Make CountWalker use COUNT(*) when $distinct is explicitly set to false (#11552)

This change makes CountWalker use COUNT(*) instead of
COUNT(tbl.id), when the user declared that their query
does not need to use (SELECT) DISTINCT, which is
commonly the case when there are no JOINs in the query,
or when the JOINs are only *ToOne.

Research showed that COUNT(*) allows databases to use
index(-only) scans more eagerly from any of the
indexed columns, especially when the query is using
a WHERE-condition that filters on an indexed column.

Author: d-ph

src/Tools/Pagination/CountWalker.php

@@ -37,27 +37,31 @@ public function walkSelectStatement(SelectStatement $selectStatement): void
             throw new RuntimeException('Cannot count query which selects two FROM components, cannot make distinction');
         }
 
-        $fromRoot            = reset($from);
-        $rootAlias           = $fromRoot->rangeVariableDeclaration->aliasIdentificationVariable;
-        $rootClass           = $this->getMetadataForDqlAlias($rootAlias);
-        $identifierFieldName = $rootClass->getSingleIdentifierFieldName();
+        $distinct = $this->_getQuery()->getHint(self::HINT_DISTINCT);
 
-        $pathType = PathExpression::TYPE_STATE_FIELD;
-        if (isset($rootClass->associationMappings[$identifierFieldName])) {
-            $pathType = PathExpression::TYPE_SINGLE_VALUED_ASSOCIATION;
-        }
+        $countPathExpressionOrLiteral = '*';
+        if ($distinct) {
+            $fromRoot            = reset($from);
+            $rootAlias           = $fromRoot->rangeVariableDeclaration->aliasIdentificationVariable;
+            $rootClass           = $this->getMetadataForDqlAlias($rootAlias);
+            $identifierFieldName = $rootClass->getSingleIdentifierFieldName();
+
+            $pathType = PathExpression::TYPE_STATE_FIELD;
+            if (isset($rootClass->associationMappings[$identifierFieldName])) {
+                $pathType = PathExpression::TYPE_SINGLE_VALUED_ASSOCIATION;
+            }
 
-        $pathExpression       = new PathExpression(
-            PathExpression::TYPE_STATE_FIELD | PathExpression::TYPE_SINGLE_VALUED_ASSOCIATION,
-            $rootAlias,
-            $identifierFieldName,
-        );
-        $pathExpression->type = $pathType;
+            $countPathExpressionOrLiteral       = new PathExpression(
+                PathExpression::TYPE_STATE_FIELD | PathExpression::TYPE_SINGLE_VALUED_ASSOCIATION,
+                $rootAlias,
+                $identifierFieldName,
+            );
+            $countPathExpressionOrLiteral->type = $pathType;
+        }
 
-        $distinct                                         = $this->_getQuery()->getHint(self::HINT_DISTINCT);
         $selectStatement->selectClause->selectExpressions = [
             new SelectExpression(
-                new AggregateExpression('count', $pathExpression, $distinct),
+                new AggregateExpression('count', $countPathExpressionOrLiteral, $distinct),
                 null,
             ),
         ];

tests/Tests/ORM/Tools/Pagination/CountWalkerTest.php

@@ -27,6 +27,20 @@ public function testCountQuery(): void
         );
     }
 
+    public function testCountQueryWithoutDistinctUsesCountStar(): void
+    {
+        $query = $this->entityManager->createQuery(
+            'SELECT p, c, a FROM Doctrine\Tests\ORM\Tools\Pagination\BlogPost p JOIN p.category c JOIN p.author a',
+        );
+        $query->setHint(Query::HINT_CUSTOM_TREE_WALKERS, [CountWalker::class]);
+        $query->setHint(CountWalker::HINT_DISTINCT, false);
+
+        self::assertEquals(
+            'SELECT count(*) AS sclr_0 FROM BlogPost b0_ INNER JOIN Category c1_ ON b0_.category_id = c1_.id INNER JOIN Author a2_ ON b0_.author_id = a2_.id',
+            $query->getSQL(),
+        );
+    }
+
     public function testCountQueryMixedResultsWithName(): void
     {
         $query = $this->entityManager->createQuery(