Thread: How do I bump a row to the front of sort efficiently

I have this query:

select * from topics
order by case when id=1 then 0 else 1 end, bumped_at desc
limit 30

It works fine, bumps id 1 to the front of the sort fine but is
terribly inefficient and scans

OTH

"select * from topics where id = 1" is super fast

"select * from topics order by bumped_at desc limit 30" is super fast

Even this is fast, and logically equiv as id is primary key unique

select * from topic
where id = 1000
union all
select * from (
  select * from topics
  where id <> 1000
  order by bumped_at desc
  limit 30
) as x
limit 30


However, the contortions on the above query make it very un-ORM
friendly as I would need to define a view for it but would have no
clean way to pass limits and offsets in.

Is there any clean technique to bump up particular rows to the front
of a sort if a certain condition is met without paying a huge
performance hit?

sam.saffron wrote
> I have this query:
> 
> select * from topics
> order by case when id=1 then 0 else 1 end, bumped_at desc
> limit 30
> 
> It works fine, bumps id 1 to the front of the sort fine but is
> terribly inefficient and scans
> 
> OTH
> 
> "select * from topics where id = 1" is super fast
> 
> "select * from topics order by bumped_at desc limit 30" is super fast
> 
> Even this is fast, and logically equiv as id is primary key unique
> 
> select * from topic
> where id = 1000
> union all
> select * from (
>   select * from topics
>   where id <> 1000
>   order by bumped_at desc
>   limit 30
> ) as x
> limit 30
> 
> 
> However, the contortions on the above query make it very un-ORM
> friendly as I would need to define a view for it but would have no
> clean way to pass limits and offsets in.
> 
> Is there any clean technique to bump up particular rows to the front
> of a sort if a certain condition is met without paying a huge
> performance hit?

CREATE FUNCTION ...?

Probably with a VARIADIC argument.

David J.



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