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lines changed Original file line number Diff line number Diff line change @@ -44,10 +44,17 @@ collection scan followed by a random sort to select N documents. In
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this case, the :pipeline:`$sample` stage is subject to the
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:ref:`sort memory restrictions <sort-memory-limit>`.
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- .. warning::
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+ MMAPV1 May Return Duplicate Documents
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+ ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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- :pipeline:`$sample` may output the same document more than once in
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- its result set.
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+ If you are using the:
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+
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+ - MMAPV1 storage engine, :pipeline:`$sample` may return the same
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+ document more than once in the result set.
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+ - :ref:`WiredTiger storage engine <storage-wiredtiger>`,
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+ :pipeline:`$sample` does not return duplicate documents. WiredTiger
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+ is the default storage engine as of MongoDB 3.2.
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Example
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--------
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