use concise correlated subquery syntax for relations with single column mapping #65
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Describe your changes
This generates a simpler query plan when there is only one column mapping. Previously a
$lookuplooked like this:{ "$lookup": { "from": "students", "let": { "v__id": "$_id" }, "pipeline": [ { "$match": { "$expr": { "$eq": ["$$v__id", "$classId"] } } }, { "$replaceWith": { "student_name": { "$ifNull": ["$name", null] }, }, } ], "as": "students", }, },After the change it looks like this:
{ "$lookup": { "from": "students", "localField": "_id", "foreignField": "classId", "pipeline": [ { "$replaceWith": { "student_name": { "$ifNull": ["$name", null] }, }, } ], "as": "students", }, },I think this will give better query performance due to avoiding the use of a variable in an expression. I need to do some more research to confirm that.
Cases with multiple column mappings still use the first form. I'm going to check to see if we can avoid a variable in those cases too.
Issue ticket number and link
MDB-125