Hi , suppose I have a thousand data in my example collection then if I create an index after that point , would this new index that comes afterhand affect prior datas that were taking place in the collection ?
No. Indexes do not modify data. The creation of a unique index will fail if your data has duplicate values on the indexed field.
I mean , can I use this new index on old datas I have inserted beforehand ?
it is not a unique index btw
Yes the existing documents will be indexed.
Refer to the following for more details.