diff options
Diffstat (limited to 'select.go')
| -rw-r--r-- | select.go | 45 |
1 files changed, 36 insertions, 9 deletions
@@ -27,17 +27,44 @@ func (s Store[R]) Select(ctx context.Context, db Handle, query string, args ...a var result []R slots := make([]any, len(indexes)) for rows.Next() { - var target R - err = collectRow(rows, s.plan, reflect.ValueOf(&target).Elem(), slots, indexes) + var target *R + result, target = growRecordSlice(result) + err = collectRow(rows, s.plan, reflect.ValueOf(target).Elem(), slots, indexes) if err != nil { return nil, err } - result = append(result, target) } return result, newIOError(err, "Rows.Err", rows.Err()) } +// Appends an empty R to the slice and returns a pointer to it, as well as the updated slice. +// It is more efficient to write: +// +// var result []R +// for rows.Next() { +// var target *R +// result, target = growRecordSlice(result) +// doSomethingWith(rows, reflect.ValueOf(target).Elem()) +// } +// +// Instead of the more obvious: +// +// var result []R +// for rows.Next() { +// var target R +// doSomethingWith(rows, reflect.ValueOf(&target).Elem()) +// result = append(result, target) +// } +// +// In the second phrasing, `target` escapes to the heap because of `reflect.ValueOf(&target)`, +// causing an additional allocation for `target` as well as a memcpy of `target` during `append()`. +func growRecordSlice[R any](records []R) (newRecords []R, target *R) { + var zero R + newRecords = append(records, zero) + return newRecords, &newRecords[len(newRecords)-1] +} + // SelectWhere is like [Store.Select], but you only provide the part of the SELECT query that comes after the WHERE. // The initial part ("SELECT ... FROM ... WHERE") is autogenerated and prepended to partialQuery. // This has two benefits: @@ -63,12 +90,12 @@ func (s Store[R]) SelectWhere(ctx context.Context, db Handle, partialQuery strin var result []R slots := make([]any, len(indexes)) for rows.Next() { - var target R - err = collectRow(rows, s.plan, reflect.ValueOf(&target).Elem(), slots, indexes) + var target *R + result, target = growRecordSlice(result) + err = collectRow(rows, s.plan, reflect.ValueOf(target).Elem(), slots, indexes) if err != nil { return nil, err } - result = append(result, target) } return result, newIOError(err, "Rows.Err", rows.Err()) @@ -232,12 +259,12 @@ func (q PreparedSelectQuery[R]) Select(ctx context.Context, db Handle, args ...a var result []R slots := make([]any, len(indexes)) for rows.Next() { - var target R - err = collectRow(rows, q.store.plan, reflect.ValueOf(&target).Elem(), slots, indexes) + var target *R + result, target = growRecordSlice(result) + err = collectRow(rows, q.store.plan, reflect.ValueOf(target).Elem(), slots, indexes) if err != nil { return nil, err } - result = append(result, target) } return result, newIOError(err, "Rows.Err", rows.Err()) |