Indexes can be created using one or more columns of a database table, providing the basis for both rapid random lookups and efficient access of ordered records.
An index is a copy of selected columns of data, from a table, that is designed to enable very efficient search.
Suppose a database contains N data items and one must be retrieved based on the value of one of the fields.
Since databases may contain many objects, and since lookup is a common operation, it is often desirable to improve performance.
An index may be declared as UNIQUE, which creates an implicit constraint on the underlying table.
An index supporting fast searching for records satisfying the predicate is required to police such a constraint.
The data rows may be spread throughout the table regardless of the value of the indexed column or expression.
Since the physical records are in this sort order on disk, the next row item in the sequence is immediately before or after the last one, and so fewer data block reads are required.
The records for the tables sharing the value of a cluster key shall be stored together in the same or nearby data blocks.
This may improve the joins of these tables on the cluster key, since the matching records are stored together and less I/O is required to locate them.
The data block where the table record is stored is defined by the value of the cluster key.
However, it is not possible or efficient (on most databases) to retrieve the set of row identifiers using only the second or greater indexed column.
So, to improve the performance, one must ensure that the index is created on the order of search columns.
With a wildcard at the beginning of the search-term, the database software is unable to use the underlying index data structure (in other words, the WHERE-clause is not sargable).
This puts the wild-card at the right-most part of the query (now gro.aidepikiw@%), which the index on reverse(email_address) can satisfy.
In contrast, the bitmap index is designed for cases where the values of a variable repeat very frequently.
For example, the sex field in a customer database usually contains at most three distinct values: male, female or unknown (not recorded).
For such variables, the bitmap index can have a significant performance advantage over the commonly used trees.
An inverted index maps a content word to the document containing it, thereby allowing full-text searches.
It is created on a column that contains unique values, such as a primary key or email address.
Indexes are one of the physical parts of database conception among others like storage (tablespace or filegroups).
[clarify] RDBMS vendors all give a CREATE INDEX syntax with some specific options that depend on their software's capabilities.