Efficient use of Query Builder

Avoiding the N+1 Query Problem

The N+1 Query Problem occurs when the system executes too many sub-queries for each row of data retrieved.

To avoid this, you can use leftJoinAndSelect or innerJoinAndSelect to combine tables in a single query instead of executing multiple queries.

const users = await dataSource
    .getRepository(User)
    .createQueryBuilder("user")
    .leftJoinAndSelect("user.posts", "post")
    .getMany()

Here, leftJoinAndSelect helps retrieve all user posts in a single query rather than many small queries.

Use getRawMany() when only raw data is needed

In cases where full objects aren't required, you can use getRawMany() to fetch raw data and avoid TypeORM processing too much information.

const rawPosts = await dataSource
    .getRepository(Post)
    .createQueryBuilder("post")
    .select("post.title, post.createdAt")
    .getRawMany()

Limit fields using select

To optimize memory usage and reduce unnecessary data, select only the required fields using select.

const users = await dataSource
    .getRepository(User)
    .createQueryBuilder("user")
    .select(["user.name", "user.email"])
    .getMany()