From e60fbf260f4b67907925bf637bc64516d401b42e Mon Sep 17 00:00:00 2001
From: Gagan Goel <gagan.nith@gmail.com>
Date: Wed, 21 Aug 2019 13:21:12 -0400
Subject: [PATCH] Disable processing of a union in a subquery by the
 select_handler

---
 dbcon/mysql/ha_mcs_pushdown.cpp | 10 ++++++++++
 1 file changed, 10 insertions(+)

diff --git a/dbcon/mysql/ha_mcs_pushdown.cpp b/dbcon/mysql/ha_mcs_pushdown.cpp
index 713b70145..9d3b6d79a 100644
--- a/dbcon/mysql/ha_mcs_pushdown.cpp
+++ b/dbcon/mysql/ha_mcs_pushdown.cpp
@@ -462,6 +462,16 @@ create_columnstore_select_handler(THD* thd, SELECT_LEX* select_lex)
         return handler;
     }
 
+    // Disable SH processing for UNION in a subquery.
+    // This way, the server falls back to using the DH for executing
+    // the UNION, if enabled.
+    TABLE_LIST *tbl_ptr = select_lex->get_table_list();
+    if (tbl_ptr && tbl_ptr->derived &&
+        (tbl_ptr->derived->is_unit_op() || tbl_ptr->derived->fake_select_lex))
+    {
+        return handler;
+    }
+
     bool unsupported_feature = false;
     // Select_handler use the short-cut that effectively disables
     // INSERT..SELECT and LDI