Optimize memory allocations in query execution (#20)

* perf: reduce allocations in Exec, execIntoMaps and Next

This change optimizes the main query execution paths to reduce memory
allocations and improve performance.

- In `QuerySet.Exec`, the slice for scanning row values is now
  allocated only once outside the loop.
- In `execIntoMaps`, the slice for scannable values is also created
  only once.
- In `resultIterator.Next`, the slice for scan values is now part of the
  iterator struct and is reused for each row, avoiding reallocation
  on each call.

These changes significantly reduce the garbage collector pressure when
processing large result sets.

* perf: reduce allocations in Exec and Next

This change optimizes the main query execution paths to reduce memory
allocations and improve performance.

- In `QuerySet.Exec`, the slice for scanning row values is now
  allocated only once outside the loop.
- In `resultIterator.Next`, the slice for scan values is now part of the
  iterator struct and is reused for each row, avoiding reallocation
  on each call.

These changes significantly reduce the garbage collector pressure when
processing large result sets.

An incorrect optimization for `execIntoMaps` was reverted.

---------

Co-authored-by: google-labs-jules[bot] <161369871+google-labs-jules[bot]@users.noreply.github.com>

Author: google-labs-jules[bot]

iterator.go

@@ -11,6 +11,7 @@ type resultIterator[T any] struct {
 	rows             pgx.Rows
 	orderedSelectors []ColumnAccessor
 	params           []any
+	scanValues       []any
 }
 
 // Close closes the rows, making the connection ready for use again. It is safe
@@ -55,11 +56,10 @@ func (i *resultIterator[T]) HasNext() bool {
 
 func (i *resultIterator[T]) Next() (*T, error) {
 	entity := new(T)
-	toScan := []any{}
-	for _, each := range i.orderedSelectors {
-		toScan = append(toScan, each.FieldValueToScan(entity))
+	for j, each := range i.orderedSelectors {
+		i.scanValues[j] = each.FieldValueToScan(entity)
 	}
-	if err := i.rows.Scan(toScan...); err != nil {
+	if err := i.rows.Scan(i.scanValues...); err != nil {
 		return nil, err
 	}
 	return entity, nil

iterator_test.go

@@ -25,6 +25,7 @@ func TestResultIterator_Next(t *testing.T) {
 		i := &resultIterator[testEntity]{
 			rows:             rows,
 			orderedSelectors: selectors,
+			scanValues:       make([]any, len(selectors)),
 		}
 		if !i.HasNext() {
 			t.Fatal("expected next")

queryset.go

@@ -158,6 +158,7 @@ func (d QuerySet[T]) Iterate(ctx context.Context, conn querier, parameters ...*Q
 		rows:             rows,
 		orderedSelectors: ordered,
 		params:           params,
+		scanValues:       make([]any, len(ordered)),
 	}, err
 }
 
@@ -168,11 +169,11 @@ func (d QuerySet[T]) Exec(ctx context.Context, conn querier, parameters ...*Quer
 		return
 	}
 	defer rows.Close()
+	sw := make([]any, len(d.selectors))
 	for rows.Next() {
 		entity := new(T)
-		sw := []any{}
-		for _, each := range d.selectors {
-			sw = append(sw, each.FieldValueToScan(entity))
+		for i, each := range d.selectors {
+			sw[i] = each.FieldValueToScan(entity)
 		}
 		if err := rows.Scan(sw...); err != nil {
 			return list, err