Utilizing jOOQ’s DiagnosticsConnection to detect N+1 Queries – Java, SQL and jOOQ.

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N+1 queries are a preferred drawback in lots of functions that run SQL queries. The issue might be described simply as follows:

  • 1 question fetching a father or mother worth is run
  • N queries fetching every particular person baby values are run

This drawback isn’t restricted to SQL, it might occur with any poorly designed API that doesn’t enable for batch and/or bulk processing (even saved procedures). However with SQL, it’s significantly painful, as a result of in lots of circumstances, working tons of logic in a single question can be completely doable, particularly with jOOQ’s MULTISET and SQL/XML or SQL/JSON assist.

Within the worst case, the N+1 drawback is brought on by a 3rd get together ORM – or somewhat, its poor implementation / configuration, however some ORMs make it very easy to shoot oneself within the foot with N+1 issues…

An instance

Let’s stick to JDBC for now as an example how N+1 queries occur.

attempt (Assertion stmt = conn.createStatement()) {

    // The father or mother question, fetching actors
    attempt (ResultSet r1 = stmt.executeQuery(
        """
        SELECT actor_id, first_name, last_name
        FROM actor
        LIMIT 5
        """
    )) {
        whereas (r1.subsequent()) {
            System.out.println();
            System.out.println(
                "Actor: " + r1.getString(2) + " " + r1.getString(2));

            // The kid question, fetching movies per actor
            attempt (PreparedStatement pstmt = conn.prepareStatement(
                """
                SELECT depend(*) FROM film_actor WHERE actor_id = ?
                """
            )) {
                pstmt.setInt(1, r1.getInt(1));

                attempt (ResultSet r2 = pstmt.executeQuery()) {
                    whereas (r2.subsequent()) {
                        System.out.println("Movies: " + r2.getInt(1));
                    }
                }
            }
        }
    }
}

When run in opposition to the sakila database, the above prints:

Actor: PENELOPE PENELOPE
Movies: 19

Actor: NICK NICK
Movies: 25

Actor: ED ED
Movies: 22

Actor: JENNIFER JENNIFER
Movies: 22

Actor: JOHNNY JOHNNY
Movies: 29

Clearly appropriate, however we might have simply run this in a single question:

SELECT
  a.first_name,
  a.last_name,
  depend(fa.film_id)
FROM actor AS a
LEFT JOIN film_actor AS fa USING (actor_id)
GROUP BY a.actor_id

On condition that we have now 200 actors in whole, which do you like? Operating 1+200 queries or simply 1 question? If you happen to’re accountable for your SQL, this error is way much less more likely to occur, however what in the event you’re not in (full) management, as a result of the SQL is generated primarily based on keen/lazy loading configurations and complicated entity graph annotations, then you definately’ll be comfortable you possibly can simply plug in jOOQ’s DiagnosticsConnection’s repeated statements diagnostic into your integration take a look at setting (not essentially in manufacturing, as there’s some overhead to parse and normalise all of the SQL strings).

Utilized to the above JDBC instance:

DSLContext ctx = DSL.utilizing(connection);
ctx.configuration().set(new DefaultDiagnosticsListener() {
    @Override
    public void repeatedStatements(DiagnosticsContext c) {

        // Customized callback, might also throw exceptions, and many others.
        System.out.println(
            "Repeated assertion: " + c.normalisedStatement());
    }
});

Connection conn = ctx.diagnosticsConnection();

You’re now getting the next output:

Actor: PENELOPE PENELOPE
Movies: 19

Actor: NICK NICK
Repeated assertion: choose depend(*) from film_actor the place actor_id = ?;
Movies: 25

Actor: ED ED
Repeated assertion: choose depend(*) from film_actor the place actor_id = ?;
Movies: 22

Actor: JENNIFER JENNIFER
Repeated assertion: choose depend(*) from film_actor the place actor_id = ?;
Movies: 22

Actor: JOHNNY JOHNNY
Repeated assertion: choose depend(*) from film_actor the place actor_id = ?;
Movies: 29

As you possibly can see, the diagnostics connection begins logging after the primary repetition of the assertion, the belief being that inside a transaction, it’s typically pointless to ever repeat a press release greater than as soon as, as a result of there’s nearly at all times a greater method.

Utilizing this with JPA / Hibernate

You most likely don’t write JDBC statements manually like this, but it surely doesn’t matter who calls JDBC (you, jOOQ, JdbcTemplate, Hibernate, and many others.). If you happen to proxy your connection (or DataSource) with jOOQ’s DiagnosticsConnection or DiagnosticsDataSource, then you possibly can intercept such occasions simply, regardless of the trigger.

Future variations of jOOQ will add much more diagnostics by way of https://github.com/jOOQ/jOOQ/points/7527.

To see what’s out there in jOOQ already, discuss with the guide.

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