Question

Hi I’m Integrating memcache in my codeigniter application,
My query change on the user selected values. some thing like this

$sql='select * from user where user_name="'.$name.'" and location='".$location."'";
$result = $this->memcached_library->get(md5($sql));
if(!$result ){
       /*  execute query and get $result    */
       $this->memcached_library->add(md5($sql), $result );
}else{
       return $result;
}

This is my approach to handle the keys for each different query. But I have heard some where that md5() is not always unique.?

1> Is md5() always unique?

2> if md5() is not unique than what shoud be the other option….

3> what about crc32() is that unique??

Thanks….

Answer 1

A. Your SQL is wrong it should be

 $sql = "select * from user where user_name = '{$name}' and location = '{$location}'";

B. You code might have errors since MD5 is case sensitive

See

var_dump(md5("A"),md5("a"));

Output

string '7fc56270e7a70fa81a5935b72eacbe29' (length=32)
string '0cc175b9c0f1b6a831c399e269772661' (length=32)

Better Approach us using strtolower

  $result = $this->memcached_library->get(md5(strtolower($sql)));

C, Is MD5 unique

MD5 cannot guarantee total uniqueness, however there are approximately 3.402823669209387e+38 different values in a 32 digit hex value (16^32). That means that, assuming the math behind the algorithm gives a good distribution, your odds are phenomenally small that there will be a duplicate.

D. Better option is using sha1

$betterKey = "user" .  sha1(strtolower($sql));
               ^- identify request for each table