MongoDB - Aggregation Pipeline

MongoDB - Aggregation
Pipeline
Jason Terpko
DBA @ Rackspace/ObjectRocket
linkedin.com/in/jterpko
1

Background
www.objectrocket.com
2

Overview
3
o Aggregation Framework
o Pipeline Stages
o Operators
o Performance
o New Features

Aggregation
Pipeline
4
o Overview
o Stages
o Operators
o Multiple Stage Example

What is the Aggregation Pipeline?
5
A framework for data visualization and or manipulation using one ore multiple stages in
order (i.e. pipeline).
• Framework - Allows for the transformation of data through stages, the result can be
an array, cursor, or even a collection
• Visualization – Data transformation is not required at all times, this framework can
be used for basic counts, summations, and grouping
• Manipulation – Using stages the documents can be transformed as they pass
through each stage, this prepares the data for the next stage or the final result set
• Output – The result can be iterated over using a cursor or saved to a collection
within the same database
• Expandable – New stages and operators are added with each major version and in
3.4 views leverage the aggregation framework

All Stages
6
$collStats
$project
$match
$redact
$limit
$skip
$unwind
$group
$sample
$sort
$geoNear
$lookup
$out
$indexStats
$facet
$bucket
$bucketAuto
$sortByCount
$addFields
$replaceRoot
$count
$graphLookup

Common Stages
7
$match
$group
$project
$sort
$limit
$unwind
$out
- Filter (reduce) the number of documents that is passed to the next stage
- Group documents by a distinct key, the key can also be a compound key
- Pass documents with specific fields or newly computed fields to the next stage
- Returns the input documents in sorted order
- Limit the number of documents for the next stage
- Splits an array into into one document for each element in the array
- As the last stage, creates/replaces an unsharded collection with the input documents

Common Operators
8
Group Operators
$sum
$avg
$max
$min
$first
$last
Date Operators
$year
$month
$week
$hour
$minute
$second
Arithmetic Operators
$abs
$add
$multiply
$subtract
$trunc
Operators that return a value based on document data.
Operators that return true or false based on document data.
Comparison Operators
$eq $gt
$lt $gte
$lte
Boolean Operators
$and
$or

Aggregate()
9
db.changelog.aggregate([
{$match : {"details.note":"success", "details.step 6 of 6": {$gte:0}}},
{$sort: {time:-1}},
{$limit: 100},
{$project : {'totalTime' : { '$add' : [ "$details.step 1 of 6","$details.step 2 of 6",
"$details.step 3 of 6","$details.step 4 of 6",
"$details.step 5 of 6","$details.step 6 of 6" ] } } },
{$group: {_id: null, averageTotalTime: {$avg: "$totalTime"} } }
]);
Collection
Purpose: Return the average number of milliseconds to move a chunk for the last one
hundred moves.

$match
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{$sort: {time:-1}},
{$limit: 100},
]);
Stage 1
Purpose: In the first stage filter only the chunks that moved successfully.
Comparison
Operator

$sort
11
{$sort: {time:-1}},
{$limit: 100},
]);
Stage 2
Purpose: Sort descending so we are prioritizing the most recent moved chunks.

$limit
12
{$sort: {time:-1}},
{$limit: 100},
]);
Stage 3
Purpose: Further reduce the number of moves being analyzed because time to move a
chunk varies by chunk and collection.

$project
13
{$sort: {time:-1}},
{$limit: 100},
]);
Stage 4
Purpose: For each moveChunk document project the sum of the steps to the next stage.
Arithmetic
Operator

$group
14
{$sort: {time:-1}},
{$limit: 100},
]);
Stage 5
Purpose: Return the average number of milliseconds to move a chunk for the last one
hundred moves.
Arithmetic
Operator

Optimizations
15
o Projections
o Sequencing
o Indexing
o Sorting

Projections
16
When using $project stage Mongo will read and pass less data to the next stage. By doing this it will
require less CPU, RAM, and reduce the disk IO to process the aggregation.
db.jobs.aggregate([
{$match : {"type": "import"}},
{$sort: {"cluster": 1}},
{$project : { cluster: 1, type:1, seconds:1, _id: 0} },
{$group: {_id: {cluster: "$cluster", type: "$type"}, avgExecTime: {$avg: "$seconds"} } }
]);
Stage 3
By default Mongo will try to determine if a subset of fields are required, if so it will request only those
fields and optimize the stage for you.

Sequencing
17
When stages can be ordered more efficiently, Mongo will reorder those stages for you to improve
execution time.
db.jobs.aggregate([
{$project : { cluster: 1, type:1, seconds:1, _id: 0} },
]);
By filtering documents first the number of documents to be sorted is reduced.

Sequencing
18
When stages can be ordered more efficiently, Mongo will reorder those stages for you to improve
execution time.
db.jobs.aggregate([
{$project : { cluster: 1, type:1, seconds:1} },
]);
In addition to sequence optimizations Mongo can also coalesce stages, for example a $match stage
followed by another $match will become one stage. A full list of sequence and coalesce optimizations
can be viewed at Aggregation Pipeline Optimization.

Indexing and Data Merging
19
Only two stages have the ability to utilize indexes, the $match stage and the $sort stage. Starting in
version 3.2 an index can cover an aggregation. Like find() you can generate an explain plan for an
aggregation to view a more detail execution plan.
To use an index, these stages must be the first stages in the pipeline.
Also released in version 3.2 for aggregations:
• Data that does not require the primary shard no longer has to be merged on the primary shard.
• Aggregations that include the shard key in the $match stage and don’t require data from other
shards can execute entirely on the target shard.

