Add support for identity columns in PostgreSQL 10+

Since PostgreSQL 10 the preferred method of defining auto increment
columns is to use SQL standard identity columns instead of SERIAL psedo-types
COLUMN TYPE GENERATED { ALWAYS | BY DEFAULT } AS IDENTITY [ ( sequence_options ) ]
See https://www.postgresql.org/docs/10/static/sql-createtable.html

It fixes issues with SERIAL psudo-types:
- CREATE TABLE / LIKE copies default but refers to same sequence
- cannot add/drop serialness with ALTER TABLE
- dropping default does not drop sequence
- need to grant separate privileges to sequence
- other slight weirdnesses because serial is some kind of special macro
As explained by author of the patch in
https://git.postgresql.org/gitweb/?p=postgresql.git;a=commitdiff;h=3217327053638085d24dd4d276e7c1f7ac2c4c6b

src/Illuminate/Database/Schema/ColumnDefinition.php

@@ -19,6 +19,8 @@
  * @method ColumnDefinition unsigned() Set the INTEGER column as UNSIGNED (MySQL)
  * @method ColumnDefinition useCurrent() Set the TIMESTAMP column to use CURRENT_TIMESTAMP as default value
  * @method ColumnDefinition virtualAs(string $expression) Create a virtual generated column (MySQL)
+ * @method ColumnDefinition generatedAs($expression) Create a SQL compliant identity column (PostgreSQL)
+ * @method ColumnDefinition always() Used as a modifier for generatedAs() (PostgreSQL)
  */
 class ColumnDefinition extends Fluent
 {

src/Illuminate/Database/Schema/Grammars/PostgresGrammar.php

@@ -437,6 +437,38 @@ protected function typeLongText(Fluent $column)
         return 'text';
     }
 
+    private function generatableColumn($type, Fluent $column)
+    {
+        if (! $column->autoIncrement && $column->generatedAs === null) {
+            return $type;
+        }
+
+        if ($column->autoIncrement && $column->generatedAs === null) {
+            $serial = [
+                'integer' => 'serial',
+                'bigint' => 'bigserial',
+                'smallint' => 'smallserial',
+            ];
+
+            return $serial[$type];
+        }
+
+        // Identity column but not a primary key
+        $always = $column->always ? 'always' : 'by default';
+        $options = '';
+
+        if (! is_bool($column->generatedAs) && ((string) $column->generatedAs !== '')) {
+            $options = sprintf(' (%s)', $column->generatedAs);
+        }
+
+        return sprintf(
+            '%s generated %s as identity%s',
+            $type,
+            $always,
+            $options
+        );
+    }
+
     /**
      * Create the column definition for an integer type.
      *
@@ -445,7 +477,7 @@ protected function typeLongText(Fluent $column)
      */
     protected function typeInteger(Fluent $column)
     {
-        return $column->autoIncrement ? 'serial' : 'integer';
+        return $this->generatableColumn('integer', $column);
     }
 
     /**
@@ -456,7 +488,7 @@ protected function typeInteger(Fluent $column)
      */
     protected function typeBigInteger(Fluent $column)
     {
-        return $column->autoIncrement ? 'bigserial' : 'bigint';
+        return $this->generatableColumn('bigint', $column);
     }
 
     /**
@@ -467,7 +499,7 @@ protected function typeBigInteger(Fluent $column)
      */
     protected function typeMediumInteger(Fluent $column)
     {
-        return $column->autoIncrement ? 'serial' : 'integer';
+        return $this->generatableColumn('integer', $column);
     }
 
     /**
@@ -478,7 +510,7 @@ protected function typeMediumInteger(Fluent $column)
      */
     protected function typeTinyInteger(Fluent $column)
     {
-        return $column->autoIncrement ? 'smallserial' : 'smallint';
+        return $this->generatableColumn('smallint', $column);
     }
 
     /**
@@ -489,7 +521,7 @@ protected function typeTinyInteger(Fluent $column)
      */
     protected function typeSmallInteger(Fluent $column)
     {
-        return $column->autoIncrement ? 'smallserial' : 'smallint';
+        return $this->generatableColumn('smallint', $column);
     }
 
     /**
@@ -854,7 +886,7 @@ protected function modifyDefault(Blueprint $blueprint, Fluent $column)
      */
     protected function modifyIncrement(Blueprint $blueprint, Fluent $column)
     {
-        if (in_array($column->type, $this->serials) && $column->autoIncrement) {
+        if ((in_array($column->type, $this->serials) || ($column->generatedAs !== null)) && $column->autoIncrement) {
             return ' primary key';
         }
     }

tests/Database/DatabasePostgresSchemaGrammarTest.php

@@ -660,6 +660,33 @@ public function testAddingUuid()
         $this->assertEquals('alter table "users" add column "foo" uuid not null', $statements[0]);
     }
 
+    public function testAddingGeneratedAs()
+    {
+        $blueprint = new Blueprint('users');
+        $blueprint->increments('foo')->generatedAs();
+        $statements = $blueprint->toSql($this->getConnection(), $this->getGrammar());
+        $this->assertCount(1, $statements);
+        $this->assertEquals('alter table "users" add column "foo" integer generated by default as identity primary key not null', $statements[0]);
+        // With always modifier
+        $blueprint = new Blueprint('users');
+        $blueprint->increments('foo')->generatedAs()->always();
+        $statements = $blueprint->toSql($this->getConnection(), $this->getGrammar());
+        $this->assertCount(1, $statements);
+        $this->assertEquals('alter table "users" add column "foo" integer generated always as identity primary key not null', $statements[0]);
+        // With sequence options
+        $blueprint = new Blueprint('users');
+        $blueprint->increments('foo')->generatedAs('increment by 10 start with 100');
+        $statements = $blueprint->toSql($this->getConnection(), $this->getGrammar());
+        $this->assertCount(1, $statements);
+        $this->assertEquals('alter table "users" add column "foo" integer generated by default as identity (increment by 10 start with 100) primary key not null', $statements[0]);
+        // Not a primary key
+        $blueprint = new Blueprint('users');
+        $blueprint->integer('foo')->generatedAs();
+        $statements = $blueprint->toSql($this->getConnection(), $this->getGrammar());
+        $this->assertCount(1, $statements);
+        $this->assertEquals('alter table "users" add column "foo" integer generated by default as identity not null', $statements[0]);
+    }
+
     public function testAddingIpAddress()
     {
         $blueprint = new Blueprint('users');