Welcome.

This will have least requirements.

Note: SERIAL is: BIGINT UNSIGNED UNIQUE AUTO_INCREMENT

Table channel has following fields:

• id SERIAL, not primary
• name VARCHAR(255)

Optional fields:

• logo_blob BLOB
• logo_url VARCHAR(2048)

Table messages has following fields:

• id SERIAL, not primary
• uid BIGINT, attribute: UNSIGNED
• message TEXT, or VARCHAR
• channel_id BIGINT UNIQUE, attribute: UNSIGNED

Table last_message
This table does not spam with INSERT, it's done only once,
Meaning: every time new message is sent, UPDATE must be called immediately.

Has following fields:

• last_id BIGINT, attribute: UNSIGNED
• channel_id BIGINT UNIQUE, attribute: UNSIGNED
• uid BIGINT, attribute: UNSIGNED

The channel_id will crash INSERT if duplicate id is found. Thus UPDATE must be called instead.

Assuming that php will be used, you would want to use usleep(256e3) for the MINIMUM delay between each fetchall.
However if you are sure your server is capable of higher stress, use usleep(128e3) instead.
For free hosting, about usleep(500e3) is recommended.

This is how you do realtime fetch:

// dummy functions, let's assume you wrote actual code
function fetchAll($last_msg){
  $last_id = $last_msg->last_id;
  $q = "SELECT * from messages where id>$last_id";
  //...
}
function get_last_message($channel_id){
  $q = "SELECT * from last_message where channel_id='$channel_id'";
  //...
}
function sendmessage(){
  // or use INSERT, if not inserted!
  $q = "UPDATE last_message set ...";
  // you know what to do here:
  $q2 = "INSERT into messages set ...";
  // ...
}

$channel_id = 1;
$maxticks = 32;
$last_msg = get_last_message($channel_id);
// last_msg contains last_id which is later used by fetchAll to fecth messages AFTER that id
while ($maxticks--) {
  $curr_msg = get_last_message($channel_id);
  if ($last_msg->last_id != $curr_msg->last_id) {
    $last_msg = $curr_msg;
    break;
  }
  usleep(500e3);
}
echo fetchAll($last_msg);