NEW Add exists as a method to the DataQuery API that will generate an SQL "exists" query

The exists query in SQL allows the query optimiser (engine specific) to execute these queries much faster - often only needing the presence of an index to return "yes it exists".

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 +453,35 @@ 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()
+    {
+        // Grab a statement selecting "everything" - the engine shouldn't care what's being selected in an "EXISTS"
+        // statement anyway
+        $statement = $this->getFinalisedQuery();
+        $statement->setSelect('*');
+
+        // Clear limit, distinct, grouping, and order as it's not relevant for an exists query
+        $statement->setDistinct(false);
+        $statement->setOrderBy(null);
+        $statement->setGroupBy(null);
+        $statement->setLimit(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 === 1 || $result === 't';
+    }
+
     /**
      * Return the maximum value of the given field in this DataList
      *