{
  "_comment": [
    "Inputs only. Expectations are measured -- see record.js.",
    "`partialFilterExpression`: which filters may be written, what the index",
    "then reports about itself, and the one behaviour that made ignoring the",
    "option a wrong answer rather than a missing feature -- a unique partial",
    "index constrains only the documents its filter selects."
  ],
  "documents": [
    { "_id": 1, "a": 1, "s": "x" },
    { "_id": 2, "a": 5, "s": "y" },
    { "_id": 3, "s": "z" },
    { "_id": 4, "a": 5, "s": "x" }
  ],
  "cases": [
    {
      "description": "a partial index is created and reports its filter",
      "ops": [
        { "name": "createIndex", "arguments": { "keys": { "a": 1 }, "partialFilterExpression": { "a": { "$gte": 5 } } } },
        { "name": "listIndexes", "arguments": {} }
      ]
    },
    {
      "description": "a query the filter covers still answers every match",
      "ops": [
        { "name": "createIndex", "arguments": { "keys": { "a": 1 }, "partialFilterExpression": { "a": { "$gte": 5 } } } },
        { "name": "find", "arguments": { "filter": { "a": { "$gte": 5 } }, "sort": { "_id": 1 } } }
      ]
    },
    {
      "description": "a query the filter excludes still answers every match",
      "ops": [
        { "name": "createIndex", "arguments": { "keys": { "a": 1 }, "partialFilterExpression": { "a": { "$gte": 5 } } } },
        { "name": "find", "arguments": { "filter": { "a": 1 }, "sort": { "_id": 1 } } }
      ]
    },
    {
      "description": "a query straddling the filter still answers every match",
      "ops": [
        { "name": "createIndex", "arguments": { "keys": { "a": 1 }, "partialFilterExpression": { "a": { "$gte": 5 } } } },
        { "name": "find", "arguments": { "filter": { "a": { "$gte": 1 } }, "sort": { "_id": 1 } } },
        { "name": "countDocuments", "arguments": { "filter": {} } }
      ]
    },
    {
      "description": "a unique partial index accepts duplicates its filter excludes",
      "documents": [],
      "ops": [
        { "name": "createIndex", "arguments": { "keys": { "a": 1 }, "unique": true, "partialFilterExpression": { "t": true } } },
        { "name": "insertMany", "arguments": { "documents": [{ "_id": 1, "a": 1, "t": false }, { "_id": 2, "a": 1, "t": false }] } },
        { "name": "find", "arguments": { "filter": {}, "sort": { "_id": 1 } } }
      ]
    },
    {
      "description": "a unique partial index refuses duplicates its filter selects",
      "documents": [],
      "ops": [
        { "name": "createIndex", "arguments": { "keys": { "a": 1 }, "unique": true, "partialFilterExpression": { "t": true } } },
        { "name": "insertMany", "arguments": { "documents": [{ "_id": 1, "a": 9, "t": true }, { "_id": 2, "a": 9, "t": true }] } }
      ]
    },
    {
      "description": "a document leaving the filter frees the value it held",
      "documents": [],
      "ops": [
        { "name": "createIndex", "arguments": { "keys": { "a": 1 }, "unique": true, "partialFilterExpression": { "t": true } } },
        { "name": "insertMany", "arguments": { "documents": [{ "_id": 1, "a": 9, "t": true }] } },
        { "name": "updateOne", "arguments": { "filter": { "_id": 1 }, "update": { "$set": { "t": false } } } },
        { "name": "insertOne", "arguments": { "document": { "_id": 2, "a": 9, "t": true } } },
        { "name": "find", "arguments": { "filter": {}, "sort": { "_id": 1 } } }
      ]
    },
    {
      "description": "a document entering the filter takes the value it names",
      "documents": [],
      "ops": [
        { "name": "createIndex", "arguments": { "keys": { "a": 1 }, "unique": true, "partialFilterExpression": { "t": true } } },
        { "name": "insertMany", "arguments": { "documents": [{ "_id": 1, "a": 9, "t": true }, { "_id": 2, "a": 9, "t": false }] } },
        { "name": "updateOne", "arguments": { "filter": { "_id": 2 }, "update": { "$set": { "t": true } } } }
      ]
    },
    {
      "_comment": [
        "The reads below exist for the implication test: a partial index may",
        "only answer a query that cannot match a document its filter left",
        "out. Each pairs a query that implies the filter with one that does",
        "not and touches the same field, so an implication test that says yes",
        "too readily loses the documents outside the filter -- which is a",
        "wrong answer a result comparison can see, unlike the index being",
        "used at all, which no client can observe."
