the indexes described in the preceding paragraphs. From a conceptual point of view, an index is composed of an index key and a set of pointers. The index key is, in effect, the index’s reference point. More formally, an index is an ordered arrangement of keys and pointers. Each key points to the location of the data identified by the key.
DBMSs use indexes for many different purposes. You just learned that an index can be used to retrieve data more efficiently. But indexes can also be used by a DBMS to retrieve data ordered by a specific attribute or attributes.
Indexes play an important role in DBMSs for the implementation of primary keys. When you define a table’s primary key, the DBMS automatically creates a unique index on the primary key column(s) you declared.
A unique index, as its name implies, is an index in which the index key can have only one pointer value (row) associated with it.
A table can have many indexes, but each index is associated with only one table. The index key can have multiple attributes (composite index)
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Codd’s Relational Database Rules
