How to implement keyset based pagination in postgres?

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Imagine you have one million record in a table, and you need to display them in a web page. Would you fetch 1 million records in a one shot and send it through an API? Probably not. Offset based pagination is one of the most common technique for fetching small chunk of data for one page at a time. But it has its own performance bottlenecks. checkout my previous article to know more about offset based pagination https://sureshdsk.dev/offset-based-pagination-in-postgres

In this article, we will explore about keyset based pagination. This method does not use count or offset clauses.

How it works

  • Unique auto increment column is used as a key column which is mostly primary key.
  • This key column is sorted so that it will produce consistent results.
  • Instead using offset to skip records, it used ketset_column > last record pk from previous page condition is used
  • by using ketset_column in where clause, the database does index only scan to skip records because primary key has an index by default.

Postgres keyset based pagination example

-- to fetch each page
select * from table_name where {{keyset_column}} > {{keyset_value}}  order by {{keyset_column}} LIMIT {{per_page}};

-- fetch first page
select * from employee order by emp_id LIMIT 100;

-- fetch second page
-- first 100 records are skipped based on index only scan
select * from employee where emp_id > 100 LIMIT 100;

-- fetch third page
-- first 200 records are skipped based on index only scan
select * from employee where emp_id > 200 LIMIT 100;

Analyze query plan

Lets take a look at the query plan for this keyset based pagination,

explain analyze select * from employee where emp_id > 200 LIMIT 100;

-- index scan is done on employee table to fetch the results
-- index scan is faster than seq scan
Limit  (cost=200.76..210.80 rows=100 width=930) (actual time=0.502..0.530 rows=100 loops=1)
  ->  Index Scan on employee  (cost=0.00..1878.10 rows=18710 width=930) (actual time=0.010..0.445 rows=2100 loops=1)
         Index Cond: (emp_id > 200)
Planning Time: 0.073 ms
Execution Time: 0.550 ms

Pros

  • Faster than offset based pagination
  • No expensive count query and offsets
  • Pagination uses index only scan instead of sequential table scan
  • Inserting/deleting a row on page n doesn't cause missing or duplicate record in page navigation

Cons

  • Client needs to remember the keyset column and previous highest key value
  • If sorting key is different than keyset column, we can't utilise keyset pagination

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