// An optimized join uses indexes on both sides of the join condition.
// Without indexes, the database does a nested loop full scan.

// PostgreSQL — ensure foreign key columns are indexed
// CREATE INDEX idx_posts_user_id ON posts(user_id);
// CREATE INDEX idx_comments_post_id ON comments(post_id);

// Prisma — index foreign keys in schema
// model Post {
//   userId String
//   user   User   @relation(fields: [userId], references: [id])
//   @@index([userId])
// }

// Mongoose — index reference fields
const PostSchema = new Schema({
  authorId: { type: ObjectId, ref: 'User', index: true }, // ← index here
});