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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()