Merge pull request #8915 from creative-commoners/pulls/4.4/helping-performance-exist

NEW DataQuery::exists now generates EXISTS SQL statements

src/ORM/DataList.php

@@ -963,7 +963,7 @@ class DataList extends ViewableData implements SS_List, Filterable, Sortable, Li
      */
     public function exists()
     {
-        return $this->count() > 0;
+        return $this->dataQuery->exists();
     }
 
     /**

src/ORM/DataQuery.php

@@ -453,6 +453,41 @@ class DataQuery
         return $this->getFinalisedQuery()->count("DISTINCT {$quotedColumn}");
     }
 
+    /**
+     * Return whether this dataquery will have records. This will use `EXISTS` statements in SQL which are more
+     * performant - especially when used in combination with indexed columns (that you're filtering on)
+     *
+     * @return bool
+     */
+    public function exists(): bool
+    {
+        // Grab a statement selecting "everything" - the engine shouldn't care what's being selected in an "EXISTS"
+        // statement anyway
+        $statement = $this->getFinalisedQuery();
+
+        // Clear limit, distinct, and order as it's not relevant for an exists query
+        $statement->setDistinct(false);
+        $statement->setOrderBy(null);
+        $statement->setLimit(null);
+
+        // We can remove grouping if there's no "having" that might be relying on an aggregate
+        // Additionally, the columns being selected no longer matter
+        $having = $statement->getHaving();
+        if (empty($having)) {
+            $statement->setSelect('*');
+            $statement->setGroupBy(null);
+        }
+
+        // Wrap the whole thing in an "EXISTS"
+        $sql = 'SELECT EXISTS(' . $statement->sql($params) . ')';
+        $result = DB::prepared_query($sql, $params);
+        $row = $result->first();
+        $result = reset($row);
+
+        // Checking for 't' supports PostgreSQL before silverstripe/postgresql@2.2
+        return $result === true || $result === 1 || $result === 't';
+    }
+
     /**
      * Return the maximum value of the given field in this DataList
      *

tests/php/ORM/DataQueryTest.php

@@ -6,6 +6,7 @@ use SilverStripe\ORM\DataQuery;
 use SilverStripe\ORM\DataObject;
 use SilverStripe\ORM\DB;
 use SilverStripe\Dev\SapphireTest;
+use SilverStripe\ORM\Tests\DataQueryTest\ObjectE;
 use SilverStripe\Security\Member;
 
 /**
@@ -386,11 +387,7 @@ class DataQueryTest extends SapphireTest
         $query = new DataQuery(DataQueryTest\ObjectC::class);
         $query->sort('"SortOrder"');
         $query->where(
-            [
+            ['"DataQueryTest_C"."Title" = ? OR "DataQueryTest_E"."SortOrder" > ?' => ['First', 2]]
-            '"DataQueryTest_C"."Title" = ? OR "DataQueryTest_E"."SortOrder" > ?' => [
-                'First', 2
-            ]
-            ]
         );
         $result = $query->getFinalisedQuery(['Title']);
         $from = $result->getFrom();
@@ -466,4 +463,11 @@ class DataQueryTest extends SapphireTest
         $this->assertEquals('Second', $titles[1]);
         $this->assertEquals('Last', $titles[2]);
     }
+
+    public function testExistsCreatesFunctionalQueries()
+    {
+        $this->assertTrue(ObjectE::get()->exists());
+        $this->assertFalse(ObjectE::get()->where(['"Title" = ?' => 'Foo'])->exists());
+        $this->assertTrue(ObjectE::get()->dataQuery()->groupby('"SortOrder"')->exists());
+    }
 }