Risk of calling next_result() when there is no multi-result set

When using PHP to manipulate MySQL databases, developers may use mysqli extensions to handle complex queries, especially when calling stored procedures or executing multiple SQL statements, the next_result() function is particularly important. This function is used for processing, that is, when multiple results are returned in a query (such as the output of multiple SELECTs or stored procedures), they can be traversed through next_result() .

However, calling next_result() without multiple result sets is risky.

The role of next_result()

When you execute SQL like the following:

 CALL get_user_and_orders(1);

This stored procedure may return two result sets:

1. The first is user information;

2. The second is the user's order information.

To get these two result sets correctly, you need to handle this:

 $mysqli = new mysqli("localhost", "user", "password", "database");

if ($mysqli->multi_query("CALL get_user_and_orders(1)")) {
    do {
        if ($result = $mysqli->store_result()) {
            while ($row = $result->fetch_assoc()) {
                print_r($row);
            }
            $result->free();
        }
    } while ($mysqli->next_result());
}

Here, next_result() makes sense - it will try to read the next result set.

What happens when there are no multiple result sets?

If the SQL you execute does not return multiple results, such as a simple query:

 $mysqli->multi_query("SELECT * FROM users WHERE id = 1");

At this time you call next_result() :

 $mysqli->next_result();

This can pose the following risks:

1. Trigger an error or warning

Although next_result() itself does not directly report an error (it returns false ), if you do not consume the current result set correctly first (such as calling store_result() ), calling it may cause a "Commands out of sync" error.

This is because the connection of MySQLi is in an incorrect state, and the internal PHP logic does not know whether you have processed the current result.

2. Resource leakage

If you do not explicitly process the current result set and go directly to next_result() , the MySQL server still believes that you have not read the previous result yet and will retain resources, especially in loops or batch calls that can easily cause connection resource leakage .

3. Code logic errors

The meaningless next_result() may make the code look like it is expected to have multiple results, but it is actually a logical error that increases maintenance costs and misleading .

The correct way to use it

Make sure to use next_result() only if:

• Use multi_query() or execute stored procedures;

• Confirm that your SQL does return multiple results;

• The previous result set is consumed before next_result() is called.

You can also encapsulate the following logic:

 if ($mysqli->multi_query($sql)) {
    do {
        if ($result = $mysqli->store_result()) {
            // Process the current result set
            $result->free();
        }
    } while ($mysqli->more_results() && $mysqli->next_result());
}

This writing ensures that next_result() is called only if there are more result sets, which is safer.

Summarize

Calling next_result() without multiple result sets can easily lead to problems such as unrelease of resources, out-of-synchronization errors, and logical confusion. Be sure to understand your query structure, determine whether multiple result sets are really needed to be processed, and avoid unnecessary calls.

In short, although next_result() is an important tool for dealing with multi-result sets, "using it right" is more important than "using it more".