Select query with offset limit is much too slow

Question

I have read from Internet resources that a query will be slow when the offset increases. But in my case I think its much too slow. I am using postgres 9.3.

Here is the query (id is primary key):

select * from test_table offset 3900000 limit 100;

It returns the data in around 10 seconds. And I think its much too slow. I have around 4 million records in the table. Overall size of the database is 23GB.

Machine configuration:

RAM: 12 GB
CPU: 2.30 GHz
Core: 10

A few values from postgresql.conf file which I have changed are as below. Others are default.

shared_buffers = 2048MB
temp_buffers = 512MB
work_mem = 1024MB
maintenance_work_mem = 256MB
dynamic_shared_memory_type = posix
default_statistics_target = 10000
autovacuum = on
enable_seqscan = off   ## its not making any effect as I can see from Analyze doing seq-scan

Apart from these, I also have tried by changing the values of random_page_cost = 2.0 and cpu_index_tuple_cost = 0.0005 and the result is the same.

Explain (analyze, buffers) result over the query is as below:

"Limit  (cost=10000443876.02..10000443887.40 rows=100 width=1034) (actual time=12793.975..12794.292 rows=100 loops=1)"
"  Buffers: shared hit=26820 read=378984"
"  ->  Seq Scan on test_table  (cost=10000000000.00..10000467477.70 rows=4107370 width=1034) (actual time=0.008..9036.776 rows=3900100 loops=1)"
"        Buffers: shared hit=26820 read=378984"
"Planning time: 0.136 ms"
"Execution time: 12794.461 ms"

How do people around the world negotiates with this problem with Postgres? Any alternate solution will be helpful for me as well.

UPDATE:: Adding order by id (tried with other indexed column as well) and here is the explain:

"Limit  (cost=506165.06..506178.04 rows=100 width=1034) (actual time=15691.132..15691.494 rows=100 loops=1)"
"  Buffers: shared hit=110813 read=415344"
"  ->  Index Scan using test_table_pkey on test_table  (cost=0.43..533078.74 rows=4107370 width=1034) (actual time=38.264..11535.005 rows=3900100 loops=1)"
"        Buffers: shared hit=110813 read=415344"
"Planning time: 0.219 ms"
"Execution time: 15691.660 ms"

Answer 1

It's slow because it needs to locate the top offset rows and scan the next 100. No amounts of optimization will change that when you're dealing with huge offsets.

This is because your query literally instruct the DB engine to visit lots of rows by using offset 3900000 -- that's 3.9M rows. Options to speed this up somewhat aren't many.

Super-fast RAM, SSDs, etc. will help. But you'll only gain by a constant factor in doing so, meaning it's merely kicking the can down the road until you reach a larger enough offset.

Ensuring the table fits in memory, with plenty more to spare will likewise help by a larger constant factor -- except the first time. But this may not be possible with a large enough table or index.

Ensuring you're doing index-only scans will work to an extent. (See velis' answer; it has a lot of merit.) The problem here is that, for all practical purposes, you can think of an index as a table storing a disk location and the indexed fields. (It's more optimized than that, but it's a reasonable first approximation.) With enough rows, you'll still be running into problems with a larger enough offset.

Trying to store and maintain the precise position of the rows is bound to be an expensive approach too.(This is suggested by e.g. benjist.) While technically feasible, it suffers from limitations similar to those that stem from using MPTT with a tree structure: you'll gain significantly on reads but will end up with excessive write times when a node is inserted, updated or removed in such a way that large chunks of the data needs to be updated alongside.

As is hopefully more clear, there isn't any real magic bullet when you're dealing with offsets this large. It's often better to look at alternative approaches.

If you're paginating based on the ID (or a date field, or any other indexable set of fields), a potential trick (used by blogspot, for instance) would be to make your query start at an arbitrary point in the index.

Put another way, instead of:

example.com?page_number=[huge]

Do something like:

example.com?page_following=[id]

That way, you keep a trace of where you are in your index, and the query becomes very fast because it can head straight to the correct starting point without plowing through a gazillion rows:

select * from foo where ID > [id] order by ID limit 100

Naturally, you lose the ability to jump to e.g. page 3000. But give this some honest thought: when was the last time you jumped to a huge page number on a site instead of going straight for its monthly archives or using its search box?

If you're paginating but want to keep the page offset by any means, yet another approach is to forbid the use of larger page number. It's not silly: it's what Google is doing with search results. When running a search query, Google gives you an estimate number of results (you can get a reasonable number using explain), and then will allow you to brows the top few thousand results -- nothing more. Among other things, they do so for performance reasons -- precisely the one you're running into.

Answer 2

There are two simple approaches to solve such a problem

• Splitting the query into two subqueries that the first one do all the heavy job on index-only scan as described here
• Create calculated index that holds the offset as described here, this can be enhanced using window functions.

Answer 3

To avoid slow pagination with big tables always use auto-increment primary key then use the query below:

SELECT * FROM test_table WHERE id > (SELECT min(id) FROM test_table WHERE id > ((1 * 10) - 10)) ORDER BY id DESC LIMIT 10

1: is the page number
10: is the records per page

Tested and work well with 50 millions records.

Answer 4

you can optimise in two steps

First get maximum id out of 3900000 records

select max(id) (select id from test_table order by id limit 3900000);

Then use this maximum id to get the next 100 records.

select * from test_table id > {max id from previous step) order by id limit 100 ;

It will be faster as both queries will do index scan by id.

