并发性：并发执行目标的线程数。

Concurrency: the number of threads concurrently executing the target.

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当根据活动表或索引空间执行时，会降低并发性和可用性。

They can reduce concurrency and availability when executed against a live table or index space.

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但是，习惯这种新的执行模型可能有点儿困难，因为这是一种全新的并发性考虑方式。

But getting used to this new model of execution can be a bit awkward, if only because it represents an entirely different way of thinking about concurrency.

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