A database index is a data structure that improves the speed of operations in a table. Indices can be created using one or more columns, providing the basis for both rapid random lookups and efficient ordering of access to records. The disk space required to store the index is typically less than the storage of the table (since indices usually contain only the key-fields according to which the table is to be arranged, and excludes all the other details in the table), yielding the possibility to store indices into memory from tables that would not fit into it. In a relational databasean index is a copy of part of a table. Some databases extend the power of indexing by allowing indices to be created on functions or expressions. For example, an index could be created on upper(last_name), which would only store the uppercase versions of the last_name field in the index. Another option sometimes supported is the use of "filtered" indices, where index entries are created only for those records that satisfy some conditional expression. A further aspect of flexibility is to permit indexing on user-defined functions, as well as expressions formed from an assortment of built-in functions.

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Boxing converts a value-type to a reference-type, thus storing the object on the heap.  Unboxing converts a reference-type to a value-type, thus storing the value on the stack.

Deletes it from the memory.
To Do: answer better.  The current answer is not entirely correct.

StringBuilder is more efficient in cases where there is a large amount of string manipulation.  Strings are immutable, so each time a string is changed, a new instance in memory is created.

The data value may not be changed.  Note: The variable value may be changed, but the original immutable data value was discarded and a new data value was created in memory.