Indexing and hashing in Databases

A database index is a data structure that improves the write speed of data recovery operations on a database table at the expense of the slower and more memory. Indexes can be created with one or more columns in a database table, which is the basis for both rapid random lookups and efficient access of ordered records. The disk space required to store the index is usually less than the required by the table (since indices usually contain only the key fields by which the table is arranged, and excludes all other information in the table), raising the possibility to store indices in memory for a table whose data is too large to store in memory.

In a relational database, an index is a copy of a portion of a table. Some databases extend the power of indexing by indexes are created on functions or expressions. For example, an index on upper (last_name) are created, which only store the uppercase versions of the last_name field in the index. Another option sometimes supported the use of "filtered" indices, where index entries for only those records that satisfy the conditional expression to be created. Another aspect of flexibility is the indexing on user-defined functions, as well as expressions formed from an assortment of built-in functions allow.

Indexes can be defined as unique or not unique. A unique index acts as a constraint on the table by preventing duplicate entries in the index and hence the support table