Question

I am doing, in postgresql, something like this:

select A.first, 
       count(B.second) as count, 
       array_agg(A.second) as second,
       array_agg(A.third) as third, 
       array_agg(B.kids) as kids 
from A join B on A.first=B.second 
group by A.first;

And it’s taking forever (also because the tables are pretty big). Limiting the output to 10 row and looking with explain analyze told me there’s a nested loop which is huge and takes most of the time.

Is there any way in which I can write this query (which I’ll then use in CREATE TABLE AS to create a new table) to speed it up, while conserving the same output, which is what I want?

Thanks!

Answer 1

Ensure the column bring used as a foreign key is indexed:

create index b_second on b(second);

Without such an index, every row of a would cause a table scan of b, which would make your query crawl.