Memory
20
Stages have a limit of 100MB of RAM, this restriction is the most common restriction one encounters
when using the aggregation framework.
To exceed this limitation use the allowDiskUse option to allow stages like $sort to use temporary files.
db.jobs.aggregate([
{$project : { cluster: 1, type:1, seconds:1} },
], {allowDiskUse: true});
This option should be used with caution in production due to added resource consumption.

New In 3.4
21
o Recursive Search
o Faceted Search
o Views

Recursive Search
22
Recursively search a collection using $graphLookup. This stage in the pipeline takes input from
either the collection or a previous stage (e.g. $match).
{
$graphLookup: {
from: "users",
startWith: "$connections",
connectFromField: "connections",
connectToField: "name",
as: "connections",
}
}
Considerations
• This stage is limited to 100M of
RAM even with allowDiskUse
option
• maxDepth of zero is equivilent to
$lookup
• Collation must be consistent when
involving multiple views

Recursive Search
23
Users Collection:
{ "_id" : 101, "name" : "John”, "connections" : ["Jane", "David"] }
{ "_id" : 102, "name" : "David”, "connections" : ["George"] }
{ "_id" : 103, "name" : "George", "connections" : ["Melissa"] }
{ "_id" : 104, "name" : "Jane", "connections" : ["Jen"] }
{ "_id" : 105, "name" : "Melissa”, "connections" : ["Jason"] }
{ "_id" : 106, "name" : "Nick", "connections" : ["Derek"] }

Recursive Search
24
db.users.aggregate( [
{ $match: { "name": "John" } },
{ $graphLookup: {
from: "users",
startWith: "$connections",
connectFromField: "connections",
connectToField: "name",
as: "connections",
} },
{ $project: {
"_id": 0,
"name": 1,
"known connections": "$connections.name"
} } ] ).pretty();
Aggregation:
{
"name": "John",
"known connections": [
"Melissa",
"George",
"Jane",
"David”
]
}
Result:

Faceted Search
25
{ "_id" : 101, "name" : "Perf T", "price" : NumberDecimal("19.99"), "colors" : [ "red","white" ],
"sizes" : ["M", "L", "XL"] }
{ "_id" : 102, "name" : "Perf V-Neck", "price" : NumberDecimal("24.99"), "colors" : [ "white", "blue" ],
"sizes" : ["M", "L", "XL"] }
{ "_id" : 103, "name" : "Perf Tank", "price" : NumberDecimal("14.99"), "colors" : [ "red", "blue" ],
"sizes" : ["M", "L"] }
{ "_id" : 104, "name" : "Perf Hoodie", "price" : NumberDecimal("34.99"), "colors" : [ "blue" ],
"sizes" : ["M", "L", "XL"] }
Sample Data:
$facet allows you to process multiple pipelines with in a single aggregation stage. The
sub-pipelines take the same input documents and output one document in the stage
output.

Faceted Search
26
db.store.aggregate( [
{
$facet: {
"categorizedByColor": [
{ $unwind: "$colors" }, { $sortByCount: "$colors" }
],
"categorizedBySize": [
{ $unwind: "$sizes" }, { $sortByCount: "$sizes" }
],
"categorizedByPrice": [
{ $bucketAuto: {
groupBy: "$price”, buckets: 2
} } ] } }
]).pretty()
Command:

Faceted Search
27
{
"categorizedByColor": [ {
"_id": "blue",
"count": 3
},
{
"_id": "white",
"count": 2
},
{
"_id": "red",
"count": 2
}
],
……….
"categorizedBySize": [{
"_id": "L",
"count": 4
},
{
"_id": "M",
"count": 4
},
{
"_id": "XL",
"count": 3
}
],
……….
"categorizedByPrice": [{
"_id": {
"min": NumberDecimal("14.99"),
"max": NumberDecimal("24.99")
},
"count": 2
}, {
"_id": {
"min": NumberDecimal("24.99"),
"max": NumberDecimal("34.99")
},
"count": 2
} ]
}

Views
28
A read-only object that can be queried like the underlying collection. A view is created
using an aggregation pipeline and can be used to transform data or limit data access
from another collection.
• Computed on demand for each read operation
• Use indexes from the underlying collection
• Names are immutable, to change the name drop and recreate
• Can be created on sharded collections
• Are listed as collections in getCollectionNames()
• Allows for more granular access controls than RBAC via views

Example View
29
{
"_id": 101,
"first_name": "John",
"last_name": "Doe",
”dept": ”123",
”role": ”DBA",
”expense_id": 1234,
”amt": ”35.00",
”c_date": ISODate("2017-02-25T17:08:46.166Z")
}
Documents:

Example View
30
db.createView(
"recentExpenses",
"expenses",
[
{ $match: {"c_date": {$gte: new Date(new Date()-86400*1000)}}},
{ $project: { "_id": 1, "first_name": 1, "last_name": 1, "amt": 1 } },
{ $sort: {"c_date": -1}}
]
);
Create View:

Example View
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> show collections
system.views
expenses
recentExpenses
> db.system.views.find()
{ "_id" : ”mydb.recentExpenses", "viewOn" : "expenses", "pipeline" : [ { ”$match ……
> db.recentExpenses.find()
{ "_id" : 103, "first_name" : "John", "last_name" : "Doe", "amt" : "35.00" }
{ "_id" : 102, "first_name" : ”Jane", "last_name" : ”Smith", "amt" : ”36.00" }
{ "_id" : 101, "first_name" : ”Mike", "last_name" : ”Adams", "amt" : ”33.00" }
Collections:
Metadata:
Usage:

Questions?
32

33
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34

MongoDB - Aggregation Pipeline

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