目录为什么要建立索引？哪些情况适合建立索引？那么哪些情况下适合建索引？哪些情况下不适合建索引？为什么索引是使用B+树？（重点）索引分为那几类？什么是聚簇索引？（重点）使用聚簇索引的优缺点？（知道）为什么
Assuming I have a class with a method that takes a System.Linq.Expressions.Expression as a parameter, how much value is there in unit testing it?
mysql> EXPLAIN SELECT * FROM urls ORDER BY RAND() LIMIT 1; +----+-------------+-------+------+---------------+------+---------+------+-------+---------------------------------+
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