备忘单给出了相对“时间成本”各种数据库操作？
这可能被认为是一个奇怪的问题 - 但我正在寻找一个“备忘单”来描述（例如）各种数据库操作的“昂贵”程度。
我正在寻找一种理论理解（也许有统计数据支持）......例如，我可以猜测带有 order by 语句的选择操作比普通的选择操作慢。
但我听说 JOIN 是最“昂贵”的，但我不确定这一点......所以最好能得到一些证明......
This might be considered an odd question - but i'm looking for a "cheat sheet" that describes (for example) how "expensive" various database operations are.
I'm looking for a theoretical understanding (perhaps backed by statistical data).... for example I can guess that a select operation with an order by statement is slower than a plain select one.
But i've heard that JOINs are the moste "expensive" ones but i'm not sure of that... so it would be good to get some proof of that...
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不确定我是否见过这样的物品。这是一个相当复杂的领域，如果本地便宜的操作阻止在整个查询中使用更便宜的策略，那么它可能不是全局最优的。
例如，表扫描应该是昂贵的，但如果需要选择超过百分之几的记录，则表扫描通常是大型表上最便宜的操作。这是因为表扫描主要由顺序 I/O 组成，与（例如）书签操作可能使用的大量随机 I/O 相比，它可以相对有效地覆盖大量流数据。
查询优化教程确实太大了，无法放入 stackoverflow 帖子中（至少在我有时间的情况下），但是网络上有很多资源，并且有很多印刷书籍可以很好地处理它。尽管这些原则通常适用于跨平台，但它在某种程度上也依赖于平台。
SQL Server 是一个很好的实验系统，因为它附带的前端工具具有查询计划可视化功能，即使在免费的“快速”版本上也是如此。
Not sure I've ever seen such an item. It's quite a complex field, and a locally cheap operation might not be globally optimal if it precludes a cheaper strategy from being used across the whole query.
For example, table scans are supposed to be expensive, but are often the cheapest operation on a large table if more than a few percent of records need to be selected. This is because a table scan is largely composed of sequential I/O, which can cover a large volume of streamed data relatively efficiently in comparison to a large number of random I/Os that (for example) a bookmarking operation might use.
A query optimisation tutorial is really too big to fit into a stackoverflow posting (at least in the time I have available) but there are many resources on the web, and quite a few print books that give it a good treatment. It is also somewhat platform dependent, although the principles often apply across platforms.
SQL Server is a good system to experiment with as the front-end tool shipped with it has a query plan visualisation feature, even on the free 'express' versions.