SUM of ever increasing column I have a table of "transactions" where each transaction has an amount:
http://sqlfiddle.com/#!15/42849/1

Records in the table are never REMOVE'ed or UPDATE'ed. Only new transactions are added.

I wish to calculate the SUM of the amounts. The calculation doesn't have to be 100 percent up to date for every request.

On a dataset of about a million rows this takes about 400 ms on my database. This is way too slow for my application and I am trying to find the best solution for speeding this up.

What I have tried so far


Materialized view: Adds complexity of having to have a cronjob running which updates the view every X seconds.
Caching on the application server: Every X request will be slow when the cache needs an update.
Storing results of queries on an old subset: Store the SUM of previous request and use these to calculate the correct total. Adds complexity.


Question

Does PostgreSQL provide a solution for speeding this type of query up?

Update 1

The SUM query is just a basic sum on a single column so I don't believe that this query in itself can get any faster. The solution is probably to do some kind of caching/precalculation or similar. Does PostgreSQL have any features in this regard?

Update 2

Table in question:

CREATE TABLE transactions
(
  id bigserial NOT NULL,
  amount bigint NOT NULL
);


Query in question:

SELECT SUM(amount) FROM transactions;


Update 3

I found that I actually need a "type" as well.

Updated table:

CREATE TABLE transactions
(
  id bigserial NOT NULL,
  amount bigint NOT NULL,
  type int NOT NULL
);


Updated query:

SELECT SUM(amount) FROM transactions GROUP BY type;


SQL Fiddle:
http://sqlfiddle.com/#!15/77e67/2
Here's one idea that you can evaluate:

CREATE TABLE last_transaction
(    last_id bigserial NOT NULL
,    cumulative_amount bigint NOT NULL
);  

INSERT INTO last_transaction (last_id, cumulative_amount) VALUES (-1,0);


The current amount should be something like:

SELECT coalesce(SUM(t.amount),0) + coalesce(lt.cumulative_amount,0) 
FROM transactions t
RIGHT JOIN last_transaction lt
    ON t.id &gt; lt.last_id
GROUP BY lt.cumulative_amount;


On a regular basis you can refresh last_transaction similar to:

update last_transaction
    set last_id = (select max(id) from transactions)
      , cumulative_amount = (select sum(amount) from transactions);


The version of PostgreSQL in your fiddle does not support (perhaps no version does?)

set (last_id, cumulative_amount) = (select ...)


Just an idea, that may or may not fit your needs.

Edit: added type

If a type is to be included (consider naming it transaction_type or something similar) we can extend last_transaction:

CREATE TABLE last_transaction
(    type int not null
,    last_id bigserial NOT NULL
,    cumulative_amount bigint NOT NULL
,        constraint pk_last_transaction primary key (type)
);  

INSERT INTO last_transaction (type, last_id, cumulative_amount) 
SELECT distinct type, -1, 0
FROM transactions;


To get the current_amount we need to add type to the GROUP BY clause as well as to the ON clause.

SELECT lt.type
     , coalesce(SUM(t.amount),0) + coalesce(lt.cumulative_amount,0) 
FROM transactions t
RIGHT JOIN last_transaction lt
    ON t.id &gt; lt.last_id
   AND t.type = lt.type
GROUP BY lt.type, lt.cumulative_amount;


To do a full refresh (according to @Andriy M suggestion) of last_transaction:

UPDATE last_transaction AS lt
    SET last_id = t.last_id
      , cumulative_amount = t.cumulative_amount
FROM (
    SELECT TYPE
         , MAX(id)
         , SUM(amount)
    FROM transactions
    GROUP BY TYPE
) AS t (type, last_id, cumulative_amount)
WHERE t.type = lt.type;


I have yet to examine @YperSillyCubeᵀᴹ suggestion.

I added about a million rows to the transaction table and what I believe would be relevant indexes, but the plan in sqlfiddle looks kind of disappointing.