Indexes

Indexes are Isars most powerful feature. Many embedded databases offer "normal" indexes (if at all) but Isar also has composite and multi-entry indexes. Understanding how indexes work is essential to optimize query performance. Isar lets you choose which index you want to use and how you want to use it. We'll start with a quick introduction to what indexes are.

What are indexes?

When a collection is unindexed, the order of the rows will likely not be discernible by the query as optimized in any way, and your query will therefore have to search through the objects linearly. In other words, the query will have to search through every object to find the ones matching the conditions. As you can imagine, that can take some time. Looking through every single object is not very efficient.

For example, this Product collection is completely unordered.

@collection
class Product {
  Id? id;

  late String name;

  late int price;
}

Data:

idnameprice
1Book15
2Table55
3Chair25
4Pencil3
5Lightbulb12
6Carpet60
7Pillow30
8Computer650
9Soap2

A query that tries to find all products that cost more than €30 has to search through all nine rows. That's not an issue for nine rows but it might become a problem for 100k rows.

final expensiveProducts = await isar.products.filter()
  .priceGreaterThan(30)
  .findAll();

To improve the performance of this query we index the price property. An index is like a sorted lookup table:

@collection
class Product {
  Id? id;

  late String name;

  @Index()
  late int price;
}

Generated index:

priceid
29
34
125
151
253
307
552
606
6508

Now the query can be executed a lot faster. The executor can directly jump to the last three index rows and find the corresponding objects by their id.

Sorting

Another cool thing indexes can do is super fast sorting. Sorted queries are very expensive because the database has to load all results in memory before sorting them. Even if you specify an offset or limit because they are applied after sorting.

Let's imagine we want to find the four cheapest products. We could use the following query:

final cheapest = await isar.products.filter()
  .sortByPrice()
  .limit(4)
  .findAll();

In this example, the database would have to load all (!) objects, sort them by price and return the four products with the lowest price.

As you can probably imagine, this can be done a lot more efficiently with the index from before. The database takes the first four rows of the index and returns the corresponding objects since they are already in the correct order.

To use the index for sorting we would write the query like this:

final cheapestFast = await isar.products.where()
  .anyPrice()
  .limit(4)
  .findAll();

The .anyX() where clause tells Isar to use an index just for sorting. You can also use a where clause like .priceGreaterThan() and still get sorted results.

Unique indexes

A unique index ensures the index does not contain any duplicate values. It may consist of one or multiple properties. If a unique index has one property, the values in this property will be unique. In case the unique index has multiple properties, the combination of values in these properties is unique.

@collection
class User {
  Id? id;

  @Index(unique: true)
  late String username;

  late int age;
}

Any attempt to insert or update data into the unique index that causes a duplicate will result in an error:

final user1 = User()
  ..id = 1
  ..username = 'user1'
  ..age = 25;

await isar.users.put(user1); // -> ok

final user2 = User()
  ..id = 2;
  ..username = 'user1'
  ..age = 30;

// try to insert user with same username
await isar.users.put(user2); // -> error: unique constraint violated
print(await isar.user.where().findAll());
// > [{id: 1, username: 'user1', age: 25}]

Replace indexes

It is sometimes not preferable to throw an error if a unique constraint is violated. Instead, you may want to replace the existing object with the new one. This can be achieved by setting the replace property of the index to true.

@collection
class User {
  Id? id;

  @Index(unique: true, replace: true)
  late String username;
}

Now when we try to insert a user with a username that already exists, the existing user will be replaced with the new one.

final user1 = User()
  ..id = 1
  ..username = 'user1'
  ..age = 25;

await isar.users.put(user1);
print(await isar.user.where().findAll());
// > [{id: 1, username: 'user1', age: 25}]

final user2 = User()
  ..id = 2;
  ..username = 'user1'
  ..age = 30;

await isar.users.put(user2);
print(await isar.user.where().findAll());
// > [{id: 2, username: 'user1' age: 30}]

Replace indexes also generate putBy() methods that allow you to update objects instead of replacing them. The existing id is reused and links are still populated.

final user1 = User()
  ..id = 1
  ..username = 'user1'
  ..age = 25;

// user does not exist so this is the same as put()
await isar.users.putByUsername(user1); 
await isar.user.where().findAll(); // -> [{id: 1, username: 'user1', age: 25}]

final user2 = User()
  ..id = 2;
  ..username = 'user1'
  ..age = 30;

await isar.users.put(user2);
await isar.user.where().findAll(); // -> [{id: 1, username: 'user1' age: 30}]

As you can see, the id of the first inserted user is reused.

