First, let us consider the time limit. In config.inc.php, the $cfg['ExecTimeLimit'] configuration directive assigns, by default, a maximum execution time of 300 seconds (five minutes) for any phpMyAdmin script, including the scripts that process data after the file has been uploaded. A value of 0 removes the limit, and in theory, gives us infinite time to complete the import operation. If the PHP server is running in safe mode, modifying $cfg['ExecTimeLimit'] will have no effect. This is because the limits set in php.ini or in user-related web server configuration files (such as .htaccess or virtual host configuration files), take precedence over this parameter.

Of course, the time it effectively takes depends on two key factors:

The system administrator can use the php.ini file or the web server's virtual host configuration file to control uploads on the server.

The upload_max_filesize parameter specifies the upper limit or maximum file size that can be uploaded via HTTP. This one is obvious, but another less obvious parameter is post_max_size. As HTTP uploading is done via the POST method, this parameter may limit our transfers. For more details about the POST method, please refer to http://en.wikipedia.org/wiki/Http#Request_methods.

The memory_limit parameter is provided to prevent web server child processes from grabbing too much of the server's memory—phpMyAdmin runs inside a child process. Thus, the handling of normal file uploads, especially compressed dumps, can be compromised by giving this parameter a small value. Here, no preferred value can be recommended; the value depends on the size of uploaded data we want to handle and on the size of the physical memory. The memory limit can also be tuned via the $cfg['MemoryLimit'] parameter in config.inc.php, as seen in Chapter 6.

Finally, file uploads must be allowed by setting file_uploads to On; otherwise, phpMyAdmin won't even show a dialog to choose a file. It would be useless to display this dialog as the connection would be refused later by the PHP component of the web server.

If the file is too big, there are ways in which we can resolve the situation. If the original data is still accessible via phpMyAdmin, we could use phpMyAdmin to generate smaller export files, choosing the Dump some row(s) dialog. If this were not possible, we could use a spreadsheet program or a text editor to split the file into smaller sections. Another possibility is to use the upload directory mechanism, which accesses the directory defined in $cfg['UploadDir']. This feature is explained later in this chapter.

In recent phpMyAdmin versions, the Partial import feature can also solve this file size problem. By selecting the Allow the interruption… checkbox, the import process will interrupt itself if it detects that it is close to the time limit. We can also specify a number of queries to skip from the start, in case we successfully import a number of rows and wish to continue from that point.

On a server, a PHP security feature called open_basedir (which limits the files that can be opened by PHP to the specified directory tree) can impede the upload mechanism. In this case, or for any other reason, when uploads are problematic, the $cfg['TempDir'] parameter can be set with the value of a temporary directory. This is probably a sub-directory of phpMyAdmin's main directory, into which the web server is allowed to put the uploaded file.