// SERVER-13946: When possible, place skip stages before fetch stages to avoid unnecessarily
// fetching documents that will be immediately discarded.

// The skip operation in a sharded query always occurs in the mongoS, so this test doesn't make
// sense on a sharded collection.
// @tags: [
//   assumes_unsharded_collection,
//   operations_longer_than_stepdown_interval_in_txns,
// ]

(function() {
"use strict";

load("jstests/libs/analyze_plan.js");

const coll = db.add_skip_stage_before_fetch;

coll.drop();
const testIndex = {
    a: 1,
    b: 1,
    c: 1
};
assert.commandWorked(coll.createIndex(testIndex));

const bulk = coll.initializeUnorderedBulkOp();
for (let i = 0; i < 10000; i++) {
    bulk.insert({
        a: i % 2,
        b: i % 4,
        c: Math.floor(Math.random() * 1000),
        d: Math.floor(Math.random() * 1000)
    });
}
assert.commandWorked(bulk.execute());

// The {a: 0, b: 2} query will match exactly one quarter of the documents in the collection:
// 2500 in total. In the test queries below, we skip the first 2400, returning exactly 100
// documents.

// This find can be computed using the index, so we should only need to fetch the 100 documents
// that get returned to the client after skipping the first 2400.
let explainResult = coll.find({a: 0, b: 2}).hint(testIndex).skip(2400).explain("executionStats");
assert.gte(explainResult.executionStats.totalKeysExamined, 2500);
assert.eq(explainResult.executionStats.totalDocsExamined, 100);

// This sort can also be computed using the index.
explainResult =
    coll.find({a: 0, b: 2}).hint(testIndex).sort({c: 1}).skip(2400).explain("executionStats");
assert.gte(explainResult.executionStats.totalKeysExamined, 2500);
assert.eq(explainResult.executionStats.totalDocsExamined, 100);

// This query is covered by the index, so there should be no fetch at all.
explainResult = coll.find({a: 0, b: 2}, {_id: 0, a: 1})
                    .hint(testIndex)
                    .sort({c: 1})
                    .skip(2400)
                    .explain("executionStats");
assert.gte(explainResult.executionStats.totalKeysExamined, 2500);
assert.eq(explainResult.executionStats.totalDocsExamined, 0);
assert(isIndexOnly(db, explainResult.queryPlanner.winningPlan));

// This sort requires a field that is not in the index, so we should be fetching all 2500
// documents that match the find predicate.
explainResult =
    coll.find({a: 0, b: 2}).hint(testIndex).sort({d: 1}).skip(2400).explain("executionStats");
assert.gte(explainResult.executionStats.totalKeysExamined, 2500);
assert.eq(explainResult.executionStats.totalDocsExamined, 2500);
})();