Optimize aggregation on sharded setup if first stage is exact match on shard key with $in

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    • Type: Improvement
    • Resolution: Duplicate
    • Priority: Major - P3
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    • Affects Version/s: None
    • Component/s: Aggregation Framework
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    • Fully Compatible
    • Query 12 (04/04/16), Query 13 (04/22/16)
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      When a $match contains an $in array with a single exact shard key match, the aggregation is not targetted to a single shard.

      The listed repro produces the following output on 3.0.10:

      $ mongo aggregation.js
MongoDB shell version: 3.0.10
connecting to: test
{ "e1" : null }
{
	"e2" : {
		"shardsPart" : [
			{
				"$match" : {
					"a" : {
						"$in" : [
							1
						]
					}
				}
			}
		],
		"mergerPart" : [ ]
	}
}

      And the following output on 3.2.4:

      $ mongo aggregation.js                                             
MongoDB shell version: 3.2.4
connecting to: test
{ "e1" : null }
{
	"e2" : {
		"shardsPart" : [
			{
				"$match" : {
					"a" : {
						"$in" : [
							1
						]
					}
				}
			}
		],
		"mergerPart" : [ ]
	}
}

      Notes:

      1. a find with / without the $in is correctly targetted to a single shard.
      2. I haven't tested multiple $in all targetted but it is likely that will also invoke a merge
      3. this issue is unrelated to SERVER-18940 as it is an exact shard key match.
      4. This issue is an related but distinct from SERVER-7656.

            Assignee:
            Benjamin Murphy (Inactive)
            Reporter:
            James O'Leary
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              Created:
              Updated:
              Resolved: