Optimizing query instead of using order by

I want to run a simple开发者_StackOverflow社区 query to get the "n" oldest records in the table. (It has a creation_date column).

How can i get that without using "order-by". It is a very big table and using order by on entire table to get only "n" records is not so convincing.

(Assume n << size of table)

When you are concerned about performance, you should probably not discard the use of order by too early.

Queries like that can be implemende as Top-N query supported by an appropriate index, that's running very fast because it doesn't need to sort the entire table, not even the selecte rows, because the data is already sorted in the index.

example:

select *
  from table
 where A = ?
 order by creation_date 
 limit 10;

without appropriate index it will be slow if you are having lot's of data. However, if you create an index like that:

create index test on table (A, creation_date );

The query will be able to start fetching the rows in the correct order, without sorting, and stop when the limit is reached.

Recipe: put the where columns in the index, followed by the order by columns.

If there is no where clause, just put the order by into the index. The order by must match the index definition, especially if there are mixed asc/desc orders.

The indexed Top-N query is the performance king--make sure to use them.

I few links for further reading (all mine):

How to use index efficienty in mysql query

http://blog.fatalmind.com/2010/07/30/analytic-top-n-queries/ (Oracle centric)

http://Use-The-Index-Luke.com/ (not yet covering Top-N queries, but that's to come in 2011).

I haven't tested this concept before but try and create an index on the creation_date column. Which will automatically sort the rows is ascending order. Then your select query can use the orderby creation_date desc with the Limit 20 to get the first 20 records. The database engine should realize the index has already done the work sorting and wont actually need to sort, because the index has already sorted it on save. All it needs to do is read the last 20 records from the index.

Worth a try.

Create an index on creation_date and query by using order by creation_date asc|desc limit n and the response will be very fast (in fact it cannot be faster). For the "latest n" scenario you need to use desc.

If you want more constraints on this query (e.g where state='LIVE') then the query may become very slow and you'll need to reconsider the indexing strategy.

You can use Group By if your grouping some data and then Having clause to select specific records.