path: root/select.go

// SPDX-FileCopyrightText: 2026 Stefan Majewsky <majewsky@gmx.net>
// SPDX-License-Identifier: Apache-2.0

package oblast

import (
	"context"
	"database/sql"
	"errors"
	"fmt"
	"reflect"

	. "go.xyrillian.de/gg/option"
	"go.xyrillian.de/oblast/handle"
)

// Select executes the provided SQL query and fills an instance of the record type R for each row in the result set,
// according to the column names reported by the database as part of the result set.
//
// An error is returned if any column name in the result set does not correspond to an addressable field in R.
func (s Store[R]) Select(ctx context.Context, db Handle, query string, args ...any) ([]R, error) {
	// NOTE: This function body should be as short as possible to reduce the binary size after monomorphization.
	//       Any expression that does not depend on type R should be factored out into a reusable function.

	rows, indexes, err := startSelectQuery(ctx, db, s.plan, query, args...)
	if err != nil {
		return nil, err
	}

	var result []R
	slots := make([]any, len(indexes))
	for rows.Next() {
		var target *R
		result, target = growRecordSlice(result)
		err = collectRow(rows, s.plan, reflect.ValueOf(target).Elem(), slots, indexes)
		if err != nil {
			return nil, err
		}
	}

	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:
//   - It is more efficient because the strategy for loading result rows into the record type R has already been precomputed during [NewStore],
//     whereas a regular [Store.Select] must inspect the column names in the result set for each [Store.Select] call.
//   - For record types that contain only some of the columns of the corresponding database table,
//     the autogenerated SELECT query will only load exactly the necessary fields and nothing else.
//
// partialQuery is implied to start right after the WHERE keyword, which is added automatically.
// To select all records unconditionally, provide a partialQuery of "TRUE", leading to a full query of "SELECT ... FROM ... WHERE TRUE".
// Besides a condition for the WHERE clause, it may contain additional clauses, such as ORDER BY or LIMIT.
//
// Returns an error if [NewStore] was called without the [TableNameIs] option, which is required to generate a query for this method.
func (s Store[R]) SelectWhere(ctx context.Context, db Handle, partialQuery string, args ...any) ([]R, error) {
	// NOTE: This function body should be as short as possible to reduce the binary size after monomorphization.
	//       Any expression that does not depend on type R should be factored out into a reusable function.

	rows, indexes, err := startSelectWhereQuery(ctx, db, s.plan, partialQuery, args...)
	if err != nil {
		return nil, err
	}

	var result []R
	slots := make([]any, len(indexes))
	for rows.Next() {
		var target *R
		result, target = growRecordSlice(result)
		err = collectRow(rows, s.plan, reflect.ValueOf(target).Elem(), slots, indexes)
		if err != nil {
			return nil, err
		}
	}

	return result, newIOError(err, "Rows.Err", rows.Err())
}

func startSelectQuery(ctx context.Context, db Handle, plan plan, query string, args ...any) (handle.Rows, [][]int, error) {
	rows, err := db.Query(ctx, query, args)
	if err != nil {
		return nil, nil, fmt.Errorf("during Query(): %w", err)
	}

	columnNames, err := rows.Columns()
	if err != nil {
		err = fmt.Errorf("during rows.Columns(): %w", err)
		return nil, nil, newIOError(err, "Rows.Close", rows.Close())
	}
	indexes := make([][]int, len(columnNames))
	for idx, columnName := range columnNames {
		var ok bool
		indexes[idx], ok = plan.IndexByColumnName[columnName]
		if !ok {
			err := fmt.Errorf(
				"result has column %q in position %d, but no field in type %s has `db:%[1]q`",
				columnName, idx, plan.TypeName,
			)
			return nil, nil, newIOError(err, "Rows.Close", rows.Close())
		}
	}

	return rows, indexes, nil
}

func startSelectWhereQuery(ctx context.Context, db Handle, plan plan, partialQuery string, args ...any) (rows handle.Rows, indexes [][]int, err error) {
	if plan.Select.Query == "" {
		return nil, nil, errors.New("cannot execute SelectWhere() because query could not be autogenerated")
	}
	query := plan.Select.Query + partialQuery
	rows, err = db.Query(ctx, query, args)
	if err != nil {
		err = fmt.Errorf("during Query(): %w", err)
	}
	return rows, plan.Select.ScanIndexes, err
}

