Fix yet another bug in the [/info/e33da6d5dc964db8|EXISTS-to-JOIN]

optimization, reported by
[forum:/forumpost/2026-01-03T14:05:48z|forum post 2026-01-03T14:05:48z].

Author: D. Richard Hipp

src/select.c

@@ -7465,6 +7465,7 @@ static SQLITE_NOINLINE void existsToJoin(
        && (pSub->selFlags & SF_Aggregate)==0
        && !pSub->pSrc->a[0].fg.isSubquery
        && pSub->pLimit==0
+       && pSub->pPrior==0
       ){
         /* Before combining the sub-select with the parent, renumber the 
         ** cursor used by the subselect. This is because the EXISTS expression

test/existsexpr.test

@@ -496,5 +496,22 @@ do_execsql_test 10.2 {
   );
 }
 
-finish_test
+# https://sqlite.org/forum/forumpost/2026-01-03T14:05:48z
+do_execsql_test 11.0 {
+  CREATE TABLE parent (id TEXT PRIMARY KEY);
+  CREATE TABLE child_a (id TEXT);
+  CREATE TABLE child_b (id TEXT);
+  INSERT INTO parent VALUES ('p1');
+  INSERT INTO child_a VALUES ('p1');
+}
+do_execsql_test 11.1 {
+  SELECT count(*), parent.id FROM parent
+   WHERE EXISTS (
+      SELECT 1 FROM child_a WHERE child_a.id = parent.id
+      UNION
+      SELECT 1 FROM child_b WHERE child_b.id = parent.id
+   )
+   GROUP BY id;
+} {1 p1}
 
+finish_test