      ],
      "description": "a query that leaves the filter's field out still sees past it",
      "documents": [
        { "_id": 1, "a": 1, "t": true },
        { "_id": 2, "a": 1, "t": false },
        { "_id": 3, "a": 2, "t": true }
      ],
      "ops": [
        { "name": "createIndex", "arguments": { "keys": { "a": 1 }, "partialFilterExpression": { "t": true } } },
        { "name": "find", "arguments": { "filter": { "a": 1 }, "sort": { "_id": 1 } } },
        { "name": "find", "arguments": { "filter": { "a": 1, "t": true }, "sort": { "_id": 1 } } },
        { "name": "find", "arguments": { "filter": { "a": 1, "t": false }, "sort": { "_id": 1 } } }
      ]
    },
    {
      "description": "an $in straddling the filter still answers every match",
      "ops": [
        { "name": "createIndex", "arguments": { "keys": { "a": 1 }, "partialFilterExpression": { "a": { "$gte": 5 } } } },
        { "name": "find", "arguments": { "filter": { "a": { "$in": [1, 5] } }, "sort": { "_id": 1 } } },
        { "name": "find", "arguments": { "filter": { "a": { "$in": [5, 9] } }, "sort": { "_id": 1 } } }
      ]
    },
    {
      "description": "a query for null does not belong to an index that requires the field",
      "documents": [
        { "_id": 1, "a": 1 },
        { "_id": 2 },
        { "_id": 3, "a": null }
      ],
      "ops": [
        { "name": "createIndex", "arguments": { "keys": { "a": 1 }, "partialFilterExpression": { "a": { "$exists": true } } } },
        { "name": "find", "arguments": { "filter": { "a": null }, "sort": { "_id": 1 } } },
        { "name": "find", "arguments": { "filter": { "a": 1 }, "sort": { "_id": 1 } } }
      ]
    },
    {
      "description": "an equality inside the filter's range, and one outside it",
      "ops": [
        { "name": "createIndex", "arguments": { "keys": { "a": 1 }, "partialFilterExpression": { "a": { "$gt": 3 } } } },
        { "name": "find", "arguments": { "filter": { "a": 5 }, "sort": { "_id": 1 } } },
        { "name": "find", "arguments": { "filter": { "a": 1 }, "sort": { "_id": 1 } } },
        { "name": "find", "arguments": { "filter": { "a": { "$gt": 4 } }, "sort": { "_id": 1 } } },
        { "name": "find", "arguments": { "filter": { "a": { "$gt": 0 } }, "sort": { "_id": 1 } } }
      ]
    },
    {
      "description": "a sort a partial index could serve",
      "documents": [
        { "_id": 1, "a": 3, "t": true },
        { "_id": 2, "a": 1, "t": false },
        { "_id": 3, "a": 2, "t": true }
      ],
      "ops": [
        { "name": "createIndex", "arguments": { "keys": { "a": 1 }, "partialFilterExpression": { "t": true } } },
        { "name": "find", "arguments": { "filter": { "t": true }, "sort": { "a": 1 } } },
        { "name": "find", "arguments": { "filter": {}, "sort": { "a": 1 } } }
      ]
    },
    {
      "description": "a unique partial index answers the read it constrains",
      "documents": [
        { "_id": 1, "a": 9, "t": true },
        { "_id": 2, "a": 9, "t": false }
      ],
      "ops": [
        { "name": "createIndex", "arguments": { "keys": { "a": 1 }, "unique": true, "partialFilterExpression": { "t": true } } },
        { "name": "find", "arguments": { "filter": { "a": 9, "t": true }, "sort": { "_id": 1 } } },
        { "name": "find", "arguments": { "filter": { "a": 9 }, "sort": { "_id": 1 } } }
      ]
    },
    {
      "description": "a filter on $exists",
      "ops": [
        { "name": "createIndex", "arguments": { "keys": { "a": 1 }, "partialFilterExpression": { "a": { "$exists": true } } } },
        { "name": "listIndexes", "arguments": {} }
      ]
    },
    {
      "description": "a filter with two predicates",
      "ops": [
        { "name": "createIndex", "arguments": { "keys": { "a": 1 }, "partialFilterExpression": { "a": { "$gt": 0 }, "s": "x" } } },
        { "name": "listIndexes", "arguments": {} }
      ]
    },
    {
