Showing posts with label Query. Show all posts
Showing posts with label Query. Show all posts

Monday, 18 February 2019

MongoDB Index in Python - Simple Index


Like RDBMS Systems MongoDB also provide Indexes to improve it's performance to process the query quicker and return the resultset. Mongo supports different type of indexes such as SingleKey, Compound, MultiKey, PartialKey and Text Indexes. We will look into these ones one by one.

Starting with Simple Index or One Key Index which use only one key from the collection/document [quivalent as  Table/Row in RDBMS systems], Let's see how -


Mongo Shell Command:  db.<collectionName>.createIndex({<field>:<direction>})
pyMongo Command:      db.<collectionName>.create_index([(<field>, <direction>)

Let's analyze the impact of Index creation on Query Performance, first via mongo shell, second in python - 

In MongoShell:

In our example, we are taking the collection 'people' as an example which has the field 'last_name'

db.people.find({last_name:'Tucker'}).explain('executionStats')

The above command will generate the executions stats for a query where last_name == Tuker .


as the execution plan shows, mongoDB scanned the whole collection (total 50747 documents for fetching 65 records) to fetch the result which is costly when your collection is big.

Now, Creating a Simple Index or Single Key Index

db.people.createIndex({"last_name":1}) 



Now, querying again the same - 

db.people.find({last_name:'Tucker'}).explain('executionStats')


This time MongoDB finds that there is an Index available on last_name columns which has been used to fetch the result. It scanned only 65 index keys to fetch 65 records. 

Single Key Index can be used in below scenarios - 
   - Querying on the range of Indexed Key values
   - Querying on selected values of Indexed Key

Advantage:
  - Returned result will be sorted by Index Key, no need to put a sort operation if sorting on the index key
  - Index key can be used in any sort order - Ascending or Descending

Consideration while Designing Single Key Index:
  - Do not create Single Key Index on each field available on collections, it will slow down the performance of select and write query both.





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Friday, 4 January 2019

How to Query on MongoDB by _id - INTERMEDIATE II


What if we have to query on MongoDB collections based on the "_id" field, Can we really query on "_id" field ? If so, what is the syntax ? Let's try this out -

Let's first fetch a document's id -

$ db.user.find_one()

{'_id': ObjectId('5c16e863817810ed3fc5e5f9'),
 'Fname': 'atul',
 'Lname': 'Singh',
 'Grade': 12.0,
 'College': 'SGM',
 'Job': 'Student',
 'Address': 'Young St.'}



Now, we will pick the object id and use this to fetch the same document from collection

$ db.user.find_one({'_id':'5c16e863817810ed3fc5e5f9'}) #this will return nothing

$ db.user.find_one({'_id':ObjectId('5c16e863817810ed3fc5e5f9')})
---------------------------------------------------------------------------
NameError                                 Traceback (most recent call last)
 in 
----> 1 db.user.find_one({'_id':ObjectId('5c16e863817810ed3fc5e5f9')})

NameError: name 'ObjectId' is not defined


We have received this error because ObjectId is not the same as its string representation, it must be converted to ObjectId from a string before it is passed to find command.

$ from bson.objectid import ObjectId
$ db.user.find_one({'_id':ObjectId('5c16e863817810ed3fc5e5f9')})

{'_id': ObjectId('5c16e863817810ed3fc5e5f9'),
 'Fname': 'atul',
 'Lname': 'Singh',
 'Grade': 12.0,
 'College': 'SGM',
 'Job': 'Student',
 'Address': 'Young St.'}



IPYTHON Notebook can be found HERE








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Thursday, 3 January 2019

MongoDB with Python - Basics IV - Update & Delete Operation


Welcome to one more quick session on MongoDB CRUD Basics with Update and Delete Operations. MDB Provides below methods under these operations -

** Update
      * update_one
      * update_many
      * replace_one

** Delete
      * delete_one
      * delete_many

For on Mongo DB -> Link


As it is quite clear from the name itself (_one and _many) that these methods perform the operation on single or multiple records based on the passed condition.
There are so many operator supported by update statement, few are as below -

$set - Add new or update field value
$unset  - Remove field
$inc - increment the current value
$push - push element into array field
$push with $each - push multiple elements into array field
$pop - pull out last value from array field

There are many more Update operators support by MongoDB, Full list can be found HERE


CRUD Operation - (Update & Delete) : Link






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Monday, 31 December 2018

MongoDB with Python - Basics III - Find/Select Operation


Hoping you guys are enjoying the NoSql journey so far (previous posts links), till now we have seen basic CRUD operation. From this post onward, I am diving in details of these operations and starting with FIND or SELECT operation in MongoDB. We will learn what are the ways and options provided by MongoDB to select or project the data.

When you start working with complex queries you might {as I have said "Might"} face difficulties with tracking of braces {([ as I've experienced the same with me/my team/students and colleagues. But no worries, Jupyter Notebook provides a  couple highlighter for braces when selected or you can use notepad++ also (which I think is not so useful as you are not gonna copy/paste the syntax so frequently).

I advise everyone to avoid the writing queries directly on mongo shell prompt as it doesn't provide any intelligence and not so good in fixing queries if made mistake.

I am sure you will love this post as well and if have any question feel free to ask in comment section below.

For on Mongo DB -> Link


There are many more Find/Read Operators supported by MongoDB, Full list can be found HERE

CRUD Operation - (Read) : Link




Next Post on this Series and more on MongoDB can be find here -> LINK




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Saturday, 29 December 2018

MongoDB with Python - Basics II - CRUD Operations


In this post, we will learn about the Advance Find and Create Operations with Sort, Skip and Limit functionality. Pymongo driver support almost same kind of syntax for python which mongo shell used.
The benefit of python (or any programming language) + mongo is to use both langauge/db functionality to work with mongo. Though, to perform the same operation is faster then performing by python but it depends on the activity you are performing.



CRUD Operation - (Create, Read, Update & Delete) : Link




Next Post on this Series and more on MongoDB can be find here -> LINK




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Friday, 28 December 2018

MongoDB with Python - Basics I - CRUD Operations


In Previous few posts (Link), We have learnt about MongoDB Cloud Setup, Installation and Basic commands to do CRUD operation with MongoDB. It can be accessed by programming language such as python, java, and node.js by using respective native drivers. We will start with PyMongo (python driver) to access mongo from python.



CRUD Operation - (Create, Read, Update & Delete) : Link




Next Post on this Series and more on MongoDB can be find here -> LINK





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