From cfb47ddde2bbbdc3065ed2b959bd4719247fbd30 Mon Sep 17 00:00:00 2001
From: Sergei Golubchik <serg@mariadb.org>
Date: Wed, 23 Nov 2022 00:58:07 +0100
Subject: [PATCH] MDEV-30066 (limit + offset) union all (...) limit = incorrect
 result

select_union_direct::send_data() only sends a record when
the LIMIT ... OFFSET clause of the individual select won't skip it.

Thus, select_union_direct::send_data() should not do any actions
related to a sending a record if the offset of a select isn't
reached yet
---
 mysql-test/main/union.result | 15 +++++++++++++++
 mysql-test/main/union.test   |  9 +++++++++
 sql/sql_union.cc             | 13 ++++++++-----
 3 files changed, 32 insertions(+), 5 deletions(-)

diff --git a/mysql-test/main/union.result b/mysql-test/main/union.result
index bf65bbd44d1..8d9d9752e1d 100644
--- a/mysql-test/main/union.result
+++ b/mysql-test/main/union.result
@@ -2748,5 +2748,20 @@ a	b	c	d
 3	4	2	197
 drop table t1,t2;
 #
+# MDEV-30066 (limit + offset) union all (...) limit = incorrect result
+#
+create table t1(id int primary key auto_increment, c1 int);
+insert into t1(c1) values(1),(2),(3);
+(select * from t1 where c1>=1 order by c1 desc limit 2,1) union all (select * from t1 where c1>1 order by c1 desc);
+id	c1
+1	1
+2	2
+3	3
+(select * from t1 where c1>=1 order by c1 desc limit 2,1) union all (select * from t1 where c1>1 order by c1 desc) limit 2;
+id	c1
+1	1
+2	2
+drop table t1;
+#
 # End of 10.3 tests
 #
diff --git a/mysql-test/main/union.test b/mysql-test/main/union.test
index eff3cfd260b..ba9ee278228 100644
--- a/mysql-test/main/union.test
+++ b/mysql-test/main/union.test
@@ -1982,6 +1982,15 @@ union (select 0 as a, 99 as b,
 
 drop table t1,t2;
 
+--echo #
+--echo # MDEV-30066 (limit + offset) union all (...) limit = incorrect result
+--echo #
+create table t1(id int primary key auto_increment, c1 int);
+insert into t1(c1) values(1),(2),(3);
+(select * from t1 where c1>=1 order by c1 desc limit 2,1) union all (select * from t1 where c1>1 order by c1 desc);
+(select * from t1 where c1>=1 order by c1 desc limit 2,1) union all (select * from t1 where c1>1 order by c1 desc) limit 2;
+drop table t1;
+
 --echo #
 --echo # End of 10.3 tests
 --echo #
diff --git a/sql/sql_union.cc b/sql/sql_union.cc
index e831e2a479c..902e25d084e 100644
--- a/sql/sql_union.cc
+++ b/sql/sql_union.cc
@@ -551,14 +551,17 @@ int select_union_direct::send_data(List<Item> &items)
 {
   if (!limit)
     return false;
-  limit--;
-  if (offset)
+  if (!unit->offset_limit_cnt)
   {
-    offset--;
-    return false;
+    limit--;
+    if (offset)
+    {
+      offset--;
+      return false;
+    }
+    send_records++;
   }
 
-  send_records++;
   fill_record(thd, table, table->field, items, true, false);
   if (unlikely(thd->is_error()))
     return true; /* purecov: inspected */