Question

I have a table I’m doing an ORDER BY on before a LIMIT and OFFSET in order to paginate.

Adding an index on the ORDER BY column makes a massive difference to performance (when used in combination with a small LIMIT). On a 500,000 row table, I saw a 10,000x improvement adding the index, as long as there was a small LIMIT.

However, the index has no impact for high OFFSETs (i.e. later pages in my pagination). This is understandable: a b-tree index makes it easy to iterate in order from the beginning but not to find the nth item.

It seems that what would help is a counted b-tree index, but I’m not aware of support for these in PostgreSQL. Is there another solution? It seems that optimizing for large OFFSETs (especially in pagination use-cases) isn’t that unusual.

Unfortunately, the PostgreSQL manual simply says “The rows skipped by an OFFSET clause still have to be computed inside the server; therefore a large OFFSET might be inefficient.”

Answer 1

You might want a computed index.

Let’s create a table:

create table sales(day date, amount real);

And fill it with some random stuff:

insert into sales 
    select current_date + s.a as day, random()*100 as amount
    from generate_series(1,20);

Index it by day, nothing special here:

create index sales_by_day on sales(day);

Create a row position function. There are other approaches, this one is the simplest:

create or replace function sales_pos (date) returns bigint 
   as 'select count(day) from sales where day <= $1;' 
   language sql immutable;

Check if it works (don’t call it like this on large datasets though):

select sales_pos(day), day, amount from sales;

     sales_pos |    day     |  amount  
    -----------+------------+----------
             1 | 2011-07-08 |  41.6135
             2 | 2011-07-09 |  19.0663
             3 | 2011-07-10 |  12.3715
    ..................

Now the tricky part: add another index computed on the sales_pos function values:

create index sales_by_pos on sales using btree(sales_pos(day));

Here is how you use it. 5 is your “offset”, 10 is the “limit”:

select * from sales where sales_pos(day) >= 5 and sales_pos(day) < 5+10;

        day     | amount  
    ------------+---------
     2011-07-12 | 94.3042
     2011-07-13 | 12.9532
     2011-07-14 | 74.7261
    ...............

It is fast, because when you call it like this, Postgres uses precalculated values from the index:

explain select * from sales 
  where sales_pos(day) >= 5 and sales_pos(day) < 5+10;

                                    QUERY PLAN                                
    --------------------------------------------------------------------------
     Index Scan using sales_by_pos on sales  (cost=0.50..8.77 rows=1 width=8)
       Index Cond: ((sales_pos(day) >= 5) AND (sales_pos(day) < 15))

Hope it helps.