diff --git a/UPGRADE.md b/UPGRADE.md
index 3592fe5b6..8aba870c6 100644
--- a/UPGRADE.md
+++ b/UPGRADE.md
@@ -1,3 +1,17 @@
+# Upgrade to 2.6
+
+## Minor BC BREAK: removed `Doctrine\ORM\Query\Parser#isInternalFunction`
+
+Method `Doctrine\ORM\Query\Parser#isInternalFunction` was removed because 
+the distinction between internal function and user defined DQL was removed.
+[#6500](https://github.com/doctrine/doctrine2/pull/6500)
+
+## Minor BC BREAK: removed `Doctrine\ORM\ORMException#overwriteInternalDQLFunctionNotAllowed`
+
+Method `Doctrine\ORM\Query\Parser#overwriteInternalDQLFunctionNotAllowed` was 
+removed because of the choice to allow users to overwrite internal functions, ie
+`AVG`, `SUM`, `COUNT`, `MIN` and `MAX`. [#6500](https://github.com/doctrine/doctrine2/pull/6500)
+
 # Upgrade to 2.5
 
 ## Minor BC BREAK: removed `Doctrine\ORM\Query\SqlWalker#walkCaseExpression()`
diff --git a/lib/Doctrine/ORM/Configuration.php b/lib/Doctrine/ORM/Configuration.php
index 104a4f4b2..6073fbe00 100644
--- a/lib/Doctrine/ORM/Configuration.php
+++ b/lib/Doctrine/ORM/Configuration.php
@@ -420,15 +420,9 @@ class Configuration extends \Doctrine\DBAL\Configuration
      * @param string|callable $className Class name or a callable that returns the function.
      *
      * @return void
-     *
-     * @throws ORMException
      */
     public function addCustomStringFunction($name, $className)
     {
-        if (Query\Parser::isInternalFunction($name)) {
-            throw ORMException::overwriteInternalDQLFunctionNotAllowed($name);
-        }
-
         $this->_attributes['customStringFunctions'][strtolower($name)] = $className;
     }
 
@@ -478,15 +472,9 @@ class Configuration extends \Doctrine\DBAL\Configuration
      * @param string|callable $className Class name or a callable that returns the function.
      *
      * @return void
-     *
-     * @throws ORMException
      */
     public function addCustomNumericFunction($name, $className)
     {
-        if (Query\Parser::isInternalFunction($name)) {
-            throw ORMException::overwriteInternalDQLFunctionNotAllowed($name);
-        }
-
         $this->_attributes['customNumericFunctions'][strtolower($name)] = $className;
     }
 
@@ -536,15 +524,9 @@ class Configuration extends \Doctrine\DBAL\Configuration
      * @param string|callable $className Class name or a callable that returns the function.
      *
      * @return void
-     *
-     * @throws ORMException
      */
     public function addCustomDatetimeFunction($name, $className)
     {
-        if (Query\Parser::isInternalFunction($name)) {
-            throw ORMException::overwriteInternalDQLFunctionNotAllowed($name);
-        }
-
         $this->_attributes['customDatetimeFunctions'][strtolower($name)] = $className;
     }
 
diff --git a/lib/Doctrine/ORM/ORMException.php b/lib/Doctrine/ORM/ORMException.php
index 039ef2eaa..3ce9ce9dc 100644
--- a/lib/Doctrine/ORM/ORMException.php
+++ b/lib/Doctrine/ORM/ORMException.php
@@ -323,16 +323,6 @@ class ORMException extends Exception
         );
     }
 
