Question

//get the messages from the database
$messagesID_query = mysql_query("SELECT
                                   messageID,
                                   posterID,
                                   messageTime,
                                   message
                                 FROM
                                   chat_messages
                                 ORDER BY
                                   messageTime DESC
                                 LIMIT
                                   20");

The above mysql_query returns the latest 20 entries from a table. However, I ideally need to reverse the resource before fetching the rows.

At the moment, I run 2 loops:
1 – one to fetch all the results in a php array
2 – then another to process the array to print once reversed.

I am trying to optimize the way this works… is it possible to reverse the mysql resource before i start fetching the rows so i do not have to reverse the array in php?

Or, is there a way that I have over looked of rewriting the query to return the said rows in reverse order.. but still getting the latest 20 results as the above does?

Answer 1

This is best done in the query. Wrap the whole thing in a subquery and reorder that back into ascending order by messageTime. This could save considerable CPU resources on the database server over looping a bunch of calls to mysql_data_seek() to fetch rows backward on a larger rowset than your 20.

SELECT * FROM 
(
  SELECT
    messageID,
    posterID,
    messageTime,
    message
  FROM
  chat_messages
  /* Subquery is used to get the most recent 20 by messageTime */
  ORDER BY messageTime DESC
  LIMIT 20
) subq
/* Reorder the result of the subquery to put them back into ascending order */
ORDER BY messageTime ASC