Having time out issue while querying COSMOS DB with filter

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Having time out issue while querying COSMOS DB with filter

PANDEY, RISHABH U138084 (EXTERN) 20

Hi I hava a COSMOS Database with mongodb core with around 5.5 Lac records. The query is giving consistent timeouts when using a filter long with vector Search. We have configured to use DISKANN for vector search. The Filter query is a simple check like {"field_name":"String Value"}. I have not created any text index in the database.

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Answer 1

Amira Bedhiafi 42,846 MVP Volunteer Moderator

Hello !

Thank you for posting on MS Learn Q&A.

I think your filter field is not indexed because a text index is not needed for {"field_name": "String Value"}. You need a normal MongoDB ascending index on the filter field in addition to the DiskANN vector index.

For exact match filtering with Cosmos DB MongoDB vCore vector search you need to create a normal index on the filter field. DiskANN only indexes the vector path and it does not automatically optimize metadata filters.

db.yourCollection.createIndex({ field_name: 1 })

Then keep the filter inside the $search.cosmosSearch.filter section:

db.yourCollection.aggregate([
  {
    "$search": {
      "cosmosSearch": {
        "path": "contentVector",
        "vector": queryVector,
        "k": 10,
        "lSearch": 100,
        "filter": {
          "field_name": { "$eq": "String Value" }
        }
      }
    }
  }
])

DiskANN filtering creates both the vector index and a separate index on the filter field for example is_open: 1 specifically so filtered vector search can be optimized.

https://learn.microsoft.com/en-us/azure/cosmos-db/mongodb/vcore/vector-search

PANDEY, RISHABH U138084 (EXTERN) 20 Reputation points

2026-05-26T06:11:35.62+00:00

Thanks @Amira Bedhiafi
I have created an Index on the filter field,
can you please let me know what happens if my query has a regex in the filter and there is an index created for the field.
Also the above link in not reachable for me. Please help me reach the appropriate documentation for vector search in cosmos DB.
Amira Bedhiafi 42,846 Reputation points MVP Volunteer Moderator

2026-05-26T16:33:40.3933333+00:00

Hi ! Yes , $regex is supported in filtered vector search but having an index on the field does not guarantee good performance.

Filters can use supported operators including $lt, $lte, $eq, $neq, $gte, $gt, $in, $nin,and $regex and that you should create a standard index on the property used in the filter in addition to the vector index.

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Having time out issue while querying COSMOS DB with filter

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