-    /**
-     * @param string $functionName
-     *
-     * @return ORMException
-     */
-    public static function overwriteInternalDQLFunctionNotAllowed($functionName)
-    {
-        return new self("It is not allowed to overwrite internal function '$functionName' in the DQL parser through user-defined functions.");
-    }
-
     /**
      * @return ORMException
      */
diff --git a/lib/Doctrine/ORM/Query/AST/Functions/AvgFunction.php b/lib/Doctrine/ORM/Query/AST/Functions/AvgFunction.php
new file mode 100644
index 000000000..6cb8d92b6
--- /dev/null
+++ b/lib/Doctrine/ORM/Query/AST/Functions/AvgFunction.php
@@ -0,0 +1,54 @@
+<?php
+/*
+ * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
+ * "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
+ * LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
+ * A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
+ * OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
+ * SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
+ * LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
+ * DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
+ * THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
+ * (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
+ * OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+ *
+ * This software consists of voluntary contributions made by many individuals
+ * and is licensed under the MIT license. For more information, see
+ * <http://www.doctrine-project.org>.
+ */
+
+namespace Doctrine\ORM\Query\AST\Functions;
+
+use Doctrine\ORM\Query\Parser;
+use Doctrine\ORM\Query\SqlWalker;
+use Doctrine\ORM\Query\AST\AggregateExpression;
+
+/**
+ * "AVG" "(" ["DISTINCT"] StringPrimary ")"
+ *
+ * @since   2.6
+ * @author  Mathew Davies <thepixeldeveloper@icloud.com>
+ */
+final class AvgFunction extends FunctionNode
+{
+    /**
+     * @var AggregateExpression
+     */
+    private $aggregateExpression;
+
+    /**
+     * @inheritDoc
+     */
+    public function getSql(SqlWalker $sqlWalker): string
+    {
+        return $this->aggregateExpression->dispatch($sqlWalker);
+    }
+
+    /**
+     * @inheritDoc
+     */
+    public function parse(Parser $parser): void
+    {
+        $this->aggregateExpression = $parser->AggregateExpression();
+    }
+}
diff --git a/lib/Doctrine/ORM/Query/AST/Functions/CountFunction.php b/lib/Doctrine/ORM/Query/AST/Functions/CountFunction.php
new file mode 100644
index 000000000..f232cf43f
--- /dev/null
+++ b/lib/Doctrine/ORM/Query/AST/Functions/CountFunction.php
@@ -0,0 +1,54 @@
+<?php
+/*
+ * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
+ * "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
+ * LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
+ * A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
+ * OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
+ * SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
+ * LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
+ * DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
+ * THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
+ * (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
+ * OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+ *
+ * This software consists of voluntary contributions made by many individuals
+ * and is licensed under the MIT license. For more information, see
+ * <http://www.doctrine-project.org>.
+ */
+
+namespace Doctrine\ORM\Query\AST\Functions;
+
+use Doctrine\ORM\Query\Parser;
+use Doctrine\ORM\Query\SqlWalker;
+use Doctrine\ORM\Query\AST\AggregateExpression;
+
+/**
+ * "COUNT" "(" ["DISTINCT"] StringPrimary ")"
+ *
+ * @since   2.6
+ * @author  Mathew Davies <thepixeldeveloper@icloud.com>
+ */
+final class CountFunction extends FunctionNode
+{
+    /**
+     * @var AggregateExpression
+     */
+    private $aggregateExpression;
+
+    /**
+     * @inheritDoc
+     */
+    public function getSql(SqlWalker $sqlWalker): string
+    {
+        return $this->aggregateExpression->dispatch($sqlWalker);
+    }
+
+    /**
+     * @inheritDoc
+     */
+    public function parse(Parser $parser): void
+    {
+        $this->aggregateExpression = $parser->AggregateExpression();
+    }
+}
diff --git a/lib/Doctrine/ORM/Query/AST/Functions/MaxFunction.php b/lib/Doctrine/ORM/Query/AST/Functions/MaxFunction.php
new file mode 100644
index 000000000..eba9b8639
--- /dev/null
+++ b/lib/Doctrine/ORM/Query/AST/Functions/MaxFunction.php
@@ -0,0 +1,54 @@
+<?php
+/*
+ * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
+ * "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
+ * LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
+ * A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
+ * OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
+ * SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
+ * LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
+ * DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
+ * THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
+ * (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
+ * OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+ *
+ * This software consists of voluntary contributions made by many individuals
+ * and is licensed under the MIT license. For more information, see
+ * <http://www.doctrine-project.org>.
+ */
+
