This how it works. cc @deepkaran.salooja
scatter gather is only partition index and after partition elimination if more than 1 partition involved.
If you have 2 indexer nodes,
partition index of 8 partitions (it will distribute partitions in two nodes based on space etc partition placement algorithm)
non-partition index it become 1 partition on one node
When query executes based on predicates, Indexer client does partition elimination (for partition index only) mostly these for equality predicates on partition keys (not range predicates). After partition elimination, it issues scan on all partitions left on the corresponding nodes.
Example: node 1 has 2 partition left, node 2 one partition left
node 1 does 2 scans (Those might done in parallel) and combine results and sent to indexer client, node 2 has one partition results directly sent to indexer client.
Then indexer client again combine results if more than 1 node involved and passed to query service.
As non partitioned index there is one node one partition no scatter gather.
See Solution it explained