Wei-hao-Li commented on code in PR #18267:
URL: https://github.com/apache/iotdb/pull/18267#discussion_r3628407323
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iotdb-core/datanode/src/main/java/org/apache/iotdb/db/queryengine/execution/operator/source/relational/LastQueryAggTableScanOperator.java:
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@@ -75,6 +75,13 @@ public class LastQueryAggTableScanOperator extends
AbstractAggTableScanOperator
private final List<DeviceEntry> cachedDeviceEntries;
private final int allDeviceCount;
+ // A no-GROUP-BY global aggregation whose device set resolves to zero devices
+ // (e.g. last_by on a device that was never written) has nothing for the
device
+ // loop to iterate, so without this it would emit zero rows. Per SQL
semantics a
+ // global aggregate over empty input must return exactly one row whose value
is
+ // NULL (except count()); this one-shot flag drives that single row.
+ private boolean emptyGlobalResultEmitted = false;
Review Comment:
I think it is better to use default Agg operator when the DeviceEntries is
empty when generate Operator Tree. The empty process logic has been there.
Just modify one line in DataNodeTableOperatorGenerator is OK:
DataNodeTableOperatorGenerator -> visitAggregationTableScan ->
`OptimizeType optimizeType =
node.getDeviceEntries().isEmpty()
? OptimizeType.NOOP
: canUseLastCacheOptimize(
parameter.getTableAggregators(), node,
parameter.getTimeColumnName());`
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