+namespace Doctrine\ORM\Query\AST\Functions;
+
+use Doctrine\ORM\Query\Parser;
+use Doctrine\ORM\Query\SqlWalker;
+use Doctrine\ORM\Query\AST\AggregateExpression;
+
+/**
+ * "MAX" "(" ["DISTINCT"] StringPrimary ")"
+ *
+ * @since   2.6
+ * @author  Mathew Davies <thepixeldeveloper@icloud.com>
+ */
+final class MaxFunction extends FunctionNode
+{
+    /**
+     * @var AggregateExpression
+     */
+    private $aggregateExpression;
+
+    /**
+     * @inheritDoc
+     */
+    public function getSql(SqlWalker $sqlWalker): string
+    {
+        return $this->aggregateExpression->dispatch($sqlWalker);
+    }
+
+    /**
+     * @inheritDoc
+     */
+    public function parse(Parser $parser): void
+    {
+        $this->aggregateExpression = $parser->AggregateExpression();
+    }
+}
diff --git a/lib/Doctrine/ORM/Query/AST/Functions/MinFunction.php b/lib/Doctrine/ORM/Query/AST/Functions/MinFunction.php
new file mode 100644
index 000000000..e1c086913
--- /dev/null
+++ b/lib/Doctrine/ORM/Query/AST/Functions/MinFunction.php
@@ -0,0 +1,54 @@
+<?php
+/*
+ * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
+ * "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
+ * LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
+ * A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
+ * OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
+ * SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
+ * LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
+ * DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
+ * THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
+ * (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
+ * OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+ *
+ * This software consists of voluntary contributions made by many individuals
+ * and is licensed under the MIT license. For more information, see
+ * <http://www.doctrine-project.org>.
+ */
+
+namespace Doctrine\ORM\Query\AST\Functions;
+
+use Doctrine\ORM\Query\Parser;
+use Doctrine\ORM\Query\SqlWalker;
+use Doctrine\ORM\Query\AST\AggregateExpression;
+
+/**
+ * "MIN" "(" ["DISTINCT"] StringPrimary ")"
+ *
+ * @since   2.6
+ * @author  Mathew Davies <thepixeldeveloper@icloud.com>
+ */
+final class MinFunction extends FunctionNode
+{
+    /**
+     * @var AggregateExpression
+     */
+    private $aggregateExpression;
+
+    /**
+     * @inheritDoc
+     */
+    public function getSql(SqlWalker $sqlWalker): string
+    {
+        return $this->aggregateExpression->dispatch($sqlWalker);
+    }
+
+    /**
+     * @inheritDoc
+     */
+    public function parse(Parser $parser): void
+    {
+        $this->aggregateExpression = $parser->AggregateExpression();
+    }
+}
diff --git a/lib/Doctrine/ORM/Query/AST/Functions/SumFunction.php b/lib/Doctrine/ORM/Query/AST/Functions/SumFunction.php
new file mode 100644
index 000000000..c9fcf7b04
--- /dev/null
+++ b/lib/Doctrine/ORM/Query/AST/Functions/SumFunction.php
@@ -0,0 +1,54 @@
+<?php
+/*
+ * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
+ * "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
+ * LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
+ * A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
+ * OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
+ * SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
+ * LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
+ * DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
+ * THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
+ * (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
+ * OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+ *
+ * This software consists of voluntary contributions made by many individuals
+ * and is licensed under the MIT license. For more information, see
+ * <http://www.doctrine-project.org>.
+ */
+
+namespace Doctrine\ORM\Query\AST\Functions;
+
+use Doctrine\ORM\Query\Parser;
+use Doctrine\ORM\Query\SqlWalker;
+use Doctrine\ORM\Query\AST\AggregateExpression;
+
+/**
+ * "SUM" "(" ["DISTINCT"] StringPrimary ")"
+ *
+ * @since   2.6
+ * @author  Mathew Davies <thepixeldeveloper@icloud.com>
+ */
+final class SumFunction extends FunctionNode
+{
+    /**
+     * @var AggregateExpression
+     */
+    private $aggregateExpression;
+
+    /**
+     * @inheritDoc
+     */
+    public function getSql(SqlWalker $sqlWalker): string
+    {
+        return $this->aggregateExpression->dispatch($sqlWalker);
+    }
+
+    /**
+     * @inheritDoc
+     */
+    public function parse(Parser $parser): void
+    {
+        $this->aggregateExpression = $parser->AggregateExpression();
+    }
+}
diff --git a/lib/Doctrine/ORM/Query/Parser.php b/lib/Doctrine/ORM/Query/Parser.php
index 34679ff61..a06cd83a6 100644
--- a/lib/Doctrine/ORM/Query/Parser.php
+++ b/lib/Doctrine/ORM/Query/Parser.php
@@ -65,6 +65,13 @@ class Parser
         'date_diff' => Functions\DateDiffFunction::class,
         'bit_and'   => Functions\BitAndFunction::class,
         'bit_or'    => Functions\BitOrFunction::class,
+
+        // Aggregate functions
+        'min'       => Functions\MinFunction::class,
+        'max'       => Functions\MaxFunction::class,
+        'avg'       => Functions\AvgFunction::class,
+        'sum'       => Functions\SumFunction::class,
+        'count'     => Functions\CountFunction::class,
     ];
 
