Ad hoc querying

阅读PNUTS论文的时候看到一个词汇：limits on ad hoc queries

什么是ad hoc queries呢？

Ad hoc querying is a term in information science .

Many application software systems have an underlying database which can be accessed by only a limited number of queries and reports. Typically these are available via some sort of menu, and will have been carefully designed, pre-programmed and optimized for performance by expert programmers.

By contrast, "ad hoc" reporting systems allow the users themselves to create specific, customized queries. Typically this would be via a user-friendly GUI -based system without the need for the in-depth knowledge of SQL , or database schema that a programmer would have.

Because such reporting has the potential to severely degrade the performance of a live system, it is usually provided over a data warehouse .

Ad hoc querying/reporting is a business intelligence subtopic, along with OLAP (online analytical processing ), data warehousing, data mining and other tools.

source: http://en.wikipedia.org/wiki/Ad_hoc#Ad_hoc_querying

对于习惯了mysql的人，ad hoc query的概念还真不太好理解，因为它太普通了！打开mysql终端，随便建立一个数据库，随便输入一个查询语句，这些都体现了ad hoc query的概念。那么，什么情况下我们对数据库的管理查询等操作不是ad hoc的呢？回忆一下高中的计算机课本上，将foxPro中创建查询的情形，某种程度上讲，那种只支持固定查询模式的查询，都是非ad hoc的。（注：foxPro的例子可能还不太恰当，因为它实际上还是支持sql语句任意创建查询表的）

在互联网IDC中，为了优化性能，非ad hoc式的query更加常见。数据库操作通常以硬编码的形式融入到了程序中，而数据库内的数据格式通常也会符合某种非通用性的约定。显然，对于这种数据库，sql语句是无能为力的，它们甚至连sql语句解释器都没有呢！