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Backwards Compatibility:
Fully Compatible
Sprint:
Query Optimization 2021-07-26, QO 2021-09-06, Query Optimization 2021-08-09, QO 2021-09-20, QO 2021-08-23

Currently, 'near' has to be a literal array (or GeoJSON object):

> db.c.aggregate([ {$geoNear: {near: [0, 0]}} ]

Instead we should allow it to be an expression:

> db.c.aggregate([ {$geoNear: {near: "$$pt"}} ], {let: {pt: [0, 0]}})

It doesn't make sense to allow the query point to vary per-document, so it has to be a constant expression.

We also want to allow 'near' to be bound in a $lookup:

{$lookup: {
  from: 'coll',
  as: 'docs',
  let: {pt: "$location"},
  pipeline: [
    {$geoNear: {
      near: "$$pt",
      ...
    }}
  ]
}}

The expression "$$pt" is constant within each subquery.

To make this work I think we'll want to:

Change the DocumentSourceGeoNear parser to accept any expression, for 'near'.
Change DocumentSourceGeoNear::optimize to optimize the expression.
When we convert DocumentSourceGeoNear to DocumentSourceGeoNearCursor, raise an error if the expression is not a constant.

is depended on by

COMPASS-5103 Allow $geoNear 'near' argument to be a let variable (string with $$)

TOOLS-2954 Investigate changes in SERVER-58604: Allow $geoNear 'near' argument to be a let variable

is duplicated by

SERVER-34766 Allow $expr or $$field in the $geoNear stage

is related to

SERVER-37675 Allow $geoWithin and $geoIntersects to take their geometry from an expression