Question

I am using

SELECT * 
  FROM picture 
 INNER JOIN user ON picture.fbid = user.fbid 
 WHERE hide = 0 
   AND userhide = 0 
 ORDER BY gender, RAND( ) LIMIT 0 , 1

This executes and gives me the link to a picture of a random row where the gender is female. It takes around 15 seconds to execute.

How could I change the SQL so that it is calculated fast? Should I add a gender column in the Picture table so that it doesn’t combine both the table and then gets a random row or is there some other way to optimise the SQL?

Answer 1

[EDIT] You can do the same with a MySQL stored procedure, I just coded it here: http://sqlfiddle.com/#!2/d0e6a/2

(Note: Not having the count done in every query would aliviate the load, otherwise in this particular case it can be even slower and I would recommend to keep your query, but with index optimization. Or check this out: Is cross-table indexing possible?)

[EDIT 2] Here’s another example with index optimization using an average of 50000 random entries in each table: http://sqlfiddle.com/#!2/bfbe1/1 The query takes less than 100ms (Without indexes I left it running for several minutes and still no result!). You can create indexes on existing tables using CREATE INDEX

Here’s my solution using 2 queries from a PHP script:

$sql="SELECT COUNT(*) 
  FROM picture 
 INNER JOIN user ON picture.fbid = user.fbid 
 WHERE hide = 0 
   AND userhide = 0
   AND gender = 'female' 
   GROUP BY gender";

$result=mysql_query($sql);
$row=mysql_fetch_array($result);

$sql="SELECT * 
      FROM picture 
      INNER JOIN user ON picture.fbid = user.fbid 
      WHERE hide = 0 
      AND userhide = 0
      AND gender = 'female' 
      LIMIT FLOOR(RAND()*$row['COUNT(*)']),1";

$result=mysql_query($sql);
$picture=mysql_fetch_array($result);

This should make you query a lot faster.

Also, if your tables are big (which I guess it’s the case, since the query takes 15 secs) it would help if the conditional/join fields are indexed.