Add support for distinct database queries

refs #4180

library/Icinga/Data/BaseQuery.php

@@ -59,6 +59,13 @@ abstract class BaseQuery implements Filterable
      */
     private $limitOffset;
 
+    /**
+     * Whether its a distinct query or not
+     *
+     * @var bool
+     */
+    private $distinct = false;
+
     /**
      * The backend independent filter to use for this query
      *
@@ -294,6 +301,30 @@ abstract class BaseQuery implements Filterable
         return $this;
     }
 
+    /**
+     * Return only distinct results
+     *
+     * @param   bool    $distinct   Whether the query should be distinct or not
+     *
+     * @return  BaseQuery
+     */
+    public function distinct($distinct = true)
+    {
+        $this->distinct = $distinct;
+
+        return $this;
+    }
+
+    /**
+     * Determine whether this query returns only distinct results
+     *
+     * @return  bool    True in case its a distinct query otherwise false
+     */
+    public function isDistinct()
+    {
+        return $this->distinct;
+    }
+
     /**
      * Determine whether this query will be ordered explicitly
      *

library/Icinga/Data/Db/Query.php

@@ -85,6 +85,14 @@ class Query extends BaseQuery
         if ($this->baseQuery !== null) {
             $this->baseQuery = clone $this->baseQuery;
         }
+
+        if ($this->selectQuery !== null) {
+            $this->selectQuery = clone $this->selectQuery;
+        }
+
+        if ($this->countQuery !== null) {
+            $this->countQuery = clone $this->countQuery;
+        }
     }
 
     /**
@@ -148,6 +156,7 @@ class Query extends BaseQuery
     {
         $this->selectQuery = clone($this->baseQuery);
         $this->selectQuery->columns($this->getColumns());
+        $this->selectQuery->distinct($this->isDistinct());
         if ($this->hasOrder()) {
             foreach ($this->getOrderColumns() as $col) {
                 $this->selectQuery->order(
@@ -200,8 +209,8 @@ class Query extends BaseQuery
      */
     private function createCountQuery()
     {
-        if ($this->useSubqueryCount) {
+        if ($this->isDistinct() || $this->useSubqueryCount) {
-            $this->countQuery = $this->createCountAsSubquery();
+            $this->countQuery = $this->createCountAsSubQuery();
         } else {
             $this->countQuery = $this->createCustomCountQuery();
         }