S3 Select: Aggregate AVG/SUM as float (#8326)

Force sum/average to be calculated as a float.

As noted in #8221

> run SELECT AVG(CAST (Score as int)) FROM S3Object on

```
Name,Score
alice,80
bob,81
```

> AWS S3 gives 80.5 and MinIO gives 80.

This also makes overflows much more unlikely.

pkg/s3select/sql/aggregation.go

@@ -49,7 +49,7 @@ type aggVal struct {
 func newAggVal(fn FuncName) *aggVal {
 	switch fn {
 	case aggFnAvg, aggFnSum:
-		return &aggVal{runningSum: FromInt(0)}
+		return &aggVal{runningSum: FromFloat(0)}
 	case aggFnMin:
 		return &aggVal{runningMin: FromInt(0)}
 	case aggFnMax:
@@ -126,8 +126,14 @@ func (e *FuncExpr) evalAggregationNode(r Record) error {
 		// For all non-null values, the count is incremented.
 		e.aggregate.runningCount++
 
-	case aggFnAvg:
+	case aggFnAvg, aggFnSum:
 		e.aggregate.runningCount++
+		// Convert to float.
+		f, ok := argVal.ToFloat()
+		if !ok {
+			return fmt.Errorf("Could not convert value %v (%s) to a number", argVal.value, argVal.GetTypeString())
+		}
+		argVal.setFloat(f)
 		err = e.aggregate.runningSum.arithOp(opPlus, argVal)
 
 	case aggFnMin:
@@ -136,9 +142,6 @@ func (e *FuncExpr) evalAggregationNode(r Record) error {
 	case aggFnMax:
 		err = e.aggregate.runningMax.minmax(argVal, true, isFirstRow)
 
-	case aggFnSum:
-		err = e.aggregate.runningSum.arithOp(opPlus, argVal)
-
 	default:
 		err = errInvalidAggregation
 	}