     /**
@@ -171,23 +178,6 @@ class Parser
      */
     private $identVariableExpressions = [];
 
-    /**
-     * Checks if a function is internally defined. Used to prevent overwriting
-     * of built-in functions through user-defined functions.
-     *
-     * @param string $functionName
-     *
-     * @return bool
-     */
-    static public function isInternalFunction($functionName)
-    {
-        $functionName = strtolower($functionName);
-
-        return isset(self::$_STRING_FUNCTIONS[$functionName])
-            || isset(self::$_DATETIME_FUNCTIONS[$functionName])
-            || isset(self::$_NUMERIC_FUNCTIONS[$functionName]);
-    }
-
     /**
      * Creates a new query parser object.
      *
@@ -1978,9 +1968,6 @@ class Parser
                         // SUM(u.id) + COUNT(u.id)
                         return $this->SimpleArithmeticExpression();
 
-                    case ($this->isAggregateFunction($this->lexer->lookahead['type'])):
-                        return $this->AggregateExpression();
-
                     default:
                         // IDENTITY(u)
                         return $this->FunctionDeclaration();
@@ -2209,11 +2196,6 @@ class Parser
                         $expression = $this->ScalarExpression();
                         break;
 
-                    case ($this->isAggregateFunction($lookaheadType)):
-                        // COUNT(u.id)
-                        $expression = $this->AggregateExpression();
-                        break;
-
                     default:
                         // IDENTITY(u)
                         $expression = $this->FunctionDeclaration();
@@ -2858,10 +2840,6 @@ class Parser
                 $peek = $this->lexer->glimpse();
 
                 if ($peek['value'] == '(') {
-                    if ($this->isAggregateFunction($this->lexer->lookahead['type'])) {
-                        return $this->AggregateExpression();
-                    }
-
                     return $this->FunctionDeclaration();
                 }
 
@@ -2932,11 +2910,6 @@ class Parser
             case Lexer::T_COALESCE:
             case Lexer::T_NULLIF:
                 return $this->CaseExpression();
-
-            default:
-                if ($this->isAggregateFunction($lookaheadType)) {
-                    return $this->AggregateExpression();
-                }
         }
 
         $this->syntaxError(
@@ -3236,10 +3209,6 @@ class Parser
                 $expr = $this->CoalesceExpression();
                 break;
 
-            case $this->isAggregateFunction($this->lexer->lookahead['type']):
-                $expr = $this->AggregateExpression();
-                break;
-
             case $this->isFunction():
                 $expr = $this->FunctionDeclaration();
                 break;
@@ -3376,8 +3345,13 @@ class Parser
         $token = $this->lexer->lookahead;
         $funcName = strtolower($token['value']);
 
-        // Check for built-in functions first!
+        $customFunctionDeclaration = $this->CustomFunctionDeclaration();
+
+        // Check for custom functions functions first!
         switch (true) {
+            case $customFunctionDeclaration !== null:
+                return $customFunctionDeclaration;
+            
             case (isset(self::$_STRING_FUNCTIONS[$funcName])):
                 return $this->FunctionsReturningStrings();
 