func collectRow(rows handle.Rows, plan plan, v reflect.Value, slots []any, indexes [][]int) error {
	for _, field := range plan.TransparentPointerStructFields {
		f := v.FieldByIndex(field.Index)
		f.Set(reflect.New(f.Type().Elem()))
	}
	for idx, index := range indexes {
		slots[idx] = v.FieldByIndex(index).Addr().Interface()
	}
	err := rows.Scan(slots...)
	if err != nil {
		return newIOError(err, "Rows.Close", rows.Close())
	}
	return nil
}

// SelectOne executes the provided SQL query and fills an instance of the record type R if there is exactly one row in the result set,
// according to the column names reported by the database as part of the result set.
//
// If there are no rows in the result set, [sql.ErrNoRows] is returned.
//
// Warning: Because of limitations in the interface of database/sql, this function is built on [Store.Select] and cannot be any faster than it.
// For maximum performance, use [Store.SelectOneWhere] which avoids the overhead of potentially having to read multiple rows.
func (s Store[R]) SelectOne(ctx context.Context, db Handle, query string, args ...any) (R, error) {
	// NOTE: This function body should be as short as possible to reduce the binary size after monomorphization.
	//       Any expression that does not depend on type R should be factored out into a reusable function.
	//
	// NOTE: The "limitation in the interface of database/sql" is that type sql.Row does not have the Columns() method,
	//       which we need when mapping result columns to struct fields for user-provided queries.

	results, err := s.Select(ctx, db, query, args...)
	switch {
	case err != nil:
		var zero R
		return zero, err
	case len(results) == 0:
		var zero R
		return zero, sql.ErrNoRows
	default:
		return results[0], nil
	}
}

// SelectOneOrNone is like SelectOne, but returns [None] instead of [sql.ErrNoRows].
func (s Store[R]) SelectOneOrNone(ctx context.Context, db Handle, query string, args ...any) (Option[R], error) {
	// NOTE: This function body should be as short as possible to reduce the binary size after monomorphization.
	//       Any expression that does not depend on type R should be factored out into a reusable function.

	results, err := s.Select(ctx, db, query, args...)
	switch {
	case err != nil:
		return None[R](), err
	case len(results) == 0:
		return None[R](), nil
	default:
		return Some(results[0]), nil
	}
}

// SelectOneWhere is like [Store.SelectOne], but you only provide the part of the SELECT query that comes after the WHERE.
// See [Store.SelectWhere] for an explanation of how the full query is constructed from this partial query.
//
// This method is more efficient than [Store.SelectOne] on CPU runtime, but has a slight memory allocation overhead per call from query preparation.
// This can be avoided by using [Store.PrepareSelectQueryWhere] instead.
func (s Store[R]) SelectOneWhere(ctx context.Context, db Handle, partialQuery string, args ...any) (R, error) {
	// NOTE: This function body should be as short as possible to reduce the binary size after monomorphization.
	//       Any expression that does not depend on type R should be factored out into a reusable function.

	var result R
	err := selectOneWhere(ctx, db, s.plan, reflect.ValueOf(&result).Elem(), partialQuery, args)
	return result, err
}

// SelectOneOrNoneWhere is like SelectOneWhere, but returns [None] instead of [sql.ErrNoRows].
func (s Store[R]) SelectOneOrNoneWhere(ctx context.Context, db Handle, partialQuery string, args ...any) (Option[R], error) {
	// NOTE: This function body should be as short as possible to reduce the binary size after monomorphization.
	//       Any expression that does not depend on type R should be factored out into a reusable function.

