diff --git a/framework/db/dao/ColumnSchema.php b/framework/db/dao/ColumnSchema.php
index 3e498cb..6fb34dc 100644
--- a/framework/db/dao/ColumnSchema.php
+++ b/framework/db/dao/ColumnSchema.php
@@ -19,6 +19,24 @@ namespace yii\db\dao;
 class ColumnSchema extends \yii\base\Component
 {
 	/**
+	 * The followings are supported abstract column data types.
+	 */
+	const TYPE_STRING = 'string';
+	const TYPE_TEXT = 'text';
+	const TYPE_SMALLINT = 'smallint';
+	const TYPE_INTEGER = 'integer';
+	const TYPE_BIGINT = 'bigint';
+	const TYPE_FLOAT = 'float';
+	const TYPE_DECIMAL = 'decimal';
+	const TYPE_DATETIME = 'datetime';
+	const TYPE_TIMESTAMP = 'timestamp';
+	const TYPE_TIME = 'time';
+	const TYPE_DATE = 'date';
+	const TYPE_BINARY = 'binary';
+	const TYPE_BOOLEAN = 'boolean';
+	const TYPE_MONEY = 'money';
+
+	/**
 	 * @var string name of this column (without quotes).
 	 */
 	public $name;
diff --git a/framework/db/dao/QueryBuilder.php b/framework/db/dao/QueryBuilder.php
index 3790f9f..aa272ae 100644
--- a/framework/db/dao/QueryBuilder.php
+++ b/framework/db/dao/QueryBuilder.php
@@ -15,6 +15,9 @@ use yii\db\Exception;
 /**
  * QueryBuilder builds a SQL statement based on the specification given as a [[Query]] object.
  *
+ * QueryBuilder is often used behind the scenes by [[Query]] to build a DBMS-dependent SQL statement
+ * from a [[Query]] object.
+ *
  * @author Qiang Xue <qiang.xue@gmail.com>
  * @since 2.0
  */
@@ -22,7 +25,8 @@ class QueryBuilder extends \yii\base\Object
 {
 	/**
 	 * @var array the abstract column types mapped to physical column types.
-	 * Child classes should override this property to declare possible type mappings.
+	 * This is mainly used to support creating/modifying tables using DB-independent data type specifications.
+	 * Child classes should override this property to declare supported type mappings.
 	 */
 	public $typeMap = array();
 	/**
@@ -45,12 +49,12 @@ class QueryBuilder extends \yii\base\Object
 
 	/**
 	 * Constructor.
-	 * @param Schema $schema the database schema information
+	 * @param Connection $connection the database connection.
 	 */
-	public function __construct($schema)
+	public function __construct($connection)
 	{
-		$this->connection = $schema->connection;
-		$this->schema = $schema;
+		$this->connection = $connection;
+		$this->schema = $connection->getSchema();
 	}
 
 	/**
@@ -182,7 +186,7 @@ class QueryBuilder extends \yii\base\Object
 	 * The columns in the new  table should be specified as name-definition pairs (e.g. 'name'=>'string'),
 	 * where name stands for a column name which will be properly quoted by the method, and definition
 	 * stands for the column type which can contain an abstract DB type.
-	 * The {@link getColumnType} method will be invoked to convert any abstract type into a physical one.
+	 * The [[getColumnType()]] method will be invoked to convert any abstract type into a physical one.
 	 *
 	 * If a column is specified with definition only (e.g. 'PRIMARY KEY (name, type)'), it will be directly
 	 * inserted into the generated SQL.
@@ -242,7 +246,7 @@ class QueryBuilder extends \yii\base\Object
 	 * Builds a SQL statement for adding a new DB column.
 	 * @param string $table the table that the new column will be added to. The table name will be properly quoted by the method.
 	 * @param string $column the name of the new column. The name will be properly quoted by the method.
-	 * @param string $type the column type. The {@link getColumnType} method will be invoked to convert abstract column type (if any)
+	 * @param string $type the column type. The [[getColumnType()]] method will be invoked to convert abstract column type (if any)
 	 * into the physical one. Anything that is not recognized as abstract type will be kept in the generated SQL.
 	 * For example, 'string' will be turned into 'varchar(255)', while 'string not null' will become 'varchar(255) not null'.
 	 * @return string the SQL statement for adding a new column.
@@ -284,7 +288,7 @@ class QueryBuilder extends \yii\base\Object
 	 * Builds a SQL statement for changing the definition of a column.
 	 * @param string $table the table whose column is to be changed. The table name will be properly quoted by the method.
 	 * @param string $column the name of the column to be changed. The name will be properly quoted by the method.
-	 * @param string $type the new column type. The {@link getColumnType} method will be invoked to convert abstract column type (if any)
+	 * @param string $type the new column type. The [[getColumnType]] method will be invoked to convert abstract column type (if any)
 	 * into the physical one. Anything that is not recognized as abstract type will be kept in the generated SQL.
 	 * For example, 'string' will be turned into 'varchar(255)', while 'string not null' will become 'varchar(255) not null'.
 	 * @return string the SQL statement for changing the definition of a column.
@@ -406,7 +410,7 @@ class QueryBuilder extends \yii\base\Object
 