@@ -3388,7 +3362,7 @@ class Parser
                 return $this->FunctionsReturningDatetime();
 
             default:
-                return $this->CustomFunctionDeclaration();
+                $this->syntaxError('known function', $token);
         }
     }
 
@@ -3416,7 +3390,7 @@ class Parser
                 return $this->CustomFunctionsReturningDatetime();
 
             default:
-                $this->syntaxError('known function', $token);
+                return null;
         }
     }
 
diff --git a/lib/Doctrine/ORM/Query/SqlWalker.php b/lib/Doctrine/ORM/Query/SqlWalker.php
index ee57cf90b..a4b5089e8 100644
--- a/lib/Doctrine/ORM/Query/SqlWalker.php
+++ b/lib/Doctrine/ORM/Query/SqlWalker.php
@@ -1579,12 +1579,6 @@ class SqlWalker implements TreeWalker
                 $sql .= $this->walkPathExpression($expr);
                 break;
 
-            case ($expr instanceof AST\AggregateExpression):
-                $alias = $simpleSelectExpression->fieldIdentificationVariable ?: $this->scalarResultCounter++;
-
-                $sql .= $this->walkAggregateExpression($expr) . ' AS dctrn__' . $alias;
-                break;
-
             case ($expr instanceof AST\Subselect):
                 $alias = $simpleSelectExpression->fieldIdentificationVariable ?: $this->scalarResultCounter++;
 
diff --git a/tests/Doctrine/Tests/ORM/ConfigurationTest.php b/tests/Doctrine/Tests/ORM/ConfigurationTest.php
index ab657c5fd..92a2eeaea 100644
--- a/tests/Doctrine/Tests/ORM/ConfigurationTest.php
+++ b/tests/Doctrine/Tests/ORM/ConfigurationTest.php
@@ -265,8 +265,6 @@ class ConfigurationTest extends TestCase
         $this->assertSame(null, $this->configuration->getCustomStringFunction('NonExistingFunction'));
         $this->configuration->setCustomStringFunctions(['OtherFunctionName' => __CLASS__]);
         $this->assertSame(__CLASS__, $this->configuration->getCustomStringFunction('OtherFunctionName'));
-        $this->expectException(ORMException::class);
-        $this->configuration->addCustomStringFunction('concat', __CLASS__);
     }
 
     public function testAddGetCustomNumericFunction()
@@ -276,8 +274,6 @@ class ConfigurationTest extends TestCase
         $this->assertSame(null, $this->configuration->getCustomNumericFunction('NonExistingFunction'));
         $this->configuration->setCustomNumericFunctions(['OtherFunctionName' => __CLASS__]);
         $this->assertSame(__CLASS__, $this->configuration->getCustomNumericFunction('OtherFunctionName'));
-        $this->expectException(ORMException::class);
-        $this->configuration->addCustomNumericFunction('abs', __CLASS__);
     }
 
     public function testAddGetCustomDatetimeFunction()
@@ -287,8 +283,6 @@ class ConfigurationTest extends TestCase
         $this->assertSame(null, $this->configuration->getCustomDatetimeFunction('NonExistingFunction'));
         $this->configuration->setCustomDatetimeFunctions(['OtherFunctionName' => __CLASS__]);
         $this->assertSame(__CLASS__, $this->configuration->getCustomDatetimeFunction('OtherFunctionName'));
-        $this->expectException(ORMException::class);
-        $this->configuration->addCustomDatetimeFunction('date_add', __CLASS__);
     }
 