	return noRowsToNone(s.SelectOneWhere(ctx, db, partialQuery, args...))
}

func selectOneWhere(ctx context.Context, db Handle, plan plan, v reflect.Value, partialQuery string, args []any) error {
	if plan.Select.Query == "" {
		return errors.New("cannot execute SelectOneWhere() because query could not be autogenerated")
	}
	query := plan.Select.Query + partialQuery
	return selectOne(ctx, db, plan, v, query, args)
}

func selectOne(ctx context.Context, db Handle, plan plan, v reflect.Value, query string, args []any) error {
	for _, field := range plan.TransparentPointerStructFields {
		f := v.FieldByIndex(field.Index)
		f.Set(reflect.New(f.Type().Elem()))
	}
	slots := make([]any, len(plan.Select.ScanIndexes))
	for idx, index := range plan.Select.ScanIndexes {
		slots[idx] = v.FieldByIndex(index).Addr().Interface()
	}
	stmt, err := db.Prepare(ctx, query, false)
	if err != nil {
		return err
	}
	err = stmt.QueryRow(ctx, args, slots)
	return newIOError(err, "Stmt.Close", stmt.Close())
}

func noRowsToNone[R any](record R, err error) (Option[R], error) {
	switch {
	case err == nil:
		return Some(record), nil
	case errors.Is(err, sql.ErrNoRows):
		return None[R](), nil
	default:
		return None[R](), err
	}
}

// PrepareSelectQueryWhere performs the same query string preparation as [Store.SelectWhere] or [Store.SelectOneWhere].
// The resulting query can then be executed multiple times without incurring repeated memory allocation overhead from this preparation step.
func (s Store[R]) PrepareSelectQueryWhere(partialQuery string) (PreparedSelectQuery[R], error) {
	// NOTE: This function body should be as short as possible to reduce the binary size after monomorphization.
	//       Any expression that does not depend on type R should be factored out into a reusable function.

	query, err := prepareSelectQueryWhere(s.plan, partialQuery)
	return PreparedSelectQuery[R]{s, query}, err
}

// MustPrepareSelectQueryWhere is like [Store.PrepareSelectQueryWhere], but panics on error.
func (s Store[R]) MustPrepareSelectQueryWhere(partialQuery string) PreparedSelectQuery[R] {
	q, err := s.PrepareSelectQueryWhere(partialQuery)
	if err != nil {
		panic(err.Error())
	}
	return q
}

func prepareSelectQueryWhere(plan plan, partialQuery string) (string, error) {
	if plan.Select.Query == "" {
		return "", errors.New("cannot execute PrepareSelectQueryWhere() because query could not be autogenerated")
	}
	return plan.Select.Query + partialQuery, nil
}

// PreparedSelectQuery holds a pre-computed SELECT query that was customized by the user.
// This type is an optimization to avoid performing the same query string manipulations over and over again in hot paths.
//
// It is returned by [Store.PrepareSelectQueryWhere].
type PreparedSelectQuery[R any] struct {
	store Store[R]
	query string
}

// Select behaves the same as [Store.SelectWhere], but uses the query that was precomputed when q was constructed.
func (q PreparedSelectQuery[R]) Select(ctx context.Context, db Handle, args ...any) ([]R, error) {
	// NOTE: This function body should be as short as possible to reduce the binary size after monomorphization.
	//       Any expression that does not depend on type R should be factored out into a reusable function.

	rows, indexes, err := startSelectQuery(ctx, db, q.store.plan, q.query, args...)
	if err != nil {
		return nil, err
	}

	var result []R
	slots := make([]any, len(indexes))
	for rows.Next() {
		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
		}
	}

	return result, newIOError(err, "Rows.Err", rows.Err())
}

// SelectOne behaves the same as [Store.SelectOneWhere], but uses the query that was precomputed when q was constructed.
func (q PreparedSelectQuery[R]) SelectOne(ctx context.Context, db Handle, args ...any) (R, error) {
	// NOTE: This function body should be as short as possible to reduce the binary size after monomorphization.
	//       Any expression that does not depend on type R should be factored out into a reusable function.

	var result R
	err := selectOne(ctx, db, q.store.plan, reflect.ValueOf(&result).Elem(), q.query, args)
	return result, err
}

// SelectOneOrNone is like SelectOne, but returns [None] instead of [sql.ErrNoRows].
func (q PreparedSelectQuery[R]) SelectOneOrNone(ctx context.Context, db Handle, args ...any) (Option[R], error) {
	return noRowsToNone(q.SelectOne(ctx, db, args...))
}