Query with subquery and aggregates cannot be implemented.

This query results in the error below:
```

"SELECT (COUNT(`p`))
FROM (SELECT `p`
FROM `contains`
GROUP BY `p`) AS `t1`"
```

This works correctly:
```

"SELECT (COUNT(`p`))
FROM (SELECT `p`, CURRENT_TIME
FROM `contains`
GROUP BY `p`)
AS `t1`"
```

Error:

 
```

[nREPL-worker-5] INFO org.apache.beam.sdk.extensions.sql.impl.planner.BeamQueryPlanner - SQL:
SELECT
COUNT(`t1`.`p`)
FROM (SELECT `contains`.`p`
FROM `contains` AS `contains`
GROUP BY `contains`.`p`)
AS `t1`
[nREPL-worker-5] INFO org.apache.beam.sdk.extensions.sql.impl.planner.BeamQueryPlanner - SQLPlan>
LogicalAggregate(group=[{}],
EXPR$0=[COUNT()])
LogicalAggregate(group=[{0}])
LogicalProject(p=[$0])
LogicalTableScan(table=[[contains]])

CannotPlanException
Node [rel#157:Subset#3.BEAM_LOGICAL.[]] could not be implemented; planner state:
Root: rel#157:Subset#3.BEAM_LOGICAL.[]
Original
rel:
LogicalAggregate(subset=[rel#157:Subset#3.BEAM_LOGICAL.[]], group=[{}], EXPR$0=[COUNT()]): rowcount
= 1.0, cumulative cost = {1.125 rows, 0.0 cpu, 0.0 io}, id = 155
LogicalAggregate(subset=[rel#154:Subset#2.NONE.[]],
group=[{0}]): rowcount = 10.0, cumulative cost = {10.0 rows, 0.0 cpu, 0.0 io}, id = 153
LogicalProject(subset=[rel#152:Subset#1.NONE.[]],
p=[$0]): rowcount = 100.0, cumulative cost = {100.0 rows, 100.0 cpu, 0.0 io}, id = 151
LogicalTableScan(subset=[rel#150:Subset#0.NONE.[]],
table=[[contains]]): rowcount = 100.0, cumulative cost = {100.0 rows, 101.0 cpu, 0.0 io}, id = 146
Sets:
Set#0,
type: RecordType(VARCHAR p, VARCHAR s, BIGINT c)
rel#150:Subset#0.NONE.[], best=null, importance=0.6561

rel#146:LogicalTableScan.NONE.[](table=[contains]), rowcount=100.0, cumulative cost={inf}
rel#162:Subset#0.BEAM_LOGICAL.[],
best=rel#164, importance=0.32805
rel#164:BeamIOSourceRel.BEAM_LOGICAL.[](table=[contains]), rowcount=100.0,
cumulative cost={100.0 rows, 101.0 cpu, 0.0 io}
Set#1, type: RecordType(VARCHAR p)
rel#152:Subset#1.NONE.[],
best=null, importance=0.7290000000000001
rel#151:LogicalProject.NONE.[](input=rel#150:Subset#0.NONE.[],p=$0),
rowcount=100.0, cumulative cost={inf}
rel#159:Subset#1.BEAM_LOGICAL.[], best=rel#163, importance=0.36450000000000005

rel#163:BeamProjectRel.BEAM_LOGICAL.[](input=rel#162:Subset#0.BEAM_LOGICAL.[],p=$0), rowcount=100.0,
cumulative cost={200.0 rows, 201.0 cpu, 0.0 io}
Set#2, type: RecordType(VARCHAR p)
rel#154:Subset#2.NONE.[],
best=null, importance=0.81
rel#153:LogicalAggregate.NONE.[](input=rel#152:Subset#1.NONE.[],group={0}),
rowcount=10.0, cumulative cost={inf}
rel#161:Subset#2.BEAM_LOGICAL.[], best=rel#160, importance=0.405

rel#160:BeamAggregationRel.BEAM_LOGICAL.[](group={0},window=org.apache.beam.sdk.transforms.windowing.GlobalWindows,trigger=Repeatedly.forever(AfterWatermark.pastEndOfWindow())),
rowcount=10.0, cumulative cost={210.0 rows, 201.0 cpu, 0.0 io}
Set#3, type: RecordType(BIGINT EXPR$0)

rel#156:Subset#3.NONE.[], best=null, importance=0.9
rel#155:LogicalAggregate.NONE.[](input=rel#154:Subset#2.NONE.[],group={},EXPR$0=COUNT()),
rowcount=1.0, cumulative cost={inf}
rel#157:Subset#3.BEAM_LOGICAL.[], best=null,
importance=1.0

rel#158:AbstractConverter.BEAM_LOGICAL.[](input=rel#156:Subset#3.NONE.[],convention=BEAM_LOGICAL,sort=[]),
rowcount=1.0, cumulative cost={inf}
org.apache.beam.sdks.java.extensions.sql.repackaged.org.apache.calcite.plan.volcano.RelSubset$CheapestPlanReplacer.visit
(RelSubset.java:441)

```

Imported from Jira [BEAM-3481](https://issues.apache.org/jira/browse/BEAM-3481). Original Jira may contain additional context.
Reported by: AustinHaas.

Start by reproducing the failing query through the BeamQueryPlanner and compare its SQL plan with the working variant that selects CURRENT_TIME. Trace the CannotPlanException and verify that the original subquery with GROUP BY followed by COUNT can be planned and executed successfully.

Written by the indexing model from the issue text.