     public function testAddGetCustomHydrationMode()
diff --git a/tests/Doctrine/Tests/ORM/Functional/CustomFunctionsTest.php b/tests/Doctrine/Tests/ORM/Functional/CustomFunctionsTest.php
index 63c3e4ee9..79668b8f2 100644
--- a/tests/Doctrine/Tests/ORM/Functional/CustomFunctionsTest.php
+++ b/tests/Doctrine/Tests/ORM/Functional/CustomFunctionsTest.php
@@ -47,6 +47,23 @@ class CustomFunctionsTest extends OrmFunctionalTestCase
         $this->assertEquals(1, count($users));
         $this->assertSame($user, $users[0]);
     }
+
+    public function testCustomFunctionOverride()
+    {
+        $user = new CmsUser();
+        $user->name = 'Bob';
+        $user->username = 'Dylan';
+        $this->_em->persist($user);
+        $this->_em->flush();
+
+        $this->_em->getConfiguration()->addCustomStringFunction('COUNT', 'Doctrine\Tests\ORM\Functional\CustomCount');
+
+        $query = $this->_em->createQuery('SELECT COUNT(DISTINCT u.id) FROM Doctrine\Tests\Models\CMS\CmsUser u');
+
+        $usersCount = $query->getSingleScalarResult();
+
+        $this->assertEquals(1, $usersCount);
+    }
 }
 
 class NoOp extends FunctionNode
@@ -70,3 +87,20 @@ class NoOp extends FunctionNode
     }
 }
 
+class CustomCount extends FunctionNode
+{
+    /**
+     * @var Query\AST\AggregateExpression
+     */
+    private $aggregateExpression;
+
+    public function parse(Parser $parser): void
+    {
+        $this->aggregateExpression = $parser->AggregateExpression();
+    }
+
+    public function getSql(SqlWalker $sqlWalker): string
+    {
+        return $this->aggregateExpression->dispatch($sqlWalker);
+    }
+}
diff --git a/tests/Doctrine/Tests/ORM/Query/SelectSqlGenerationTest.php b/tests/Doctrine/Tests/ORM/Query/SelectSqlGenerationTest.php
index b674c5d38..c10afbb89 100644
--- a/tests/Doctrine/Tests/ORM/Query/SelectSqlGenerationTest.php
+++ b/tests/Doctrine/Tests/ORM/Query/SelectSqlGenerationTest.php
@@ -1031,7 +1031,7 @@ class SelectSqlGenerationTest extends OrmTestCase
     {
         $this->assertSqlGeneration(
             "SELECT u.name, (SELECT COUNT(p.phonenumber) FROM Doctrine\Tests\Models\CMS\CmsPhonenumber p WHERE p.phonenumber = 1234) pcount FROM Doctrine\Tests\Models\CMS\CmsUser u WHERE u.name = 'jon'",
-            "SELECT c0_.name AS name_0, (SELECT COUNT(c1_.phonenumber) AS dctrn__1 FROM cms_phonenumbers c1_ WHERE c1_.phonenumber = 1234) AS sclr_1 FROM cms_users c0_ WHERE c0_.name = 'jon'"
+            "SELECT c0_.name AS name_0, (SELECT COUNT(c1_.phonenumber) AS sclr_2 FROM cms_phonenumbers c1_ WHERE c1_.phonenumber = 1234) AS sclr_1 FROM cms_users c0_ WHERE c0_.name = 'jon'"
         );
     }
 
@@ -1198,7 +1198,7 @@ class SelectSqlGenerationTest extends OrmTestCase
     {
         $this->assertSqlGeneration(
             "SELECT u.name, (SELECT COUNT(cfc.id) total FROM Doctrine\Tests\Models\Company\CompanyFixContract cfc) as cfc_count FROM Doctrine\Tests\Models\CMS\CmsUser u",
-            "SELECT c0_.name AS name_0, (SELECT COUNT(c1_.id) AS dctrn__total FROM company_contracts c1_ WHERE c1_.discr IN ('fix')) AS sclr_1 FROM cms_users c0_"
+            "SELECT c0_.name AS name_0, (SELECT COUNT(c1_.id) AS sclr_2 FROM company_contracts c1_ WHERE c1_.discr IN ('fix')) AS sclr_1 FROM cms_users c0_"
         );
     }
 
