Merge pull request #18893 from kamil-tekiela/Fix-#18890

Don't optimize count for UNION queries

Fixes #18890

libraries/classes/Sql.php

@@ -752,6 +752,7 @@ class Sql
                 if (
                     $analyzedSqlResults['is_group'] === false
                     && $analyzedSqlResults['distinct'] === false
+                    && $analyzedSqlResults['union'] === false
                     && count($statement->expr) === 1
                 ) {
                     $statement->expr[0] = new Expression();

test/classes/SqlTest.php

@@ -571,6 +571,14 @@ class SqlTest extends AbstractTestCase
                 false,
                 'SELECT COUNT(*) FROM (SELECT DISTINCT country_id FROM city ) as cnt',
             ],
+            [
+                'SELECT * FROM t1 UNION SELECT * FROM t2;',
+                ['max_rows' => -1, 'pos' => 0],
+                25,
+                109,
+                false,
+                'SELECT COUNT(*) FROM (SELECT * FROM t1 UNION SELECT * FROM t2 ) as cnt',
+            ],
         ];
     }