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The subset of a $match following an $unwind that does not filter on the unwound field can be moved up in the pipeline.

For example:

[
  {'$unwind' : "$x"}, 
  {'$match' : {_id : 1, x : {'$gt': 3}  } }
]

is equivalent to:

[ 
   {'$match' : {_id : 1} }, 
   {'$unwind' : "$x"}, 
   {'$match' : {x : {'$gt': 3}} }
]

depends on

SERVER-22833 MatchExpression::toBSON does not necessarily produce a valid query object.

is documented by

DOCS-9499 Docs for SERVER-20506: Conditionally order $match with $unwind

is duplicated by

SERVER-4506 aggregation: optimize by pushing matches to earlier in a pipeline

is related to

SERVER-23444 Pessimizing moves break compile on clang 3.7+ (expression_tree.h)

SERVER-4506 aggregation: optimize by pushing matches to earlier in a pipeline

related to

SERVER-23469 $match involving field generated from $unwind is pushed ahead of $unwind in pipeline

SERVER-4506 aggregation: optimize by pushing matches to earlier in a pipeline

(2 related to)