@@ -1750,7 +1750,7 @@ class SelectSqlGenerationTest extends OrmTestCase
         );
         $this->assertSqlGeneration(
             'SELECT u1 FROM Doctrine\Tests\Models\CMS\CmsUser u1 WHERE COUNT(u1.id) = ( SELECT SUM(u2.id) FROM Doctrine\Tests\Models\CMS\CmsUser u2 )',
-            'SELECT c0_.id AS id_0, c0_.status AS status_1, c0_.username AS username_2, c0_.name AS name_3 FROM cms_users c0_ WHERE COUNT(c0_.id) = (SELECT SUM(c1_.id) AS dctrn__1 FROM cms_users c1_)'
+            'SELECT c0_.id AS id_0, c0_.status AS status_1, c0_.username AS username_2, c0_.name AS name_3 FROM cms_users c0_ WHERE COUNT(c0_.id) = (SELECT SUM(c1_.id) AS sclr_4 FROM cms_users c1_)'
         );
         $this->assertSqlGeneration(
             'SELECT u1 FROM Doctrine\Tests\Models\CMS\CmsUser u1 WHERE COUNT(u1.id) <= ( SELECT SUM(u2.id) + COUNT(u2.email) FROM Doctrine\Tests\Models\CMS\CmsUser u2 )',
diff --git a/tests/Doctrine/Tests/ORM/Tools/Pagination/LimitSubqueryOutputWalkerTest.php b/tests/Doctrine/Tests/ORM/Tools/Pagination/LimitSubqueryOutputWalkerTest.php
index 06e21173d..7e65ab7bc 100644
--- a/tests/Doctrine/Tests/ORM/Tools/Pagination/LimitSubqueryOutputWalkerTest.php
+++ b/tests/Doctrine/Tests/ORM/Tools/Pagination/LimitSubqueryOutputWalkerTest.php
@@ -330,7 +330,7 @@ ORDER BY b.id DESC'
         $query->setHint(Query::HINT_CUSTOM_OUTPUT_WALKER, LimitSubqueryOutputWalker::class);
 
         $this->assertEquals(
-            'SELECT DISTINCT id_0 FROM (SELECT b0_.id AS id_0, b0_.author_id AS author_id_1, b0_.category_id AS category_id_2 FROM BlogPost b0_ WHERE ((SELECT COUNT(b1_.id) AS dctrn__1 FROM BlogPost b1_) = 1)) dctrn_result ORDER BY id_0 DESC',
+            'SELECT DISTINCT id_0 FROM (SELECT b0_.id AS id_0, b0_.author_id AS author_id_1, b0_.category_id AS category_id_2 FROM BlogPost b0_ WHERE ((SELECT COUNT(b1_.id) AS sclr_3 FROM BlogPost b1_) = 1)) dctrn_result ORDER BY id_0 DESC',
             $query->getSQL()
         );
     }
@@ -346,7 +346,7 @@ ORDER BY b.id DESC'
         $query->setHint(Query::HINT_CUSTOM_OUTPUT_WALKER, LimitSubqueryOutputWalker::class);
 
         $this->assertEquals(
-            'SELECT DISTINCT id_0, MIN(sclr_1) AS dctrn_minrownum FROM (SELECT b0_.id AS id_0, ROW_NUMBER() OVER(ORDER BY b0_.id DESC) AS sclr_1, b0_.author_id AS author_id_2, b0_.category_id AS category_id_3 FROM BlogPost b0_ WHERE ((SELECT COUNT(b1_.id) AS dctrn__1 FROM BlogPost b1_) = 1)) dctrn_result GROUP BY id_0 ORDER BY dctrn_minrownum ASC',
+            'SELECT DISTINCT id_0, MIN(sclr_1) AS dctrn_minrownum FROM (SELECT b0_.id AS id_0, ROW_NUMBER() OVER(ORDER BY b0_.id DESC) AS sclr_1, b0_.author_id AS author_id_2, b0_.category_id AS category_id_3 FROM BlogPost b0_ WHERE ((SELECT COUNT(b1_.id) AS sclr_4 FROM BlogPost b1_) = 1)) dctrn_result GROUP BY id_0 ORDER BY dctrn_minrownum ASC',
             $query->getSQL()
         );
     }
@@ -390,7 +390,7 @@ ORDER BY b.id DESC'
         $query->setHint(Query::HINT_CUSTOM_OUTPUT_WALKER, LimitSubqueryOutputWalker::class);
 
