MongoDB Query->Exists() has no good performance

[ziaratban]

hi.
method exists() in yii\mongodb\Query by default has not projection or limit in command. so MongoDB return all document and field in result.

User::Find()->Where(['_id' => oid('5de3320034af172d38e38102')])->Exists()
#yii2 output: find({"ns":"myDB.users","filter":{"_id":"MongoDB\\BSON\\ObjectId(5de3320034af172d38e38102)"}})

vs

User::Find()->Select(['_id'])->Limit(1)->Where(['_id' => oid('5de3320034af172d38e38102')])->Exists()
#yii2 output: find({"ns":"myDB.users","filter":{"_id":"MongoDB\\BSON\\ObjectId(5de3320034af172d38e38102)"},"limit":1,"projection":{"_id":true}})

please see Benchmarking findOne() vs. find()->limit(1)->count(true)

[samdark]

We can likely add LIMIT 1 to exists by default.

[samdark]

@ziaratban want to try making a pull request?