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SQL Performance Tuning by Trudy Pelzer, Peter Gulutzan

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Chapter 15. Locks

Here's a locking analogy to consider.

Think of an office with men's and women's washrooms. Doubtless a man could actually lock the men's washroom when he goes in, but that would be inefficient for three reasons: (1) men who only want to comb their hair are excluded whether or not their purpose would conflict with another occupant's; (2) other men can't tell from a distance whether the door is locked; (3) nobody can open the door for maintenance or emergencies. To avoid these problems, it's smarter to use a system of signs. For example, a man scrawls “occupied” on the wall by the bathroom door or, better yet, he flies a flag above his cubicle so that everyone knows where he's going. Of course, there are different flags for urinals ...

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