Indexes and indexing overview in Azure Search

In Azure Search, an index is a persistent store of documents and other constructs used for filtered and full text search on an Azure Search service. A document is a single unit of searchable data in your index. For example, an e-commerce retailer might have a document for each item they sell, a news organization might have a document for each article, and so forth. Mapping these concepts to more familiar database equivalents: an index is conceptually similar to a table, and documents are roughly equivalent to rows in a table.

When you add or upload documents, or submit search queries to Azure Search, you are sending requests to a specific index in your search service. The process of adding documents to an index is called indexing.

Field types and attributes in an Azure Search index

As you define your schema, you must specify the name, type, and attributes of each field in your index. The field type classifies the data that is stored in that field. Attributes are set on individual fields to specify how the field is used. The following tables enumerate the types and attributes you can specify.

Field types

Type Description
Edm.String Text that can optionally be tokenized for full-text search (word-breaking, stemming, etc).
Collection(Edm.String) A list of strings that can optionally be tokenized for full-text search. There is no theoretical upper limit on the number of items in a collection, but the 16 MB upper limit on payload size applies to collections.
Edm.Boolean Contains true/false values.
Edm.Int32 32-bit integer values.
Edm.Int64 64-bit integer values.
Edm.Double Double-precision numeric data.
Edm.DateTimeOffset Date time values represented in the OData V4 format (e.g. yyyy-MM-ddTHH:mm:ss.fffZ or yyyy-MM-ddTHH:mm:ss.fff[+/-]HH:mm).
Edm.GeographyPoint A point representing a geographic location on the globe.

You can find more detailed information about Azure Search's supported data types here.

Field attributes

Attribute Description
Key A string that provides the unique ID of each document, used for document look up. Every index must have one key. Only one field can be the key, and its type must be set to Edm.String.
Retrievable Specifies whether a field can be returned in a search result.
Filterable Allows the field to be used in filter queries.
Sortable Allows a query to sort search results using this field.
Facetable Allows a field to be used in a faceted navigation structure for user self-directed filtering. Typically fields containing repetitive values that you can use to group multiple documents together (for example, multiple documents that fall under a single brand or service category) work best as facets.
Searchable Marks the field as full-text searchable.

You can find more detailed information about Azure Search's index attributes here.

Guidance for defining an index schema

As you design your index, take your time in the planning phase to think through each decision. It is important that you keep your search user experience and business needs in mind when designing your index as each field must be assigned the proper attributes. Changing an index after it is deployed involves rebuilding and reloading the data.

If data storage requirements change over time, you can increase or decrease capacity by adding or removing partitions. For details, see Manage your Search service in Azure or Service Limits.