 	/**
 	 * Converts an abstract column type into a physical column type.
-	 * The conversion is done using the type map specified in {@link typeMap}.
+	 * The conversion is done using the type map specified in [[typeMap]].
 	 * These abstract column types are supported (using MySQL as example to explain the corresponding
 	 * physical types):
 	 * <ul>
diff --git a/framework/db/dao/Schema.php b/framework/db/dao/Schema.php
index d5826e4..88a18ac 100644
--- a/framework/db/dao/Schema.php
+++ b/framework/db/dao/Schema.php
@@ -26,22 +26,6 @@ use yii\db\Exception;
  */
 abstract class Schema extends \yii\base\Object
 {
-	const TYPE_PK = 'pk';
-	const TYPE_STRING = 'string';
-	const TYPE_TEXT = 'text';
-	const TYPE_SMALLINT = 'smallint';
-	const TYPE_INTEGER = 'integer';
-	const TYPE_BIGINT = 'bigint';
-	const TYPE_FLOAT = 'float';
-	const TYPE_DECIMAL = 'decimal';
-	const TYPE_DATETIME = 'datetime';
-	const TYPE_TIMESTAMP = 'timestamp';
-	const TYPE_TIME = 'time';
-	const TYPE_DATE = 'date';
-	const TYPE_BINARY = 'binary';
-	const TYPE_BOOLEAN = 'boolean';
-	const TYPE_MONEY = 'money';
-
 	/**
 	 * @var Connection the database connection
 	 */
@@ -148,11 +132,13 @@ abstract class Schema extends \yii\base\Object
 	 * Returns all table names in the database.
 	 * @param string $schema the schema of the tables. Defaults to empty string, meaning the current or default schema.
 	 * If not empty, the returned table names will be prefixed with the schema name.
+	 * @param boolean $refresh whether to fetch the latest available table names. If this is false,
+	 * table names fetched previously (if available) will be returned.
 	 * @return array all table names in the database.
 	 */
-	public function getTableNames($schema = '')
+	public function getTableNames($schema = '', $refresh = false)
 	{
-		if (!isset($this->_tableNames[$schema])) {
+		if (!isset($this->_tableNames[$schema]) || $refresh) {
 			$this->_tableNames[$schema] = $this->findTableNames($schema);
 		}
 		return $this->_tableNames[$schema];
@@ -171,19 +157,25 @@ abstract class Schema extends \yii\base\Object
 
 	/**
 	 * Refreshes the schema.
-	 * This method resets the loaded table metadata and command builder
-	 * so that they can be recreated to reflect the change of schema.
+	 * This method cleans up the cached table schema and names
+	 * so that they can be recreated to reflect the database schema change.
+	 * @param string $tableName the name of the table that needs to be refreshed.
+	 * If null, all currently loaded tables will be refreshed.
 	 */
-	public function refresh()
+	public function refresh($tableName = null)
 	{
 		$db = $this->connection;
 		if ($db->schemaCachingDuration >= 0 && ($cache = \Yii::$application->getComponent($db->schemaCacheID)) !== null) {
-			foreach ($this->_tables as $name => $table) {
-				$cache->delete($this->getCacheKey($name));
+			if ($tableName === null) {
+				foreach ($this->_tables as $name => $table) {
+					$cache->delete($this->getCacheKey($name));
+				}
+				$this->_tables = array();
+			} else {
+				$cache->delete($this->getCacheKey($tableName));
+				unset($this->_tables[$tableName]);
 			}
 		}
-		$this->_tables = array();
-		$this->_tableNames = array();
 	}
 
 	/**
@@ -253,7 +245,7 @@ abstract class Schema extends \yii\base\Object
 	 */
 	public function createQueryBuilder()
 	{
-		return new QueryBuilder($this);
+		return new QueryBuilder($this->connection);
 	}
 
