Fix incorrect syntax for dropping primary indexes in PostgreSQL (#6025)

|      Q       |   A
|------------- | -----------
| Type         | bug/feature/improvement
| Fixed issues | #6024

#### Summary

The correct way of dropping primary index in PostgreSQL is not dropping
the index, but removing the primary key constraints

---------

Co-authored-by: Alexander M. Turek <me@derrabus.de>

src/Platforms/PostgreSQLPlatform.php

@@ -10,6 +10,7 @@
 use Doctrine\DBAL\Schema\Index;
 use Doctrine\DBAL\Schema\PostgreSQLSchemaManager;
 use Doctrine\DBAL\Schema\Sequence;
+use Doctrine\DBAL\Schema\Table;
 use Doctrine\DBAL\Schema\TableDiff;
 use Doctrine\DBAL\Types\BinaryType;
 use Doctrine\DBAL\Types\BlobType;
@@ -809,6 +810,36 @@ public function getDropForeignKeySQL($foreignKey, $table)
         return $this->getDropConstraintSQL($foreignKey, $table);
     }
 
+    /**
+     * {@inheritDoc}
+     */
+    public function getDropIndexSQL($index, $table = null)
+    {
+        if ($index instanceof Index && $index->isPrimary() && $table !== null) {
+            $constraintName = $index->getName() === 'primary' ? $this->tableName($table) . '_pkey' : $index->getName();
+
+            return $this->getDropConstraintSQL($constraintName, $table);
+        }
+
+        if ($index === '"primary"' && $table !== null) {
+            $constraintName = $this->tableName($table) . '_pkey';
+
+            return $this->getDropConstraintSQL($constraintName, $table);
+        }
+
+        return parent::getDropIndexSQL($index, $table);
+    }
+
+    /**
+     * @param Table|string|null $table
+     *
+     * @return string
+     */
+    private function tableName($table)
+    {
+        return $table instanceof Table ? $table->getName() : (string) $table;
+    }
+
     /**
      * {@inheritDoc}
      */

tests/Functional/Driver/DBAL6024Test.php

@@ -0,0 +1,43 @@
+<?php
+
+namespace Doctrine\DBAL\Tests\Functional\Driver;
+
+use Doctrine\DBAL\Schema\Table;
+use Doctrine\DBAL\Tests\FunctionalTestCase;
+use Doctrine\DBAL\Tests\TestUtil;
+
+class DBAL6024Test extends FunctionalTestCase
+{
+    protected function setUp(): void
+    {
+        if (TestUtil::isDriverOneOf('pdo_pgsql', 'pgsql')) {
+            return;
+        }
+
+        self::markTestSkipped('This test requires the pdo_pgsql or the pgsql driver.');
+    }
+
+    public function testDropPrimaryKey(): void
+    {
+        $table = new Table('mytable');
+        $table->addColumn('id', 'integer');
+        $table->setPrimaryKey(['id']);
+        $this->dropAndCreateTable($table);
+
+        $newTable = clone $table;
+        $newTable->dropPrimaryKey();
+
+        $schemaManager = $this->connection->createSchemaManager();
+        $diff          = $schemaManager->createComparator()->compareTables($table, $newTable);
+
+        $statements = $this->connection->getDatabasePlatform()->getAlterTableSQL($diff);
+        foreach ($statements as $statement) {
+            $this->connection->executeStatement($statement);
+        }
+
+        $validationSchema = $schemaManager->introspectSchema();
+        $validationTable  = $validationSchema->getTable($table->getName());
+
+        $this->assertNull($validationTable->getPrimaryKey());
+    }
+}

tests/Platforms/PostgreSQLPlatformTest.php

@@ -581,6 +581,44 @@ public function testDroppingConstraintsBeforeColumns(): void
         self::assertEquals($expectedSql, $sql);
     }
 
+    public function testDroppingPrimaryKey(): void
+    {
+        $oldTable = new Table('mytable');
+        $oldTable->addColumn('id', 'integer');
+        $oldTable->setPrimaryKey(['id']);
+
+        $newTable = clone $oldTable;
+        $newTable->dropPrimaryKey();
+
+        $diff = (new Comparator())->compareTables($oldTable, $newTable);
+
+        $sql = $this->platform->getAlterTableSQL($diff);
+
+        $expectedSql = ['ALTER TABLE mytable DROP CONSTRAINT mytable_pkey'];
+
+        self::assertEquals($expectedSql, $sql);
+    }
+
+    public function testDroppingPrimaryKeyWithUserDefinedName(): void
+    {
+        self::markTestSkipped('Edge case not covered yet');
+
+        $oldTable = new Table('mytable');
+        $oldTable->addColumn('id', 'integer');
+        $oldTable->setPrimaryKey(['id'], 'a_user_name');
+
+        $newTable = clone $oldTable;
+        $newTable->dropPrimaryKey();
+
+        $diff = (new Comparator())->compareTables($oldTable, $newTable);
+
+        $sql = $this->platform->getAlterTableSQL($diff);
+
+        $expectedSql = ['ALTER TABLE mytable DROP CONSTRAINT a_user_name'];
+
+        self::assertEquals($expectedSql, $sql);
+    }
+
     public function testUsesSequenceEmulatedIdentityColumns(): void
     {
         self::assertTrue($this->platform->usesSequenceEmulatedIdentityColumns());