Contents of key-value pair created by collection and index when db.collection.insertOne()

[junhan_lee]

I have a question about the contents of key-value pairs which are converted from collection and index data, when I put db.collection.insertOne() in pymongo.
I understand that the document, which is inserted from User, is converted into two key-value pairs(collection and index, respectively) and inserted into each b+ tree that is created by file schema, in default MongoDB. Then, what is the exact content of those key-value pairs? Also, what does happen to those values when I create Index?

To answer my question, I did some little experiment. I inserted 10 documents one by one, and capture contents of cursor at __curfile_insert(WT_CURSOR* cursor) using GDB, and looking at those key and value data of cursor. Also, in between 5-th and 6-th insert, I created Index to see whether the content of index is changed or not.

• Settings

MongoDB version : 4.0.9
MongoDB storage engine : wiredtiger 3.1.1 version
MongoDB storage engine configurations :
collectionConfig blockCompressor : none
indexConfig prefixCompression : false
python version: 3.6

• python script

1 from pymongo import MongoClient
  2 import bson
  3 import pprint
  4 
  5 client = MongoClient()
  6 db = client['idx_tree_check']
  7 admin_db = client.admin
  8 
  9 document0 = { "item": "canvas_1", "qty": "500"}
 10 
 11 document1 = { "item": "canvas_2", "qty": "700"}
 12 
 13 document2 = { "item": "canvas_3", "qty": "1000"}
 14 
 15 document3 = { "item": "canvas_4", "qty": "400"}
 16 
 17 document4 = { "item": "canvas_5", "qty": "100"}
 18 
 19 document5 = { "item": "canvas_6", "qty": "600"}
 20 
 21 document6 = { "item": "canvas_7", "qty": "900"}
 22 
 23 document7 = { "item": "canvas_8", "qty": "800"}
 24 
 25 document8 = { "item": "canvas_9", "qty": "200"}
 26 
 27 document9 = { "item": "canvas_10", "qty": "300"}
 28 
 29 print("write document0")
 30 db.usertable.insert_one(document0)
 31 
 32 print("write document1")
 33 db.usertable.insert_one(document1)
 34 
 35 print("write document2")
 36 db.usertable.insert_one(document2)
 37 
 38 print("write document3")
 39 db.usertable.insert_one(document3)
 40 
 41 print("write document4")
 42 db.usertable.insert_one(document4)
 43 
 44 print("create Index")
 45 db.usertable.create_index([ ("qty", 1) ])
 46 
 47 print("write document5")
 48 db.usertable.insert_one(document5)
 49 
 50 print("write document6")
 51 db.usertable.insert_one(document6)
 52 
 53 print("write document7")
 54 db.usertable.insert_one(document7)
 55 
 56 print("write document8")
 57 db.usertable.insert_one(document8)
 58 
 59 print("write document9")
 60 db.usertable.insert_one(document9)

• result using GDB

Sorry that I am new user and can post one image only

1. 1st document insert : collection kv pair
   As shown in figure above, the key of collection would be record id, and value would be concatenation of object Id(primary key), and document data.
2. 1st document insert : index kv pair
   In case of inserting kv pair to btree managed by index, the key is equal to object Id which is created before. However, I don’t get what the value is.
3. now I created index
4. 6 th document insert : collection kv pair
   As shown in figure above, the pattern is same as the first result: key is record id, and the value is the document data.
5. 6 th document insert : index kv pair
   Before the experiment, I expected that the key would be changed. However, the key of index was same: object id of document.
6. 6 th document insert : second index kv pair
   Furthermore, I observed that one more kv pair is inserted to index btree. But I cannot see this content about.

Finally, here are my questions:
When inserting one document by calling db.collection.insertOne(),
q1-1 : what is content of key-value pair which accesses btree managed by collection file schema?
q1-2 : what is content of key-value pair which accesses btree managed by index file schema?
When I created index, and inserting document again,
q2-1 : what is content of key-value pair which accesses btree managed by collection file schema?
q2-2 : what is content of key-value pair which accesses btree managed by index file schema?
q2-3 : Why index key-value pairs are created twice?

Thank you.

[junhan_lee]

My question has been solved! there were similar discussion in following link: https://groups.google.com/g/mongodb-dev/c/f7FEFlheAxQ/m/iUdhUi0IBQAJ