如果我正确理解您所说的编写迁移，它可以根据当前选定的或目标数据库处理特定于db的关键字或函数，那么您可能需要为迁移创建自己的逻辑来支持它。
我使用下面的类作为基本迁移类，并扩展从这个类到 tableOptions 删除列约束。您可以根据需要对其进行升级并添加必要的操作/功能。但它可以创建逻辑来实现你的要求。

<?php

namespace console\migrations;
use Yii;
class Migration extends \yii\db\Migration
{

    /**
     * @var string
     */
    protected $tableOptions;

    /**
     * @var string
     */
    protected $restrict = 'RESTRICT';

    /**
     * @var string
     */
    protected $cascade = 'CASCADE';

    /**
     * @var string
     */
    protected $noAction = 'NO ACTION';

    /**
     * @var mixed
     */
    protected $dbType;

    /**
     * @inheritdoc
     */
    public function init()
    {
        parent::init();

        switch ($this->db->driverName) {
            case 'mysql':
                $this->tableOptions = 'CHARACTER SET utf8 COLLATE utf8_unicode_ci ENGINE=InnoDB';
                $this->dbType = 'mysql';
                break;
            case 'pgsql':
                $this->tableOptions = null;
                $this->dbType = 'pgsql';
                break;
            case 'dblib':
            case 'mssql':
            case 'sqlsrv':
                $this->restrict = 'NO ACTION';
                $this->tableOptions = null;
                $this->dbType = 'sqlsrv';
                break;
            default:
                throw new \RuntimeException('Your database is not supported!');
        }
    }

    /**
     * Drops the constraints for the given column
     *
     * @param string $table  the table name
     * @param string $column the column name
     *
     * @return null
     */
    public function dropColumnConstraints($table, $column)
    {
        $table = Yii::$app->db->schema->getRawTableName($table);
        $cmd = Yii::$app->db->createCommand(
            'SELECT name FROM sys.default_constraints
            WHERE parent_object_id = object_id(:table)
            AND type = \'D\' AND parent_column_id = (
                SELECT column_id
                FROM sys.columns
                WHERE object_id = object_id(:table)
                and name = :column
            )', [':table' => $table, ':column' => $column]
        );

        $constraints = $cmd->queryAll();
        foreach ($constraints as $c) {
            $this->execute('ALTER TABLE ' . Yii::$app->db->quoteTableName($table) . ' DROP CONSTRAINT ' . Yii::$app->db->quoteColumnName($c['name']));
        }
    }

}

我对postgre不是很熟悉，但是从app-advanced中的基本迁移来看，你可以查看 driverName 并在此基础上创建不同的逻辑。
迁移文件

$tableOptions = null;
if ($this->db->driverName === 'mysql') {
    $tableOptions = 'CHARACTER SET utf8 COLLATE utf8_unicode_ci ENGINE=InnoDB';
}