// @tags: [requires_non_retryable_writes, requires_fastcount]

// Tests for explaining the delete command.
(function() {
"use strict";

var collName = "jstests_explain_delete";
var t = db[collName];
t.drop();

var explain;

/**
 * Verify that the explain command output 'explain' shows a DELETE stage with an nWouldDelete
 * value equal to 'nWouldDelete'.
 */
function checkNWouldDelete(explain, nWouldDelete) {
    assert.commandWorked(explain);
    assert("executionStats" in explain);
    var executionStats = explain.executionStats;
    assert("executionStages" in executionStats);

    // If passed through mongos, then DELETE stage(s) should be below the SHARD_WRITE mongos
    // stage.  Otherwise the DELETE stage is the root stage.
    var execStages = executionStats.executionStages;
    if ("SHARD_WRITE" === execStages.stage) {
        let totalToBeDeletedAcrossAllShards = 0;
        execStages.shards.forEach(function(shardExplain) {
            const rootStageName = shardExplain.executionStages.stage;
            assert.eq(rootStageName, "DELETE", tojson(execStages));
            totalToBeDeletedAcrossAllShards += shardExplain.executionStages.nWouldDelete;
        });
        assert.eq(totalToBeDeletedAcrossAllShards, nWouldDelete, explain);
    } else {
        assert.eq(execStages.stage, "DELETE", explain);
        assert.eq(execStages.nWouldDelete, nWouldDelete, explain);
    }
}

// Explain delete against an empty collection.
assert.commandWorked(db.createCollection(t.getName()));
explain = db.runCommand({explain: {delete: collName, deletes: [{q: {a: 1}, limit: 0}]}});
checkNWouldDelete(explain, 0);

// Add an index but no data, and check that the explain still works.
t.ensureIndex({a: 1});
explain = db.runCommand({explain: {delete: collName, deletes: [{q: {a: 1}, limit: 0}]}});
checkNWouldDelete(explain, 0);

// Add some copies of the same document.
for (var i = 0; i < 10; i++) {
    t.insert({a: 1});
}
assert.eq(10, t.count());

// Run an explain which shows that all 10 documents *would* be deleted.
explain = db.runCommand({explain: {delete: collName, deletes: [{q: {a: 1}, limit: 0}]}});
checkNWouldDelete(explain, 10);

// Make sure all 10 documents are still there.
assert.eq(10, t.count());

// If we run the same thing without the explain, then all 10 docs should be deleted.
var deleteResult = db.runCommand({delete: collName, deletes: [{q: {a: 1}, limit: 0}]});
assert.commandWorked(deleteResult);
assert.eq(0, t.count());
}());