PHP - Confusing Performance Issue getting data fro

问题:

Using PHP/MySQLi I wanted to extract some data to an array. The data is a few megabytes in size:

The code to get the table dump of this data follows:

error_reporting(E_ALL);

 $q =  "SELECT * FROM mytable";
 if (!$result = $mysqli->query($q)) {
      echo "Error: Our query failed to execute and here is why: \n";
      echo "Query: " . $query . "\n";
      echo "Errno: " . $mysqli->errno . "\n";
      echo "Error: " . $mysqli->error . "\n";
      return $ret;
    }
if ($result->num_rows === 0) {
   $ret = 0;
   return $ret;
  }

$ret = array();
while($row = $result->fetch_array()){
  array_push($ret, $row);
} 

echo  mb_strlen(serialize((array)$ret), '8bit');

When executing the following code I got:

But it said tried to allocate 28672 bytes which is nowhere near the limit. Why is this?

回答1:

Think of it this way.

You've got $100. You go to the candy store, and decide to buy 200 $1 candy bars, one-by-one (your while loop).

The first 100 purchases are going to go through, but the next time you try buying a $1 candy bar, you're going to get told "you tried to spend a dollar, but you've already spent $100".

PHP is reporting the amount of memory your last instruction attempted to use. In this case, it's probably the array_push($ret, $row); call.