When performing a step of exporting csv, when more than 3w pieces of data are to be exported, an error will be reported, as follows:

Allowed memory size of 134217728 bytes exhausted (tried to allocate 20480 bytes)

php stores more than 3w pieces of data. This array takes up more than 20M of memory

Solution: take data in batches and process data in batches
Problem point: how much data is the most suitable one time
Take 1w at a time to reduce the number of database IO operations, but the php array is good and large
Take 1k at a time, which increases the number of database IO operations, and the php array is very small

41000 pieces of data
Because it is a circular query of data, only null can be returned, so there will be an invalid IO query

Considering comprehensively, I think it is necessary to use 1w pieces of data per request

Code simple record

$filename = 'test';
$head  = array('Application time', 'province', 'city', 'Supermarket brand');
$field = array('created_time', 'pro_name', 'city_name', 'brand_name');
$csv   = ExportCsvService::getInstance(implode(",", $head), $field, $filename);

$param['limit'] = C('SQL_LIMIT');
$i              = 0;
while (true) {
        $param['current'] = $i + 1;
        $info             = $rainbowservice->get_list($param)['data']['list'];

        if (empty($info)) {
                $csv->end();
                break;
        } else {
                $csv->sendDate($info);
        }
        unset($info);
        $i++;
}

csv operation class

class ExportCsvService
{

    private $content;
    private $filename;
    private $field;
    private static $instance;

    private function __construct($head,$field,$filename)
    {
        $this->field = $field;
        $this->filename = $filename;
        $this->content  = iconv('UTF-8', 'GBK',$head) . PHP_EOL;
    }

    /**
     * Instantiate service class
     * @param $head
     * @param $field
     * @param $filename
     * @return ExportCsvService
     */
    public static function getInstance($head,$field,$filename) {
        if (!self::$instance instanceof self) {
            self::$instance = new self($head,$field,$filename);
        }
        return self::$instance;
    }

    /**
     * Splicing data to export
     * @param $data
     */
    public function sendDate($data) {

        foreach ($data as $key => $value) {
            $temp = [];
            foreach ($this->field as $k) {
                $temp[] = iconv('UTF-8', 'GBK', $value[$k]);
            }

            $this->content .= implode(",", $temp) . PHP_EOL; //Separated by commas
            unset($temp);
        }
    }

    /**
     * output data
     */
    public function end() {
        header("Content-type:text/csv;charset=GBK");
        header("Content-Disposition:attachment;filename=" . $this->filename);
        header('Cache-Control:must-revalidate,post-check=0,pre-check=0');
        header('Expires:0');
        header('Pragma:public');
        echo $this->content;
    }

}