yeah, it makes sense to summarize your raw data into minute-level rows, stored in hourly or daily segments. so when you run a query for a time range, druid will know which segments might have your data, so no reason to check in every segment.

Going back to original question, reindexing from druid data source into another, I am missing summary of metrics. All are showing as 0

Since the original ingestion was done without rollup or defining metrics, reindexing with rollup enabled did not find any metrics to aggregate, which is why it shows null/zero. Solution: In the Re-Indexing spec, the metrics from the original datasource can be specified as part of the inputSource spec, via something like:

"inputSource": {
    "type": "druid",
    "metrics": [
      "m1"        <--=============---
    ],
    "dataSource": "orig",
    "interval": "2000/3000"
  }

If using the web console, the UI presents the option to specify metrics during the "connect" step (default is empty). The "m1" metric can be added thereto generate the correct spec. After adding the "m1" metric, I'm able to query the reindexed data source and see the expected (non-zero) values for the sum_m1 column.

That would be in the ioConfig section, btw