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！