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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A parser that ensures that an XML document is valid in addition to being well formed. See also parser.

A valid XML document, in addition to being well formed, conforms to all the constraints imposed by a DTD. It does not contain any tags that are not permitted by the DTD, and the order of the tags conforms to the DTD's specifications.

A module that reads in XML data from an input source and breaks it into chunks so that your program knows when it is working with a tag, an attribute, or element data. A nonvalidating parser ensures that the XML data is well formed but does not verify that it is valid. See also validating parser.