implicit cursor – Java, SQL and jOOQ.

A recent question on Stack Overflow about jOOQ caught my attention. The question essentially asked: Why do both of these loops work?


// With fetch()
for (MyTableRecord rec : DSL
    .using(configuration)
    .selectFrom(MY_TABLE)
    .orderBy(MY_TABLE.COLUMN)
    .fetch()) { // fetch() here

    doThingsWithRecord(rec);
}

// Without fetch()
for (MyTableRecord rec : DSL
    .using(configuration)
    .selectFrom(MY_TABLE)
    .orderBy(MY_TABLE.COLUMN)) { // No fetch() here

    doThingsWithRecord(rec);
}

And indeed, just like in PL/SQL, you can use any jOOQ ResultQuery as a Java 5 Iterable, because that’s what it is. An Iterable<R> where R extends Record. The semantics is simple. When Iterable.iterator() is invoked, the query is executed and the Result.iterator() is returned. So, the result is materialised in the client memory just like if I called fetch(). Unsurprisingly, this is the implementation of AbstractResultQuery.iterator():


@Override
public final Iterator<R> iterator() {
    return fetch().iterator();
}

No magic. But it’s great that this works like PL/SQL:


FOR rec IN (SELECT * FROM my_table ORDER BY my_table.column)
LOOP
  doThingsWithRecord(rec);
END LOOP;

Note, unfortunately, there’s no easy way to manage resources through Iterable, i.e. there’s no AutoCloseableIterable returning an AutoCloseableIterator, which could be used in an auto-closing try-with-resources style loop. This is why the entire result set needs to be fetched at the beginning of the loop. For lazy fetching, you can still use ResultQuery.fetchLazy()


try (Cursor<MyTableRecord> cursor = DSL
    .using(configuration)
    .selectFrom(MY_TABLE)
    .orderBy(MY_TABLE.COLUMN)
    .fetchLazy()) {

    for (MyTableRecord rec : cursor)
        doThingsWithRecord(rec);
}

Happy coding!

Tag: implicit cursor

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