      "description": "a filter with an explicit $and",
      "ops": [
        { "name": "createIndex", "arguments": { "keys": { "a": 1 }, "partialFilterExpression": { "$and": [{ "a": { "$gt": 0 } }] } } },
        { "name": "listIndexes", "arguments": {} }
      ]
    },
    {
      "description": "a filter on a field the index does not name",
      "ops": [
        { "name": "createIndex", "arguments": { "keys": { "a": 1 }, "partialFilterExpression": { "s": "x" } } },
        { "name": "listIndexes", "arguments": {} }
      ]
    },
    {
      "description": "a filter using $regex is refused",
      "ops": [
        { "name": "createIndex", "arguments": { "keys": { "a": 1 }, "partialFilterExpression": { "s": { "$regex": "x" } } } }
      ]
    },
    {
      "description": "a filter using $ne is refused",
      "ops": [
        { "name": "createIndex", "arguments": { "keys": { "a": 1 }, "partialFilterExpression": { "a": { "$ne": 1 } } } }
      ]
    },
    {
      "description": "a filter using $in is refused",
      "ops": [
        { "name": "createIndex", "arguments": { "keys": { "a": 1 }, "partialFilterExpression": { "a": { "$in": [1, 2] } } } }
      ]
    },
    {
      "description": "a filter that is not a document is refused",
      "ops": [
        { "name": "createIndex", "arguments": { "keys": { "a": 1 }, "partialFilterExpression": 1 } }
      ]
    },
    {
      "description": "an empty filter",
      "ops": [
        { "name": "createIndex", "arguments": { "keys": { "a": 1 }, "partialFilterExpression": {} } },
        { "name": "listIndexes", "arguments": {} }
      ]
    },
    {
      "description": "sparse and a partial filter may not be combined",
      "ops": [
        { "name": "createIndex", "arguments": { "keys": { "a": 1 }, "sparse": true, "partialFilterExpression": { "a": { "$gt": 0 } } } }
      ]
    },
    {
      "description": "a partial filter with a TTL",
      "ops": [
        { "name": "createIndex", "arguments": { "keys": { "a": 1 }, "expireAfterSeconds": 100, "partialFilterExpression": { "a": { "$gt": 0 } } } },
        { "name": "listIndexes", "arguments": {} }
      ]
    },
    {
      "description": "a partial index on a compound key",
      "ops": [
        { "name": "createIndex", "arguments": { "keys": { "a": 1, "s": 1 }, "partialFilterExpression": { "a": { "$gt": 0 } } } },
        { "name": "listIndexes", "arguments": {} }
      ]
    },
    {
      "description": "two indexes on one key differing only in their filter",
      "ops": [
        { "name": "createIndex", "arguments": { "keys": { "a": 1 }, "name": "one", "partialFilterExpression": { "a": { "$gt": 0 } } } },
        { "name": "createIndex", "arguments": { "keys": { "a": 1 }, "name": "two", "partialFilterExpression": { "a": { "$gt": 4 } } } },
        { "name": "listIndexes", "arguments": {} }
      ]
    },
    {
      "description": "the same index twice is idempotent",
      "ops": [
        { "name": "createIndex", "arguments": { "keys": { "a": 1 }, "partialFilterExpression": { "a": { "$gt": 0 } } } },
        { "name": "createIndex", "arguments": { "keys": { "a": 1 }, "partialFilterExpression": { "a": { "$gt": 0 } } } },
        { "name": "listIndexes", "arguments": {} }
      ]
    },
    {
      "description": "the same name with a different filter conflicts",
      "ops": [
        { "name": "createIndex", "arguments": { "keys": { "a": 1 }, "partialFilterExpression": { "a": { "$gt": 0 } } } },
        { "name": "createIndex", "arguments": { "keys": { "a": 1 }, "partialFilterExpression": { "a": { "$gt": 4 } } } }
      ]
    },
    {
      "description": "a partial index is dropped by name",
      "ops": [
        { "name": "createIndex", "arguments": { "keys": { "a": 1 }, "name": "pa", "partialFilterExpression": { "a": { "$gt": 0 } } } },
        { "name": "dropIndex", "arguments": { "name": "pa" } },
        { "name": "listIndexes", "arguments": {} }
      ]
    }
  ]
}