# MongoDB Performance<br />ch2 MongoDB Indexes

###### tags: `MongoDB University M201`

## Introduction to Indexes

:::info
### index 最佳建立順序
精確匹配 > 排序條件 > 範圍匹配
equality before range
equality before sorting
:::

* Indexes is try to solve ==slow queries==
* b-tree data structure
* Indexes increase the speed of ==read== but ==decrease write, update and delete performance==

## Single Field Indexes

* Simplest indexes in MongoDB
* db.<collection>.createIndex({<field>: <direction>})
* Key features
* Keys from only one field
* Can find a single value for the indexed field
* Can find a range of values
* Can use dot notation to index field in subdocuments
* Can be used to find several distinct values in a single query

:::info
We should never index on the field that point to sub document, becuase doing so we'd have to query on the entire subdocument.
:::

## Sorting with Indexes

#### Method for sorting
* In memory
* using an index

:::info
When sorting with a single field index, sort either ascending or descending regardless of the pyhsical ordering of the index keys
:::

## Querying on Compound Indexes

* index on two or more fields
* There can be **no more than 32 fields** in a compound index

### Index Prefix

## Multikey Indexes

* Index on a field that is an array, cuz for each entry in array the server will create a seperate index key.
* Multikey indexes can be constructed over arrays that hold both **scalar values** (e.g. strings, numbers) and **nested documents**

## Partial Indexes

**only index the documents in a collection that meet a specified filter expression.** By indexing a subset of the documents in a collection, partial indexes have ==lower storage requirements== and ==reduced performance costs== for index creation and maintenance.

// partial index example
db.restaurants.createIndex(
   { cuisine: 1, name: 1 },
   { partialFilterExpression: { rating: { $gt: 5 } } }
)

## Sparse Indexes

Sparse indexes only contain entries for documents that have the indexed field, even if the index field contains a null value

db.addresses.createIndex( { "xmpp_id": 1 }, { sparse: true } )

// partial index can cover sparse index
db.addresses.createIndex( { "xmpp_id": 1 }, { "xmpp_id": { $exists: true } } )

## Text Indexes

:::warning
**A collection can have at most one text index.**
:::

* To create text index

db.<collection>.createIndex( { <field>: "text" } )

* Query

db.<collection>.find({$text: {$search: "<keyword>"})

* Create index keys for each word of string in field
* e.g "MongoDB long sleeve T-shirt"
* mongodb
* long
* sleeve
* t
* shirt
* Case insensitive

## Collation

Collation allows users to specify language-specific rules for string comparison, such as rules for ==lettercase== and ==accent marks==.

{
    locale: <string>,
    caseLevel: <boolean>,
    caseFirst: <string>,
    strength: <int>
    numericOrdering: <boolean>,
    alternate: <string>,
    maxVariable: <string>,
    backwards: <boolean>
}