         $this->assertEquals(
-            'SELECT DISTINCT id_0 FROM (SELECT a0_.id AS id_0, a0_.name AS name_1, (SELECT MIN(m1_.title) AS dctrn__1 FROM MyBlogPost m1_ WHERE m1_.author_id = a0_.id) AS sclr_2 FROM Author a0_) dctrn_result ORDER BY sclr_2 DESC',
+            'SELECT DISTINCT id_0 FROM (SELECT a0_.id AS id_0, a0_.name AS name_1, (SELECT MIN(m1_.title) AS sclr_3 FROM MyBlogPost m1_ WHERE m1_.author_id = a0_.id) AS sclr_2 FROM Author a0_) dctrn_result ORDER BY sclr_2 DESC',
             $query->getSQL()
         );
     }
@@ -415,7 +415,7 @@ ORDER BY b.id DESC'
         $query->setHint(Query::HINT_CUSTOM_OUTPUT_WALKER, LimitSubqueryOutputWalker::class);
 
         $this->assertEquals(
-            'SELECT DISTINCT id_0, MIN(sclr_3) AS dctrn_minrownum FROM (SELECT a0_.id AS id_0, a0_.name AS name_1, (SELECT MIN(m1_.title) AS dctrn__1 FROM MyBlogPost m1_ WHERE m1_.author_id = a0_.id) AS sclr_2, ROW_NUMBER() OVER(ORDER BY (SELECT MIN(m1_.title) AS dctrn__2 FROM MyBlogPost m1_ WHERE m1_.author_id = a0_.id) DESC) AS sclr_3 FROM Author a0_) dctrn_result GROUP BY id_0 ORDER BY dctrn_minrownum ASC',
+            'SELECT DISTINCT id_0, MIN(sclr_4) AS dctrn_minrownum FROM (SELECT a0_.id AS id_0, a0_.name AS name_1, (SELECT MIN(m1_.title) AS sclr_3 FROM MyBlogPost m1_ WHERE m1_.author_id = a0_.id) AS sclr_2, ROW_NUMBER() OVER(ORDER BY (SELECT MIN(m1_.title) AS sclr_5 FROM MyBlogPost m1_ WHERE m1_.author_id = a0_.id) DESC) AS sclr_4 FROM Author a0_) dctrn_result GROUP BY id_0 ORDER BY dctrn_minrownum ASC',
             $query->getSQL()
         );
     }
@@ -440,7 +440,7 @@ ORDER BY b.id DESC'
         $query->setHint(Query::HINT_CUSTOM_OUTPUT_WALKER, LimitSubqueryOutputWalker::class);
 
         $this->assertEquals(
-            'SELECT DISTINCT ID_0, MIN(SCLR_3) AS dctrn_minrownum FROM (SELECT a0_.id AS ID_0, a0_.name AS NAME_1, (SELECT MIN(m1_.title) AS dctrn__1 FROM MyBlogPost m1_ WHERE m1_.author_id = a0_.id) AS SCLR_2, ROW_NUMBER() OVER(ORDER BY (SELECT MIN(m1_.title) AS dctrn__2 FROM MyBlogPost m1_ WHERE m1_.author_id = a0_.id) DESC) AS SCLR_3 FROM Author a0_) dctrn_result GROUP BY ID_0 ORDER BY dctrn_minrownum ASC',
+            'SELECT DISTINCT ID_0, MIN(SCLR_4) AS dctrn_minrownum FROM (SELECT a0_.id AS ID_0, a0_.name AS NAME_1, (SELECT MIN(m1_.title) AS SCLR_3 FROM MyBlogPost m1_ WHERE m1_.author_id = a0_.id) AS SCLR_2, ROW_NUMBER() OVER(ORDER BY (SELECT MIN(m1_.title) AS SCLR_5 FROM MyBlogPost m1_ WHERE m1_.author_id = a0_.id) DESC) AS SCLR_4 FROM Author a0_) dctrn_result GROUP BY ID_0 ORDER BY dctrn_minrownum ASC',
             $query->getSQL()
         );
     }