Postgres Lateral Join Multiple Tables to Limit Results

Question

I have a question regarding lateral joins in Postgres.

My use case is I want to return a dataset that combines multiple tables but limits the number of publications and reviews returned. The simplified table schema is below

Table Author

• ID
• NAME

Table Review

• ID
• AUTHOR_ID
• PUBLICATION_ID
• CONTENT

Table Publication

• ID
• NAME

Table AuthorPublication

• AUTHOR_ID
• PUBLICATION_ID

So for my initial query I have this:

SELECT
  a.id,
  a.name
    json_agg (
      json_build_object (
        'id', r.id,
        'content', r.content 
       )
    ) AS reviews,
    json_agg (
      json_build_object(
        'id', p.id, 
        'name', p.name
        )
    ) AS publications
FROM
  public.author a
INNER JOIN
  public.review r ON r.author_id = a.id
INNER JOIN
  public.author_publication ap ON ap.author_id = a.id 
INNER JOIN 
  public.publication p ON p.id = ap.publication_id
WHERE 
  a.id = '1'
GROUP BY
  a.id

This returns the data I need, for example I get the author's name, id and a list of all of their reviews and publications they belong to. What I want to be able to do is limit the number of reviews and publications. For example return 5 reviews, and 3 publications.

I tried doing this with a lateral query but am running into an issue where if I do a single lateral query it works as intended.

so like:

INNER JOIN LATERAL
 (SELECT r.* FROM public.review r WHERE r.author_id = a.id LIMIT 5) r ON TRUE

This returns the dataset with only 5 reviews - but if I add a second lateral query

INNER JOIN LATERAL
 (SELECT ap.* FROM public.author_publication ap WHERE ap.author_id = a.id LIMIT 5) r ON TRUE

I now get 25 results for both reviews and publications with repeated/duplicated data.

So my question is are you allowed to have multiple lateral joins in a single PG query and if not what is a good way to go about limiting the number of results from a JOIN?

Thanks!

Answer 1

You must change your query to something like this:

SELECT
  a.id,
  a.name,
  (
    SELECT
        json_agg ( r )
      FROM (
             SELECT 
                  json_build_object (
                      'id', r.id,
                      'content', r.content 
                  ) AS r
               FROM public.review r 
              WHERE r.author_id = a.id
              ORDER BY r.id
              LIMIT 5
           ) AS a
  ) AS reviews,
  (
   SELECT
        json_agg (p)
     FROM (
            SELECT
                 json_build_object(
                    'id', p.id, 
                     'name', p.name
                 ) AS p
              FROM public.author_publication ap 
             INNER JOIN public.publication p ON p.id = ap.publication_id
             WHERE ap.author_id = a.id
             ORDER BY p.id
             LIMIT 3
        ) AS a
   ) AS publications
FROM
  public.author a
WHERE 
  a.id = '1'