PDGGK commented on PR #18267:
URL: https://github.com/apache/iotdb/pull/18267#issuecomment-5044359222
Thanks @Wei-hao-Li — that's cleaner, adopted. I switched to routing the
empty device set to the default aggregation operator in
`DataNodeTableOperatorGenerator.visitAggregationTableScan`, and reverted the
flag in `LastQueryAggTableScanOperator` entirely (net zero there now). As you
said, the empty-process logic is already there: `AbstractAggTableScanOperator`
builds an `EMPTY_DEVICE_ID` entry for an empty device set, and the default
operator's `hasNext()` is time-range-based, so the global aggregation over
empty input still runs its time range and emits the single all-NULL row.
One edge I wanted to flag: a plain `GROUP BY` tag `last_by` also qualifies
for the last-cache path (`canUseLastCacheOptimize` only NOOPs the date_bin
case, not a plain grouping key), so an unconditional `deviceEntries.isEmpty() ?
NOOP` would reroute `GROUP BY`-over-an-empty-device-set to the default operator
too — and that case was already returning 0 rows correctly via the last-cache
op. To leave that path untouched I scoped the NOOP to both conditions:
```java
OptimizeType optimizeType =
node.getDeviceEntries().isEmpty() && node.getGroupingKeys().isEmpty()
? OptimizeType.NOOP
: canUseLastCacheOptimize(
parameter.getTableAggregators(), node,
parameter.getTimeColumnName());
```
So the global (no-GROUP-BY) empty case falls to the default op and gets the
single NULL row, while `GROUP BY` over an empty device set stays on the
last-cache path (0 rows, unchanged). Happy to use the unconditional form if
you'd prefer it. The `IoTDBDeletionTableIT` cases (never-written device → one
NULL row, `GROUP BY` over empty → no rows, deleted-device unchanged) are kept
and exercise both paths. Updated the PR — thanks for the pointer.
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