Case-insensitive indexes

By default, all indexes on String and List<String> properties are case-sensitive. This means that the index will only contain entries where the property value is the same as the index value. If you want to create a case-insensitive index, you can use the caseSensitive option:

@collection
class Person {
  Id? id;

  @Index(caseSensitive: false)
  late String name;

  @Index(caseSensitive: false)
  late List<String> tags;
}

Index type

There are different types of indexes. Most of the time you'll want to use a IndexType.value index but hash indexes are more efficient.

Value index

This is the default type and also the only allowed type for all properties that don't hold Strings or Lists. Property values are used to build the index. In case of lists, the elements of the list are used. It is the most flexible but also space-consuming of the three index types.

TIP

Use IndexType.value for primitives, Strings where you need startsWith where clauses and Lists if you want to search for individual elements.

Hash index

Strings and Lists can be hashed to greatly reduce the storage required by the index. The disadvantage of hash indexes is that they can't be used for prefix scans (startsWith where clauses).

TIP

Use IndexType.hash for Strings and Lists if you don't need startsWith and elementEqualTo where clauses.

HashElements index

String lists can either be hashed completely (using IndexType.hash) or the elements of the list can be hashed (using IndexType.hashElements) effectively creating a multi-entry index with hashed elements.

TIP

Use IndexType.hashElements for List<String> where you need elementEqualTo where clauses.

Composite indexes

A composite index is an index on multiple properties. Isar allows you to create composite indexes that consist of up to three properties.

Composite indexes are also known as multiple-column indexes.

It's probably best to start with an example. We create a person collection and define a composite index on the age and name properties:

@collection
class Person {
  Id? id;

  late String name;

  @Index(composite: [CompositeIndex('name')])
  late int age;

  late String hometown;
}

Data:

idnameagehometown
1Daniel20Berlin
2Anne20Paris
3Carl24San Diego
4Simon24Munich
5David20New York
6Carl24London
7Audrey30Prague
8Anne24Paris

Generated index

agenameid
20Anne2
20Daniel1
20David5
24Anne8
24Carl3
24Carl6
24Simon4
30Audrey7

The generated composite index contains all persons sorted by their age and then by their name.

Composite indexes are great if you want to create efficient queries that are sorted by multiple properties. They also enable advanced where clauses with multiple properties:

final result = await isar.where()
  .ageNameEqualTo(24, 'Carl')
  .hometownProperty()
  .findAll() // -> ['San Diego', 'London']

The last property of a composite index also supports conditions like startsWith() or lessThan():

final result = await isar.where()
  .ageEqualToNameStartsWith(20, 'Da')
  .findAll() // -> [Daniel, David]

Multi-entry indexes

If you index a list using IndexType.value, Isar will automatically create a multi-entry index and each item in the array is indexed towards the object. It works for all types of lists.

Useful applications for multi-entry indexes are for example indexing a list of tags or creating a full-text index.

@collection
class Product {
  Id? id;

  late String description;

  @Index(type: IndexType.value, caseSensitive: false)
  List<String> get descriptionWords => Isar.splitWords(description);
}

Isar.splitWords() splits a string into words according to the Unicode Annex #29open in new window specification so it works for almost all languages correctly.

Data:

iddescriptiondescriptionWords
1comfortable blue t-shirt[comfortable, blue, t-shirt]
2comfortable, red pullover!!![comfortable, red, pullover]
3plain red t-shirt[plain, red, t-shirt]
4red necktie (super red)[red, necktie, super, red]

Entries with duplicate words only appear once in the index.

Generated index

descriptionWordsid
comfortable[1, 2]
blue1
necktie4
plain3
pullover2
red[2, 3, 4]
super4
t-shirt[1, 3]

This index can now be used for prefix (or equality) where clauses of the individual words of the description.

TIP

Instead of storing the words directly, also consider using the result of a phonectic algorithmopen in new window like Soundexopen in new window.