Very naive implementation of table truncation with postgres.

I'm fairly certain that sequence names will need to be generated/handled
as the auto generated names are very unlikely to be the names given
to addConstraint()

lib/Cake/Database/Schema/PostgresSchema.php

@@ -342,4 +342,32 @@ public function dropTableSql(Table $table) {
 		return [sprintf('DROP TABLE "%s"', $table->name())];
 	}
 
+/**
+ * Generate the SQL to truncate a table.
+ *
+ * @param Cake\Database\Schema\Table $table Table instance
+ * @return array SQL statements to drop truncate a table.
+ */
+	public function truncateTableSql(Table $table) {
+		$name = $this->_driver->quoteIdentifier($table->name());
+		$sequence = null;
+		foreach ($table->constraints() as $seq) {
+			if ($table->constraint($seq)['type'] == Table::CONSTRAINT_PRIMARY) {
+				$sequence = $this->_driver->quoteIdentifier($seq);
+				break;
+			}
+		}
+
+		$out = [];
+		if ($sequence) {
+			$out[] = sprintf(
+				'ALTER SEQUENCE %s.%s RESTART WITH 1',
+				$name,
+				$sequence
+			);
+		}
+		$out[] = sprintf('DELETE FROM %s', $name);
+		return $out;
+	}
+
 }

lib/Cake/Test/TestCase/Database/Schema/PostgresSchemaTest.php

@@ -562,6 +562,29 @@ public function testDropSql() {
 		$this->assertEquals('DROP TABLE "articles"', $result[0]);
 	}
 
+/**
+ * Test truncateSql()
+ *
+ * @return void
+ */
+	public function testTruncateSql() {
+		$driver = $this->_getMockedDriver();
+		$connection = $this->getMock('Cake\Database\Connection', array(), array(), '', false);
+		$connection->expects($this->any())->method('driver')
+			->will($this->returnValue($driver));
+
+		$table = new Table('articles');
+		$table->addColumn('id', 'integer')
+			->addConstraint('primary', [
+				'type' => 'primary',
+				'columns' => ['id']
+			]);
+		$result = $table->truncateSql($connection);
+		$this->assertCount(2, $result);
+		$this->assertEquals('ALTER SEQUENCE "articles"."primary" RESTART WITH 1', $result[0]);
+		$this->assertEquals('DELETE FROM "articles"', $result[1]);
+	}
+
 /**
  * Get a schema instance with a mocked driver/pdo instances
  *