From 2e253efcc8cdfe47246a07d23001da3f86aa6975 Mon Sep 17 00:00:00 2001
From: Dave Gosselin <dave.gosselin@mariadb.com>
Date: Mon, 12 Jan 2026 14:05:40 -0500
Subject: [PATCH] MDEV-38099: Assertion failed in
 st_select_lex::optimize_unflattened_subqueries

When optimizing unflattened subqueries we assert that, if any SELECT in a unit
sets the uncacheable flag with UNCACHEABLE_RAND, then the unit's uncacheable
flag itself should be set likewise.  Put another way, any SELECT in the unit
that is UNCACHEABLE_RAND causes the entire unit to be so marked.

However, this marking was not preserved by SELECT_LEX::register_unit when
registering a new unit.  During that method, the SELECT_LEX's uncacheable
value is updated with the registered unit's uncacheable value but, if the
UNCACHEABLE_RAND marking is present, it is not propagated to the uncacheable
flag of the enclosing unit.

If the incoming unit's uncacheable flag has the UNCACHEABLE_RAND bit set and
if the master unit exists, then set the UNCACHEABLE_RAND flag on the master
unit.
---
 mysql-test/main/cte_nonrecursive.result |  8 ++++++++
 mysql-test/main/cte_nonrecursive.test   | 13 +++++++++++++
 sql/sql_lex.cc                          |  2 ++
 3 files changed, 23 insertions(+)

diff --git a/mysql-test/main/cte_nonrecursive.result b/mysql-test/main/cte_nonrecursive.result
index 992c218a10eed..59365d71be1da 100644
--- a/mysql-test/main/cte_nonrecursive.result
+++ b/mysql-test/main/cte_nonrecursive.result
@@ -2683,3 +2683,11 @@ cte as (select t1.a as t1a, t2.a as t2a from t as t1, t as t2 where t1.a=t2.a)
 select * from cte;
 ERROR 42S02: Table 'test.t' doesn't exist
 # End of 10.4 tests
+#
+# Start of 12.2 tests
+#
+#
+# MDEV-38099:  Assertion failed in st_select_lex::optimize_unflattened_subqueries
+#
+SELECT (WITH cte AS (SELECT RAND()) SELECT * FROM cte);
+# End of 12.2 tests
diff --git a/mysql-test/main/cte_nonrecursive.test b/mysql-test/main/cte_nonrecursive.test
index 1816888626398..5950c71399141 100644
--- a/mysql-test/main/cte_nonrecursive.test
+++ b/mysql-test/main/cte_nonrecursive.test
@@ -2026,3 +2026,16 @@ drop table t,s;
 eval $q1;
 
 --echo # End of 10.4 tests
+
+--echo #
+--echo # Start of 12.2 tests
+--echo #
+
+--echo #
+--echo # MDEV-38099:  Assertion failed in st_select_lex::optimize_unflattened_subqueries
+--echo #
+--disable_result_log
+SELECT (WITH cte AS (SELECT RAND()) SELECT * FROM cte);
+--enable_result_log
+
+--echo # End of 12.2 tests
diff --git a/sql/sql_lex.cc b/sql/sql_lex.cc
index 73d97861325ef..7779411ff8a98 100644
--- a/sql/sql_lex.cc
+++ b/sql/sql_lex.cc
@@ -11130,6 +11130,8 @@ void st_select_lex::register_unit(SELECT_LEX_UNIT *unit,
   slave= unit;
   unit->master= this;
   uncacheable|= unit->uncacheable;
+  if (master && (unit->uncacheable & UNCACHEABLE_RAND))
+    master->uncacheable|= UNCACHEABLE_RAND;
 
   for(SELECT_LEX *sel= unit->first_select();sel; sel= sel->next_select())
   {