Question

i have 2 dbs: one in mySql and one in MongoDB with the same data inside…
i do the follow in mySQL:

Select tweet.testo From tweet Where tweet.testo like ‘%pizza%’

and this is the result:

1627 rows in set (2.79 sec) 

but if i exec in mongo:

Db.tweets.find({text: /pizza/ }).explain() 

this is the result:

nscannedObjects" : 1606334,
"n" : 1169,

or if i exec:

Db.tweets.find({text: /pizza/i }).explain() 

this is the result:

"nscannedObjects" : 1606334,
"n" : 1641,

Why the number of rows/document in mysql/mongo find is different?

Answer 1

Why the number of rows/document in mysql/mongo find is different??

There could be 1000000000000000 reasons including the temperature of the sun on that particular day.

MongoDB and MySQL are two completely separate techs as such if you expect to keep both in synch you will need some kind of replicator between the two. You have not made us aware as to whether this is the case.

Also we have no idea of your coding, server setup, network setup and everything else so really we cannot even begin to answer this.

A good answer would be to say that the reason you are seeing this is because the data between the two is different…

As for the difference between:

Db.tweets.find({text: /pizza/ }).explain()

and

Db.tweets.find({text: /pizza/i }).explain() 

This is because MySQL, by default, queries in lower case I believe and MongoDB (I know) does not as such it is case sensitive (this i makes it case insensitive).

However about replicators, here is a good one: https://docs.continuent.com/wiki/display/TEDOC/Replicating+from+MySQL+to+MongoDB