DoytoQuery中关于N+1查询问题解决方案详解

List<Item> items = em.createQuery("select i from Item i").getResultList();
// select * from ITEM
for (Item item : items) {
    assertTrue(item.getBids().size() > 0);
    // select * from BID where ITEM_ID = ?
}

SELECT * FROM item;
SELECT * FROM bid WHERE item_id = ?;
SELECT * FROM bid WHERE item_id = ?;
SELECT * FROM bid WHERE item_id = ?;
SELECT * FROM bid WHERE item_id = ?;

SELECT * FROM item;
SELECT ? AS item_id, b.* FROM bid b WHERE item_id = ? UNION ALL
SELECT ? AS item_id, b.* FROM bid b WHERE item_id = ? UNION ALL
SELECT ? AS item_id, b.* FROM bid b WHERE item_id = ? UNION ALL
SELECT ? AS item_id, b.* FROM bid b WHERE item_id = ?;

SELECT * FROM bid;
SELECT ? AS bid_id, i.* FROM item i WHERE id IN (SELECT item_id FROM bid WHERE id = ?) UNION ALL
SELECT ? AS bid_id, i.* FROM item i WHERE id IN (SELECT item_id FROM bid WHERE id = ?) UNION ALL
SELECT ? AS bid_id, i.* FROM item i WHERE id IN (SELECT item_id FROM bid WHERE id = ?) UNION ALL
SELECT ? AS bid_id, i.* FROM item i WHERE id IN (SELECT item_id FROM bid WHERE id = ?);

@Target(FIELD)
@Retention(RUNTIME)
public @interface DomainPath {
    String[] value();
    String localField() default "id";
    String foreignField() default "id";
}

@Getter
@Setter
public class ItemView extends AbstractPersistable<Integer> {
    // other fields in Item
    // one-to-many
    // SELECT ? AS MAIN_ENTITY_ID, b.*
    //              FROM bid b           WHERE item_id = ? [UNION ALL ...]
    @DomainPath(value = "bid", foreignField = "item_id")
    private List<BidView> bids;
}
@Getter
@Setter
public class BidView extends AbstractPersistable<Integer> {
    // other fields in Bid
    // many-to-one
    // SELECT ? AS MAIN_ENTITY_ID, i.*
    //              FROM item i           WHERE id      IN (SELECT item_id FROM bid WHERE id = ?)  [UNION ALL ...]
    @DomainPath(value = "item", foreignField = "id", localField = "item_id")
    private ItemView item;
}  

@Getter
@Setter
public class ItemView extends AbstractPersistable<Integer> {
    // other fields in Item
    // many-to-many
    @DomainPath({"item", "~", "category"})
    private List<ItemView> items;
}
@Getter
@Setter
public class CategoryView extends AbstractPersistable<Integer> {
    // other fields in Category
    // many-to-many
    @DomainPath({"category", "item"})
    private List<ItemView> items;
}