All we need is an easy explanation of the problem, so here it is.
I have two table article and article_content in PostgreSQL, the article_id in table article_content was the article table id. Now the sql look like this:
select * from article_content ac where article_id not in( select id from article a ) limit 10
find the record in article_content that not exists in article. this is the query plain:
Limit (cost=1000.00..44996.68 rows=1 width=415) -> Gather (cost=1000.00..38955542254.16 rows=885420 width=415) Workers Planned: 2 -> Parallel Seq Scan on article_content ac (cost=0.00..38955452712.16 rows=368925 width=415) Filter: (NOT (SubPlan 1)) SubPlan 1 -> Materialize (cost=0.00..101153.51 rows=1775167 width=8) -> Seq Scan on article a (cost=0.00..85342.67 rows=1775167 width=8)
now the table article and article_content have so many rows. seems this sql could not complete forever. what should I do to do remove the article content rows that did not exists in artcle?
How to solve :
I know you bored from this bug, So we are here to help you! Take a deep breath and look at the explanation of your problem. We have many solutions to this problem, But we recommend you to use the first method because it is tested & true method that will 100% work for you.
Typically NOT EXISTS is more efficient:
select ac.* from article_content ac where not exists (select * from article a where a.id = ac.article_id) limit 10;
An index on
article_content (article_id) will improve the performance. I assume there is already an index on
Note: Use and implement method 1 because this method fully tested our system.
Thank you 🙂