CHAPTER 11Nested Loops
After working out the cardinality of a join, the optimizer then has to consider the cost of the three ways of performing that join, nested loop, hash, or merge join. As I pointed out in Chapter 9, the star join and star transformation are not join mechanisms, they are join strategies, so they don't get a mention from this point onwards. The next three chapters examine each of the three join mechanisms in turn, first examining the operation and then the cost calculations.
As we go through the mechanisms, it is important to remember that several contradictory effects may appear. The run-time engine may not do exactly what the execution plan appears to say; the cost calculation may also not tally with what the plan seems ...
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