PHP MySQL pagination with random ordering

Question

This is a problem with a ordering search results on my website,

When a search is made, random results appear on the content page, this page includes pagination too.

Answer 1

The combination of

1. random ordering
2. pagination
3. HTTP (stateless)

is as ugly as it comes: 1. and 2. together need some sort of "persistent randomness", while 3. makes this harder to achieve. On top of this 1. is not a job a RDBMS is optimized to do.

My suggestion depends on how big your dataset is:

Few rows (ca. <1K):

• select all PK values in first query (first page)
• shuffle these in PHP
• store shuffled list in session
• for each page call select the data according to the stored PKs

Many rows (10K+):

This assumes, you have an AUTO_INCREMENT unique key called ID with a manageable number of holes. Use a amintenace script if needed (high delete ratio)

• Use a shuffling function that is parameterized with e.g. the session ID to create a function rand_id(continuous_id)
• If you need e.g. the records 100,000 to 100,009 calculate $a=array(rand_id(100,000), rand_id(100,001), ... rand_id(100,009));
• $a=implode(',',$a);
• $sql="SELECT foo FROM bar WHERE ID IN($a) ORDER BY FIELD(ID,$a)";
• To take care of the holes in your ID select a few records too many (and throw away the exess), looping on too few records selected.