How to handle count query in Spring Data JPA Pageable?

When using findPage from KotlinJdslJpqlExecutor with Spring Data JPA's Pageable, you might encounter issues with the automatically generated count query, especially with complex queries involving join and groupBy.

Spring Data JPA, by default, tries to generate a count query from your main query. However, this generated query is often incorrect for complex scenarios.

To solve this, you can provide a separate count query. KotlinJdslJpqlExecutor has a findPage method that accepts a separate count query lambda.

Here is an example:

interface BookRepository : JpaRepository<Book, Isbn>, KotlinJdslJpqlExecutor

val page: Page<Book> = bookRepository.findPage(pageable,
    { // Query
        select(
            entity(Book::class)
        ).from(
            entity(Book::class),
            join(Book::author)
        ).where(
            // ... some conditions
        ).groupBy(
            path(Book::isbn)
        )
    },
    { // Count Query
        select(
            countDistinct(path(Book::isbn))
        ).from(
            entity(Book::class),
            join(Book::author)
        ).where(
            // ... same conditions
        )
    }
)

By providing a separate, simplified count query, you can avoid the issues with the automatically generated one. The count query should return a single Long value. It's important to apply the same where conditions to the count query as you do to the main query to get the correct total count.

Note that when you have a groupBy clause in your main query, the count query should usually count the distinct values of the grouped expression, as shown in the example with countDistinct(path(Book::isbn)).