OFFLINE UPSERT PARTITION TABLE NOT ABLE TO FIND PARTITION METADATA IN SEGMENT

I am using pinot 1.5.0. I want to update my offline table based on id column with comparisonColumn as created_timestamp and routing to strictReplicaGroup. This config is working fine for single server, but when I configured this for multiple servers. It actually not upserting rows based on id but actually appending new rows which ends up in giving multiple rows for same id.
So I find that as we have not partitioned rows based on id, it actually selects random server say server1 and inserts segment into it. Now when I again ingest same id file, it randomly selects server say server0, now because server has not stored any metadata for that id, server stores this segment, which results into multiple rows of same id.
So I added partition based on column id with Murmur function and numPartitions = 3, which used to complete the SegmentGenerationAndPushTask, but segment stays in BAD state with error like
{
"segmentName": "RAW_WITH_PARTITION1_CLIENT_UPSERT_OFFLINE_2026-04-16_2026-04-26_0_34624b1d-c101-41aa-a374-7b11052e1abf",
"serverState": {
"Server_pinot-server-2.pinot-server-headless.cp-pinot.svc.cluster.local_8098": {
"idealState": "ONLINE",
"externalView": "ERROR",
"segmentSize": "0 bytes",
"consumerInfo": null,
"errorInfo": {
"timestamp": "2026-05-15 08:29:16 GMT",
"errorMessage": "Caught exception while adding ONLINE segment",
"stackTrace": "java.lang.NullPointerException: Failed to get partition id for segment: RAW_WITH_PARTITION1_CLIENT_UPSERT_OFFLINE_2026-04-16_2026-04-26_0_34624b1d-c101-41aa-a374-7b11052e1abf (upsert-enabled table: RAW_WITH_PARTITION1_CLIENT_UPSERT_OFFLINE). Segment must follow a naming convention that encodes partition id (e.g. LLCSegmentName, UploadedRealtimeSegmentName), or have partition metadata configured via SegmentPartitionConfig ...

and metadata of this segment is below:
{
"segment.start.time": "1776297600000",
"segment.time.unit": "MILLISECONDS",
"segment.size.in.bytes": "188134",
"segment.end.time": "1777161600000",
"segment.total.docs": "5000",
"segment.creation.time": "1778832621470",
"segment.push.time": "1778832621499",
"segment.end.time.raw": "1777161600000",
"segment.start.time.raw": "1776297600000",
"segment.index.version": "v3",
"segment.data.crc": "334204552",
"custom.map": "{\"input.data.file.uri\":\"s3://pinot-dch-test/tims/with_primary_key/cdr_2026-04-26.csv\"}",
"segment.crc": "2602756192",
"segment.partition.metadata": "{\"columnPartitionMap\":{\"client_pmn\":{\"partitions\":[0,1,2],\"functionName\":\"Murmur\",\"numPartitions\":3,\"functionConfig\":null}}}",
"segment.download.url": "http://pinot-controller-0.pinot-controller-headless.cp-pinot.svc.cluster.local:9000/segments/RAW_WITH_PARTITION1_CLIENT_UPSERT/RAW_WITH_PARTITION1_CLIENT_UPSERT_OFFLINE_2026-04-16_2026-04-26_0_34624b1d-c101-41aa-a374-7b11052e1abf"
}
**Pinot OFFLINE Upsert Partitioning Issue**
Using Apache Pinot 1.5.0 with batch ingestion via SegmentGenerationAndPushTask.
I observed different behavior between:
Normal OFFLINE partitioned table
OFFLINE upsert-enabled partitioned table

**Case 1 — Normal OFFLINE Partitioned Table (Without Upsert) (Works)**
Configuration:
partition column = client_pmn
numPartitions = 3
Generated segment metadata:
"partitions":[0,1,2]
Segment loads successfully and table works fine.
This indicates:
multi-partition segments are valid for normal OFFLINE tables
partition metadata generation works correctly

**Case 2 — OFFLINE Upsert Table (Fails)**
Configuration:
same partitioning setup
added:
upsertConfig
strictReplicaGroup
Generated segment metadata:
"partitions":[0,1]
Segment fails to load with:
Failed to get partition id for segment
Caught exception while adding ONLINE segment

**My understanding is:**
normal OFFLINE tables allow segments spanning multiple partitions
OFFLINE upsert may require:
for deterministic partition ownership with strictReplicaGroup

**Additional Observation**
prePartition=true appears to add partition metadata but does NOT physically isolate rows into partition-specific segments during SegmentGenerationAndPushTask.

**Question**
Is this expected behavior/limitation for OFFLINE upsert tables?
If yes, is external preprocessing (e.g. Spark repartition by hash(primaryKey)%N) currently the recommended approach for scalable OFFLINE upsert batch ingestion?
one segment == one partition

Start with the reported SegmentGenerationAndPushTask behavior and the SegmentPartitionConfig metadata, then trace the offline upsert segment-loading path around the reported “Failed to get partition id” error. Compare normal offline partitioned loading with strictReplicaGroup upsert loading and reproduce the configuration on Pinot 1.5.0; done means establishing the supported behavior and recording either a verified fix or a clear limitation and workaround.

Written by the indexing model from the issue text.