SelectGenerator.php

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<?php
namespace Magento\Framework\Setup\Declaration\Schema\DataSavior;

use Magento\Framework\App\ResourceConnection;
use Magento\Framework\Db\Select;

class SelectGenerator
{
    private $batchSize = 30000;

    private $baseBatchSize;

    private $resourceConnection;

    public function __construct(
        ResourceConnection $resourceConnection,
        $baseBatchSize = 30000
    ) {
        $this->baseBatchSize = $baseBatchSize;
        $this->resourceConnection = $resourceConnection;
    }

    public function generator(Select $select, $connectionName)
    {
        $page = 0;
        $select->limit($this->batchSize, $page * $this->batchSize);
        $adapter = $this->resourceConnection->getConnection($connectionName);
        $data = $adapter->fetchAll($select);
        yield $data;

        while (count($data)) {
            ++$page;
            $select->limit($this->batchSize, $page * $this->batchSize + 1);
            $data = $adapter->fetchAll($select);
            yield $data;
        }
    }
}