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MongoDB Notes

Useful Commands

  • db.help()
  • db.COLLCECTION_NAME.find(**).count()
db.stats()
show statistics of current database
  • help
  • help keys
db
show current database name
db.COLLCECTION_NAME.findOne()
Returns one document that satisfies the specified query criteria. If multiple documents satisfy the query, this method returns the first document according to the natural order
  • avoid print result, add null; after the statement;
  • > use mydb; > db.dropDatabase();
  • mongoimport –type csv –headerline weather.csv -d weather -c data
db.example.update({},{$unset: {words:1}},false,true)
delete word field from doc db.collection.update( criteria, objNew, upsert, multi )
  • totalIndex

Mongoose

Schema

  • Define document’s style, like struct in C link

Model

  • name the schema in databse, like typedef, can pre-initialize value. link

Problems

save insert or update a document. insert does only an insertion

  • get result from mongoose query

mongoose promise

find

  • search more than one field : User.find({name:’wil’, score: 80})

find and remove

Express

Miscellaneous

  • one single document should be less than 16M(BSON document size)
  • upsert (if exists, update, else create)
  • enable upsert by pass {upsert:true} in update
  • denormalization (in RDBS) improve database reading performance by add redunct data
  • durability http://www.techopedia.com/definition/27416/durability-databases
  • Capped Collection are fixed-size collections, once a collection fills its allocated space, it makes room for new documents by overwriting the oldest documents in the collection.
  • db is the variable that references the current database. The variable is automatically set to the default database test or is set when you use the use <db> to switch current database.
  • Cursor: A pointer to the result set of a query. Clients can iterate through a cursor to retrieve results.
  • GridFS
  • you cannot have parrael array for multi-key
  • compound index : used in sort and find, leftmost

for example, {a:1, b:1, c:1}, find({a:1}), find({a:1, b:1}) use compound index, find({b:1, c:1}) not use, but find({b:1,c:1}).sort({a:1,b:1}) use it

  • Index Selectivity: 最有区分度的，变化最多的，作为选择
  • hint : Tell mongodb which index to use
  • find and sort canot use sparse index(就是不是所有document都有的index)
  • _id can be a document

Types && op

  • NumberInt
  • NumberLong
  • new Date()
  • mongodb comparision will not cross types
$exists
find whether a field is exist
$type
specify whether the value match a type
  • $regex: Perl style regex (optimization problem)
  • Dot Notation, find a part of document

Operations

  1. $group
  2. $match
  3. $project
  4. $pop:{array_name: -1} :: pop from left
  5. db.COLLCECTION_NAME.drop() is more efficient than db.COLLCECTION_NAME.remove({})
  6. save :: if this doc _id exists, update, otherwise, create one.

week2 hw1

  • db.data.find({“Wind Direction”: {$gte: 180, $lte: 360}}, {“State”:1, “Temperature”:1}).sort({“Temperature”:1}).limit(1)

week3 hw1

  • db.students.update({}, {$push: {scores: { $each: [], $sort:{type: 1, score: 1} }}}, {multi: true})
  • db.students.update({}, {$unset:{‘scores.1’:1}}, {multi:true})
  • db.students.update({}, {$pull:{‘scores’:null}}, {multi:true})

week4 hw3

  • homepage: db.posts.ensureIndex({ date: -1})
  • perlink: db.posts.ensureIndex({ permalink: 1}, {unique: true})
  • pertag: db.posts.ensureIndex({ tags: 1})

week4 hw4

  • db.profile.find({op: ‘query’, ns: ‘school2.students’}, {millis: 1, _id: 0}).sort({millis:-1}).limit(1)

Final

Q1

  • db.messages.ensureIndex({From:1, To:1})
  • db.messages.find({ “headers.From”: “andrew.fastow@enron.com”, “headers.To”: “jeff.skilling@enron.com”}).count()

Q2

-

Questions

  • B Tree
  • document-level locking 2.8
  • aggregation framework
  • sort memory, limit
  • unwind will generate same _id
  • assert: command failed: {

“errmsg” : “exception: Exceeded memory limit for $group, but didn’t allow external sort. Pass allowDiskUse:true to opt in.”, “code” : 16945, “ok” : 0 } : aggregate failed

  • index can be use for sort
  • index has to be maintain, will slow the insert
  • w = 0, j = 0