SQL_CALC_FOUND_ROWS / FOUND_ROWS() does not work in PHP

Question

I use SQL_CALC_FOUND_ROWS in Mysql SELECT statement, to get the number of lines my SELECT would return without a LIMIT clause.

$sql = new mysqli('localhost', 'root', '');
$sql->select_db('mysql');
$s1 = $sql->query('select SQL_CALC_FOUND_ROWS * from db limit 0, 3');
$s2 = $sql->query('select FOUND_ROWS()');
if($row = $s2->fetch_row()) printf('%d/%d', $s1->num_rows, $row[0]);

On my WinXP dev station it return 3/0 everytime for several weeks. When I use another MySQL server from my station it return 3/0 too. On anothers PC the same code runs fine, and return the correct number (3/17 for example, if I have 17 records in mysql.db table). Every XP PC have the same PHP/Mysql version, and it ran fine in the past on my PC Using Mysql Query Browser with the same SQL queries I get the right number.

Could anyone give me an idea of solution, without re-install all?

Sorry, my previous request was awfully unclear.

Answer 1

Thank you.

When I ran something analogous to your example on the mysql command line, it would work; but running it from php, it failed. The second query has to "know about" the first one, so I figure somehow that persistence/memory linking the two queries was getting messed up by the php.

(It turns out that Wordpress uses this type of query to do its pagination - so our larger problem was that the pagination in a wordpress install suddenly stopped working when we moved to php 5.2.6 ... eventually tracked it down to the FOUND_ROWS()).

Just for the sake of posting for people who may run into this in the future... for me it was the php setting "mysql.trace_mode" - this defaulted "on" in 5.2.6 instead of "off" like previously, and for some reason prevents the FOUND_ROWS() from working.

As a "fix", we could either put this in every php page (actually, in a common "include"):

ini_set("mysql.trace_mode", "0");

or add this to the .htaccess:

php_value mysql.trace_mode "0"

Thanks again, Jerry

Answer 2

Are you using a MySQL query method that allows for multiple queries.

From MySQL documentation.

To obtain this row count, include a SQL_CALC_FOUND_ROWS option in the SELECT statement, and then invoke FOUND_ROWS() afterward

Example:

mysql> SELECT SQL_CALC_FOUND_ROWS * FROM tbl_name
    -> WHERE id > 100 LIMIT 10;
mysql> SELECT FOUND_ROWS();

Also just for fun, there's a great discussion about the race condition of FOUND_ROWS()'s usage here.