 	/**
diff --git a/framework/db/dao/mysql/ColumnSchema.php b/framework/db/dao/mysql/ColumnSchema.php
index 4ec9ce1..6b52a34 100644
--- a/framework/db/dao/mysql/ColumnSchema.php
+++ b/framework/db/dao/mysql/ColumnSchema.php
@@ -25,35 +25,35 @@ class ColumnSchema extends \yii\db\dao\ColumnSchema
 	public function initTypes($dbType)
 	{
 		static $typeMap = array(  // dbType => type
-			'tinyint' => 'smallint',
-			'bit' => 'smallint',
-			'smallint' => 'smallint',
-			'mediumint' => 'integer',
-			'int' => 'integer',
-			'integer' => 'integer',
-			'bigint' => 'bigint',
-			'float' => 'float',
-			'double' => 'float',
-			'real' => 'float',
-			'decimal' => 'decimal',
-			'numeric' => 'decimal',
-            'tinytext' => 'text',
-            'mediumtext' => 'text',
-            'longtext' => 'text',
-            'text' => 'text',
-            'varchar' => 'string',
-            'string' => 'string',
-            'char' => 'string',
-			'datetime' => 'datetime',
-			'year' => 'date',
-			'date' => 'date',
-			'time' => 'time',
-			'timestamp' => 'timestamp',
-			'enum' => 'enum',
+			'tinyint' => self::TYPE_SMALLINT,
+			'bit' => self::TYPE_SMALLINT,
+			'smallint' => self::TYPE_SMALLINT,
+			'mediumint' => self::TYPE_INTEGER,
+			'int' => self::TYPE_INTEGER,
+			'integer' => self::TYPE_INTEGER,
+			'bigint' => self::TYPE_BIGINT,
+			'float' => self::TYPE_FLOAT,
+			'double' => self::TYPE_FLOAT,
+			'real' => self::TYPE_FLOAT,
+			'decimal' => self::TYPE_DECIMAL,
+			'numeric' => self::TYPE_DECIMAL,
+            'tinytext' => self::TYPE_TEXT,
+            'mediumtext' => self::TYPE_TEXT,
+            'longtext' => self::TYPE_TEXT,
+            'text' => self::TYPE_TEXT,
+            'varchar' => self::TYPE_STRING,
+            'string' => self::TYPE_STRING,
+            'char' => self::TYPE_STRING,
+			'datetime' => self::TYPE_DATETIME,
+			'year' => self::TYPE_DATE,
+			'date' => self::TYPE_DATE,
+			'time' => self::TYPE_TIME,
+			'timestamp' => self::TYPE_TIMESTAMP,
+			'enum' => self::TYPE_STRING,
 		);
 
 		$this->dbType = $dbType;
-		$this->type = 'string';
+		$this->type = self::TYPE_STRING;
 		$this->unsigned = strpos($this->dbType, 'unsigned') !== false;
 
 		if (preg_match('/^(\w+)(?:\(([^\)]+)\))?/', $this->dbType, $matches)) {
@@ -63,7 +63,7 @@ class ColumnSchema extends \yii\db\dao\ColumnSchema
 			}
 
 			if (!empty($matches[2])) {
-				if ($this->type === 'enum') {
+				if ($type === 'enum') {
 					$values = explode(',', $matches[2]);
 					foreach ($values as $i => $value) {
 						$values[$i] = trim($value, "'");
@@ -93,7 +93,7 @@ class ColumnSchema extends \yii\db\dao\ColumnSchema
 
 	/**
 	 * Extracts the default value for the column.
-	 * The value is typecasted to correct PHP type.
+	 * The value is typecast to correct PHP type.
 	 * @param mixed $defaultValue the default value obtained from metadata
 	 */
 	public function initDefaultValue($defaultValue)
diff --git a/framework/db/dao/mysql/QueryBuilder.php b/framework/db/dao/mysql/QueryBuilder.php
index e5b241b..96ac877 100644
--- a/framework/db/dao/mysql/QueryBuilder.php
+++ b/framework/db/dao/mysql/QueryBuilder.php
@@ -22,21 +22,21 @@ class QueryBuilder extends \yii\db\dao\QueryBuilder
 	 * @var array the abstract column types mapped to physical column types.
 	 */
     public $typeMap = array(
-		Schema::TYPE_PK => 'int(11) NOT NULL AUTO_INCREMENT PRIMARY KEY',
-		Schema::TYPE_STRING => 'varchar(255)',
-		Schema::TYPE_TEXT => 'text',
-		Schema::TYPE_SMALLINT => 'smallint(6)',
-		Schema::TYPE_INTEGER => 'int(11)',
-		Schema::TYPE_BIGINT => 'bigint(20)',
-		Schema::TYPE_FLOAT => 'float',
-		Schema::TYPE_DECIMAL => 'decimal',
-		Schema::TYPE_DATETIME => 'datetime',
-		Schema::TYPE_TIMESTAMP => 'timestamp',
-		Schema::TYPE_TIME => 'time',
-		Schema::TYPE_DATE => 'date',
-		Schema::TYPE_BINARY => 'blob',
-		Schema::TYPE_BOOLEAN => 'tinyint(1)',
-		Schema::TYPE_MONEY => 'decimal(19,4)',
+		'pk' => 'int(11) NOT NULL AUTO_INCREMENT PRIMARY KEY',
+		ColumnSchema::TYPE_STRING => 'varchar(255)',
+		ColumnSchema::TYPE_TEXT => 'text',
+		ColumnSchema::TYPE_SMALLINT => 'smallint(6)',
+		ColumnSchema::TYPE_INTEGER => 'int(11)',
+		ColumnSchema::TYPE_BIGINT => 'bigint(20)',
+		ColumnSchema::TYPE_FLOAT => 'float',
+		ColumnSchema::TYPE_DECIMAL => 'decimal',
+		ColumnSchema::TYPE_DATETIME => 'datetime',
+		ColumnSchema::TYPE_TIMESTAMP => 'timestamp',
+		ColumnSchema::TYPE_TIME => 'time',
+		ColumnSchema::TYPE_DATE => 'date',
+		ColumnSchema::TYPE_BINARY => 'blob',
+		ColumnSchema::TYPE_BOOLEAN => 'tinyint(1)',
+		ColumnSchema::TYPE_MONEY => 'decimal(19,4)',
     );
 
 	/**