Answer 5

How about if paginate based on IDs instead of offset/limit?

The following query will give IDs which split all the records into chunks of size per_page. It doesn't depend on were records deleted or not

SELECT id AS from_id FROM (
  SELECT id, (ROW_NUMBER() OVER(ORDER BY id DESC)) AS num FROM test_table
) AS rn
WHERE num % (per_page + 1) = 0;

With these from_IDs you can add links to the page. Iterate over :from_ids with index and add the following link to the page:

<a href="/test_records?from_id=:from_id">:from_id_index</a>

When user visits the page retrieve records with ID which is greater than requested :from_id:

SELECT * FROM test_table WHERE ID >= :from_id ORDER BY id DESC LIMIT :per_page

For the first page link with from_id=0 will work

<a href="/test_records?from_id=0">1</a>

Answer 6

I don't know all of the details of your data, but 4 million rows can be a little hefty. If there's a reasonable way to shard the table and essentially break it up into smaller tables it could be beneficial.

To explain this, let me use an example. let's say that I have a database where I have a table called survey_answer, and it's getting very large and very slow. Now let's say that these survey answers all come from a distinct group of clients (and I also have a client table keeping track of these clients). Then something I could do is I could make it so that I have a table called survey_answer that doesn't have any data in it, but is a parent table, and it has a bunch of child tables that actually contain the data the follow the naming format survey_answer_<clientid>, meaning that I'd have child tables survey_answer_1, survey_answer_2, etc., one for each client. Then when I needed to select data for that client, I'd use that table. If I needed to select data across all clients, I can select from the parent survey_answer table, but it will be slow. But for getting data for an individual client, which is what I mostly do, then it would be fast.

This is one example of how to break up data, and there are many others. Another example would be if my survey_answer table didn't break up easily by client, but instead I know that I'm typically only accessing data over a year period of time at once, then I could potentially make child tables based off of year, such as survey_answer_2014, survey_answer_2013, etc. Then if I know that I won't access more than a year at a time, I only really need to access maybe two of my child tables to get all the data I need.

In your case, all I've been given is perhaps the id. We can break it up by that as well (though perhaps not as ideal). Let's say that we break it up so that there's only about 1000000 rows per table. So our child tables would be test_table_0000001_1000000, test_table_1000001_2000000, test_table_2000001_3000000, test_table_3000001_4000000, etc. So instead of passing in an offset of 3900000, you'd do a little math first and determine that the table that you want is table test_table_3000001_4000000 with an offset of 900000 instead. So something like:

SELECT * FROM test_table_3000001_4000000 ORDER BY id OFFSET 900000 LIMIT 100;

Now if sharding the table is out of the question, you might be able to use partial indexes to do something similar, but again, I'd recommend sharding first. Learn more about partial indexes here.

I hope that helps. (Also, I agree with Szymon Guz that you want an ORDER BY).

Edit: Note that if you need to delete rows or selectively exclude rows before getting your result of 100, then sharding by id will become very hard to deal with (as pointed out by Denis; and sharding by id is not great to begin with). But if your 'just' paginating the data, and you only insert or edit (not a common thing, but it does happen; logs come to mind), then sharding by id can be done reasonably (though I'd still choose something else to shard on).

Answer 7

I have upvoted Denis's answer, but will add a suggestion myself, perhaps it can be of some performance benefit for your specific use-case:

Assuming your actual table is not test_table, but some huge compound query, possibly with multiple joins. You could first determine the required starting id:

select id from test_table order by id offset 3900000 limit 1

This should be much faster than original query as it only requires to scan the index vs the entire table. Getting this id then opens up a fast index-search option for full fetch:

select * from test_table where id >= (what I got from previous query) order by id limit 100

Answer 8

You didn't say if your data is mainly read-only or updated often. If you can manage to create your table at one time, and only update it every now and then (say every few minutes) your problem will be easy to solve:

• Add a new column "offset_id"
• For your complete data set ordered by ID, create an offset_id simply by incrementing numbers: 1,2,3,4...
• Instead of "offset ... limit 100" use "where offset_id >= 3900000 limit 100"

Answer 9

First, you have to define limit and offset with order by clause or you will get inconsistent result.

To speed up the query, you can have a computed index, but only for these condition :

1. Newly inserted data is strictly in id order
2. No delete nor update on column id

Here's how You can do it :

1. Create a row position function

create or replace function id_pos (id) returns bigint as 'select count(id) from test_table where id <= $1;' language sql immutable;

2. Create a computed index on id_pos function

create index table_by_pos on test_table using btree(id_pos(id));

Here's how You call it (offset 3900000 limit 100):

select * from test_table where id_pos(id) >= 3900000 and sales_pos(day) < 3900100;

This way, the query will not compute the 3900000 offset data, but only will compute the 100 data, making it much faster.

Please note the 2 conditions where this approach can take place, or the position will change.

Answer 10

This way you get the rows in semi-random order. You are not ordering the results in a query, so as a result, you get the data as it is stored in the files. The problem is that when you update the rows, the order of them can change.

To fix that you should add order by to the query. This way the query will return the rows in the same order. What's more then it will be able to use an index to speed the query up.

So two things: add an index, add order by to the query. Both to the same column. If you want to use the id column, then don't add index, just change the query to something like:

select * from test_table order by id offset 3900000 limit 100;