Look at this:
NBET=> explain select * from account_transaction where users_id=123456 order by created desc limit 10;
QUERY PLAN
-------------------------------------------------------------------------------------------------------------------------------------------
Limit (cost=0.00..27.40 rows=10 width=213)
-> Index Scan Backward using account_transaction_on_user_and_timestamp on account_transaction (cost=0.00..1189.19 rows=434 width=213)
Index Cond: (users_id = 123456)
(3 rows)
NBET=> explain select * from account_transaction where users_id=123456 order by created desc, id desc limit 10;
QUERY PLAN
------------------------------------------------------------------------------------------------------------------------------
Limit (cost=1114.02..1114.04 rows=10 width=213)
-> Sort (cost=1114.02..1115.10 rows=434 width=213)
Sort Key: created, id
-> Index Scan using account_transaction_by_users_id on account_transaction (cost=0.00..1095.01 rows=434 width=213)
Index Cond: (users_id = 123456)
(5 rows)
In case the explains doesn't explain themself good enough: we have a
transaction table with ID (primary key, serial), created (a timestamp)
and a users_id. Some of the users have generated thousands of
transactions, and the above query is a simplified version of the query
used to show the users their last transactions. Since we have a large
user base hammering our servers with this request, the speed is
significant.
We have indices on the users_id field and the (users_id, created)-tuple.
The timestamp is set by the application and has a resolution of 1 second
- so there may easily be several transactions sharing the same
timestamp, but this is an exception not the rule. I suppose the
developers needed to add the ID to the sort list to come around a bug,
but still prefering to have the primary sorting by created to be able to
use the index. One workaround here is to order only by id desc and
create a new index on (users_id, id) - but I really don't like adding
more indices to the transaction table.
