原文如下:
This is closely related to both “Tip #6: Do not embed fields that have unbound
growth” on page 7 and “Tip #7: Pre-populate anything you can” on page 8. This is an
optimization for once you know that your documents usually grow to a certain size,
but they start out at a smaller size. When you initially insert the document, add a
garbage field that contains a string the size that the document will (eventually) be, then
immediately unset that field:
> collection.insert({"_id" : 123, /* other fields */, "garbage" : someLongString})
> collection.update({"_id" : 123}, {"$unset" : {"garbage" : 1}})
This way, MongoDB will initially place the document somewhere that gives it enough
room to grow (Figure 1-3).
看下unset的定义
类似占位符之类的,可能mongoDB对于unset后的字段空间不会删除,只是做一下标记,这样这部分空间已经预先分出来了,这个空间就可以被重用
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