Database Interfaces
Perl is widely favored for database applications. Its text-handling facilities are useful for generating SQL queries; arrays, hashes, and automatic memory management make it easy to collect and process the returned data. For example, in Tim Bunce's DBI application programming interface (API), the arguments to the API can be the text of SQL queries; thus it is possible to program in multiple languages at the same time (e.g., for generating a web page using HTML, JavaScript, and SQL in a here document). The use of Perl variable interpolation to programmatically customize each of the SQL queries, and the specification of Perl arrays or hashes as the structures to programmatically hold the resulting data sets from each SQL query, allows a high-level mechanism for handling large amounts of data for post-processing by a Perl subprogram. In early versions of Perl, database interfaces were created by relinking the interpreter with a client-side database library. This was sufficiently difficult that it was done for only a few of the most-important and most widely used databases, and it restricted the resulting perl executable to using just one database interface at a time.
In Perl 5, database interfaces are implemented by Perl DBI modules. The DBI (Database Interface) module presents a single, database-independent interface to Perl applications, while the DBD (Database Driver) modules handle the details of accessing some 50 different databases; there are DBD drivers for most ANSI SQL databases.
DBI provides caching for database handles and queries, which can greatly improve performance in long-lived execution environments such as mod_perl, helping high-volume systems avert load spikes as in the Slashdot effect.
In modern Perl applications, especially those written using Web application frameworks such as Catalyst, the DBI module is often used indirectly via object-relational mappers such as DBIx::Class, Class::DBI or Rose::DB::Object which generate SQL queries and handle data transparently to the application author.
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