ORM allows applications to query and manipulate data in a database using an object-oriented paradigm. However, ORM can lead to performance issues due to "greedy fetching" where unnecessary joins are performed. It is better to write custom queries using aggregation functions to retrieve data from the database in one query and return it without mapping to objects to improve performance. ORM built-in functions should be avoided in favor of writing custom queries with joins and groups to control the queries issued to the database.

1. ORM and..
Bad habits
Patryk Szlagowski

2. Patryk Szlagowski
Backend Developer at Logisfera Nova - lsn.io
@abdulklarapl
Captain at FunkyMonkeyLabs

3. 1. ORM - what is it?
2. Problem 1 - greedy fetching
3. Problem 2 - „lets map everything”
4. Performance table
5. Conclusions
6. Conclusions - moar

4. 1. ORM
In one picture
ORMDB applicationDBAL*

5. /**
* @Route("/", name="homepage")
*/
public function indexAction(Request $request)
{
return $this->render('default/index.html.twig', array(
'news' => $this->getDoctrine()
->getRepository('AppBundle:Post')
->findAll()
));
}
2. Problem 1 - greedy fetching

6. /**
* @return Post[]
*/
public function getAll()
{
$qb = $this->createQueryBuilder("post");
$qb
->select("post, author, role,
category, comments,
commentsAuthor,
commentsAuthorRole")
->join("post.category", "category")
->join("post.author", "author")
->join("author.role", "role")
->join("post.comments", "comments")
->join("comments.author", "commentsAuthor")
->join("commentsAuthor.role", "commentsAuthorRole");
return $qb->getQuery()->getResult();
}
That is nice. But wait.. How is possible, that results can be
mapped to relations from this?
Let’s add custom method in repository and control the joins
2. Problem 1 - greedy fetching

7. 2. Problem 1 - greedy fetching

8. SELECT {fields}
FROM post p0_
INNER JOIN dictionary_item d1_ ON p0_.category_id = d1_.id
INNER JOIN person p2_ ON p0_.author_id = p2_.id
INNER JOIN dictionary_item d3_ ON p2_.role_id = d3_.id
INNER JOIN COMMENT c4_ ON p0_.id = c4_.post_id
INNER JOIN person p5_ ON c4_.author_id = p5_.id
INNER JOIN dictionary_item d6_ ON p5_.role_id = d6_.id
That is I expected. 3 rows for 1 post (3 comments). ORM mapped columns to many objects,
grouped by primary key.
But let’s imagine, what if we have 100 records (A) with relation to B (10 records each) AND B-C
(with another 10 records for each B)? 10000 records to map.
2. Problem 1 - greedy fetching
What’s going on?

9. What if..
I tell you, that you can get all of the rows by one query, without multiply results rows number?
IMPOSSIBRU

10. Cons
• PostgreSQL
• JSON functions
• complicated query
• slower than simple
query
Pros
• JSON response
• whole object in one row
• what you see in results,
it’s what you get in app
SELECT array_to_json(
array_agg(
row_to_json({alias})
)
)
FROM (
{subquery}
) AS {alias}
SELECT array_to_json(array_agg(row_to_json(results)))
FROM (
SELECT
post.id, post.create_date, post.content, post.title,
(SELECT row_to_json(author)
FROM (
SELECT
person.*,
(
SELECT row_to_json(role)
FROM (
SELECT * FROM dictionary_item di WHERE di.id = person.role_id
) AS role
) AS role
FROM person WHERE person.id = post.author_id
) as author) AS author,
(
SELECT row_to_json(category)
FROM dictionary_item category WHERE category.id = post.category_id
) as category,
(
SELECT array_to_json(array_agg(row_to_json(comments)))
FROM (
SELECT comment.title, comment.content, (
SELECT row_to_json(comment_author)
FROM (
SELECT person.*, (
SELECT row_to_json(role)
FROM (
SELECT *
FROM dictionary_item di
WHERE di.id = person.role_id
) AS role
) AS role
FROM person
WHERE person.id = comment.author_id
) AS comment_author
) AS author
FROM comment where comment.post_id = post.id
) AS comments
) as comments
FROM post
) AS results

11. 3. Problem 2 - results mapping
ORMDataSource
response
application
model
REST
response
In most cases
for what?

12. 3. Problem 2 - results mapping
DataSource
response
array/hashmap response REST
response
In most cases

13. Built-in findAll
function
Simple query with
joins, mapped to
objects
array_to_json query,
mapped to objects
array_to_json query,
no mapping, return
in REST Api
query time 2326ms 52ms 101ms 111ms
memory usage 82MB 51MB 33.5MB 17.2MB
total (with render)
time
7464ms 2100ms 1755ms 364ms
number of queries 4405 1 1 1
4. All about performance
* tested with 400 posts, 10 comments per post, one author per comment, author per post, label per category

14. 5. Conclusions
Query builders
Query builders
$this->repository->findAll()
4405
trolololo
vs
$this->repository->customFindAll()
1

15. 1. Maybe we should start using aggregating functions to get data from DB
2. If you don’t have to to transform DB response to object - don’t do it
3. Don’t use built-in functions such as findBy, findAll, findOne, sth, sth
4. Write your own functions with joins, groups to take control over the query
5. Remember about lazy loading
6. Conclusions

16. Thank youI want to talk with you