Description:
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My webserver computer isn't in the same local with MySQL server computer. MySQL server computer is at the different building. I wrote the IP address of the MySQL server in web file on the webserver computer. I noticed that mysql_select_db downloaded a huge table file fully (over 10,000 rows) from the MySQL server computer to the webserver computer for about 20 seconds. After that, finally it started querying fast. When I refresh the page, mysql_select_db downloaded the file again for other 20 seconds. I already ping the webserver computer to the MySQL server computer. It was about 15ms.

source here:

$connect=mysql_pconnect(xxx.xxx.xxx.xxx(IP internet), ..., ...) or die(mysql_error());
mysql_select_db(..., ...) or die(mysql_error());