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Multi-Collection Behaviour

March 16, 2025

+ 12

Couchbase’s FTS service is the only service that can create indexes that span collections.

Multi-Collection Index: A user can search multi-collection indexes in the same way as that of a bucket-based index. Since a multi-collection index contains data from multiple source collections, it is helpful to know the source collection of every document hit in the search result.

Users can see the source collection names in the fields section of each document hit under the key _$c. See the image below for an example.

Users can also narrow their full-text search requests to only specific Collection(s) within the multi-Collection index. This focus speeds up searches on a large index.

Below is a sample Collection search request for Collections "airport".

Example

console
Copy
curl -XPOST -H “Content-Type:application/json” - u
<username>:<password> http://localhost:8094/api/index/demoindex/query -d

‘{
  “explain”: true,
  “fields”:[
  “*”
  ],
  “highlight”:{},
  “query”:{
    “query”:”france”
  },
  “size”:10,
  “from”:50,
  “collections”:[“airport”]
}’

At search time, there is no validation to determine whether or not a collection with a given name exists. As a result, users won’t receive any validation errors for the incorrect collection names within the search request. See the below example:

Example

An incorrect collection name “XYZ” is used.

console
Copy
curl -XPOST -H “Content-Type:application/json” - u
<username>:<password> http://localhost:8094/api/index/demoindex/query -d
‘{
“query”:{
“query”:”france”
},
“size”:10,
“from”:50,
“collections”:[“XYZ”]
}’

Result:

json
Copy
Result:
{
  "status": {
    "total": 1,
    "failed": 0,
    "successful": 1
  },
  "request": {
    "query": {
      "query": "france"
    },
    "size": 10,
    "from": 50,
    "highlight": null,
    "fields": null,
    "facets": null,
    "explain": false,
    "sort": [
      "-_score"
    ],
    "includeLocations": false,
    "search_after": null,
    "search_before": null
  },
  "hits": [
    {
      "index": "demoindex_6dbcc808a8278714_4c1c5584",
      "id": "hotel_21844",
      "score": 0.8255329922213157,
      "sort": [
        "_score"
      ],
      "fields": {
        "_$c": "hotel"
      }
    },
    {
      "index": "demoindex_6dbcc808a8278714_4c1c5584",
      "id": "hotel_21652",
      "score": 0.8236828315727989,
      "sort": [
        "_score"
      ],
      "fields": {
        "_$c": "hotel"
      }
    },
    {
      "index": "demoindex_6dbcc808a8278714_4c1c5584",
      "id": "hotel_1364",
      "score": 0.8232253432142588,
      "sort": [
        "_score"
      ],
      "fields": {
        "_$c": "hotel"
      }
    },
    {
      "index": "demoindex_6dbcc808a8278714_4c1c5584",
      "id": "hotel_21721",
      "score": 0.8225069701742189,
      "sort": [
        "_score"
      ],
      "fields": {
        "_$c": "hotel"
      }
    },
    {
      "index": "demoindex_6dbcc808a8278714_4c1c5584",
      "id": "hotel_21674",
      "score": 0.8218917130827247,
      "sort": [
        "_score"
      ],
      "fields": {
        "_$c": "hotel"
      }
    },
    {
      "index": "demoindex_6dbcc808a8278714_4c1c5584",
      "id": "hotel_35854",
      "score": 0.8218917094653351,
      "sort": [
        "_score"
      ],
      "fields": {
        "_$c": "hotel"
      }
    },
    {
      "index": "demoindex_6dbcc808a8278714_4c1c5584",
      "id": "hotel_21847",
      "score": 0.8212458150010249,
      "sort": [
        "_score"
      ],
      "fields": {
        "_$c": "hotel"
      }
    },
    {
      "index": "demoindex_6dbcc808a8278714_4c1c5584",
      "id": "hotel_21849",
      "score": 0.8201164200350234,
      "sort": [
        "_score"
      ],
      "fields": {
        "_$c": "hotel"
      }
    },
    {
      "index": "demoindex_6dbcc808a8278714_4c1c5584",
      "id": "hotel_21846",
      "score": 0.8197896824791812,
      "sort": [
        "_score"
      ],
      "fields": {
        "_$c": "hotel"
      }
    },
    {
      "index": "demoindex_6dbcc808a8278714_4c1c5584",
      "id": "hotel_20421",
      "score": 0.8191068922164917,
      "sort": [
        "_score"
      ],
      "fields": {
        "_$c": "hotel"
      }
    }
  ],
  "total_hits": 141,
  "max_score": 1.0743017811485551,
  "took": 999962,
  "facets": null
}

Impact of using Role-Based Access Control

The Couchbase Full Admin can administer Role-Based Access Control (RBAC) roles for full-text search indexes at a Bucket, Scope, or Collection(s) level.

FTS provides two primary roles for managing the access control:

A user must have at least search reader permissions at the source Bucket or Scope or Collection level to access the FTS index.

With multi-collection indexes, the user must have search reader roles for all source collections in order to access a multi-collection index.

Data lifecycle impact

Multi-collection indexes are deleted when any of the corresponding source collections are deleted. Therefore, multi-collection indexes are best suited for collections with similar data lifespans.