We are seeing a severe regression in azure-cosmos Python SDK 4.16.0 for single-partition COUNT queries.

[ajmehul]

• azure-cosmos:
• 4.16.0 and above:
• Windows 11:
• 3.14.5:

We are seeing a severe regression in azure-cosmos Python SDK 4.16.0 for single-partition COUNT queries.

Container:

• Container id: power_generation
• Partition key path: /documentType
• Partition key kind: Hash

Query:
SELECT VALUE COUNT(1) FROM c

SDK call:
query_items(
query="SELECT VALUE COUNT(1) FROM c",
partition_key="PowerCut",
enable_cross_partition_query=False,
populate_query_metrics=True
)

Results with azure-cosmos==4.15.0:

• Count: 30,735,986
• Elapsed: 0.049 sec
• Pages: 1
• RU: 8.82
• Continuation: None
• retrievedDocumentCount: 0
• retrievedDocumentSize: 0
• indexUtilizationRatio: 1.00
• Activity ID: 0899ada3-21d3-4273-9f83-ae075b91f69c

Results with azure-cosmos==4.16.0:

• Count: 30,725,812
• Elapsed: 191.923 sec
• Pages: 49
• Total RU: 506,961.75
• Continuation present for pages 1-48
• retrievedDocumentCount: ~626k per page
• retrievedDocumentSize: ~168 MB per page
• indexUtilizationRatio: 0.00
• Activity ID: 8a09c368-ce86-4fea-bd41-5240d24cbce6

Expected:
The query should remain an index-only count within the supplied logical partition, as in 4.15.0.

Actual:
4.16.0 scans/loads documents page-by-page and consumes extremely high RU.

[github-actions]

Thank you for your feedback. Tagging and routing to the team member(s) best able to assist.

//cc: @AbhinavTrips @bambriz @Pilchie @pjohari-ms @simorenoh

[github-actions]

🎯 Agentic Issue Triage

Summary: A severe performance regression was introduced in azure-cosmos SDK 4.16.0 where a single-partition COUNT query that previously completed in ~0.05s with 8.82 RU (index-only) now performs a full document scan taking ~192s with 506,961 RU across 49 pages.

📋 Issue Details

Package: azure-cosmos 4.16.0 (regression from 4.15.0)
Affected API: container.query_items() with partition_key and enable_cross_partition_query=False
Scenarios:

• SELECT VALUE COUNT(1) FROM c scoped to a single logical partition key (PowerCut)
• In 4.15.0: index-only count, 1 page, 8.82 RU, 0.049s
• In 4.16.0: full document scan, 49 pages, ~506k RU, 192s, indexUtilizationRatio drops from 1.00 to 0.00

Root ask: Restore the index-only COUNT behavior for single-partition queries that was present in 4.15.0.

🔎 Debugging / Reproduction Notes

The indexUtilizationRatio dropping from 1.00 to 0.00 between versions is the key diagnostic signal — the query is no longer using the index in 4.16.0. The continuation token being present for pages 1–48 and retrievedDocumentCount jumping to ~626k per page confirms a full scan is happening.

Suggested investigation steps:

1. Review changes in azure-cosmos between 4.15.0 and 4.16.0 related to query execution / partition routing logic.
2. Check if populate_query_metrics=True or enable_cross_partition_query=False handling changed in 4.16.0.
3. Reproduce with the exact query and SDK call shown in the issue against a similarly sized container.
4. Compare the request headers/options sent to the backend between the two SDK versions (especially query headers that influence index usage).

🏷️ Label Confidence

• Category: Client — azure-cosmos is a data-plane client library (not azure-mgmt-*)
• Service: Cosmos — issue explicitly references azure-cosmos package under /sdk/cosmos/
• Confidence: High — package and service are unambiguous; regression is clearly scoped to the data-plane SDK

👥 Owner Routing

• Matched CODEOWNERS entry: # ServiceLabel: %Cosmos %Service Attention (line 512) — matched after ignoring the %Service Attention token; Cosmos equals the predicted service label
• AzureSdkOwners: none listed
• ServiceOwners: @AbhinavTrips @bambriz @pilchie @pjohari-ms @simorenoh
• Routing action: Added "Service Attention" and "needs-team-attention" labels; ServiceOwners mentioned via routing comment; issue left unassigned (no AzureSdkOwners in matched entry)
• Scan notes: Scanned bottom-to-top; line 512 was the first matching entry encountered (closer to end of file than line 191 # ServiceLabel: %Cosmos)

Generated by Agentic Triage for issue #48209 · 21.7 AIC · ⌖ 5.92 AIC · ⊞ 9.6K · ◷

[Pilchie]

@dibahlfi can you take a look please?

[dibahlfi]

@ajmehul - you are right its a regression. we are working on a fix and will update once the release is out.