[SERVER-13104] Plan enumerator doesn't enumerate all possibilities for a nested $or - MongoDB Jira

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Say you have indices {a: 1} and {b: 1} and the following query:

db.coll.find({c: 1, $or: [{$and: [{a: 1}, {b: 1}]}, {a: 1}]});

Only one plan is generated for this query:

KEEP_MUTATIONS

---filter:

        $and

$or

                $and

                    a == 1.0

                    b == 1.0

                a == 1.0

            c == 1.0

---fetched = 1

---sortedByDiskLoc = 0

---getSort = []

---Child:

------FETCH

---------filter:

                c == 1.0

---------fetched = 1

---------sortedByDiskLoc = 0

---------getSort = []

---------Child:

------------OR

---------------fetched = 0

---------------sortedByDiskLoc = 0

---------------getSort = []

---------------Child 0:

------------------FETCH

---------------------filter:

                                b == 1.0

---------------------fetched = 1

---------------------sortedByDiskLoc = 1

---------------------getSort = [{ a: 1 }, ]

---------------------Child:

------------------------IXSCAN

---------------------------keyPattern = { a: 1.0 }

---------------------------direction = 1

---------------------------bounds = field #0['a']: [1.0, 1.0]

---------------------------fetched = 0

---------------------------sortedByDiskLoc = 1

---------------------------getSort = [{ a: 1 }, ]

---------------Child 1:

------------------IXSCAN

---------------------keyPattern = { a: 1.0 }

---------------------direction = 1

---------------------bounds = field #0['a']: [1.0, 1.0]

---------------------fetched = 0

---------------------sortedByDiskLoc = 1

---------------------getSort = [{ a: 1 }, ]

However, there are additional plans that have not been enumerated. For example, the first child of the OR stage above could use index {b: 1}. It does not get enumerated because of how we step through the memo (see https://github.com/mongodb/mongo/blob/20a22bdf907a0e2cd60b79f242a7375d84a3ab6a/src/mongo/db/query/plan_enumerator.cpp#L1033). The outer $and only has one "top-level" choice, and we don't consider that there are multiple enumeration choices for the nested $and.

is related to

SERVER-13732 Predicates in top-level implicit AND query not considered when generating index access plan for contained OR

related to

SERVER-13184 2.6 should explore same number of one index solns as 2.4