MongoDB的查询引擎也使得对数据应用聚合和分组函数变得非常简单。
MongoDB's query engine also makes it very simple to apply aggregation and group functions on your data.
一枚火箭可能会由多个部分组成，每一个部分称作一级，每一级又包含各自的引擎。当某一级的燃料耗尽时就回自动脱离火箭。
Rockets can be made up of multiple sections, known as stages, each of which contain an engine and can be jettisoned when their fuel runs out.
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