MDEV-24242 Query returns wrong result while using big_tables=1

When executing set operations in a pipeline using only one temporary table
additional scans of intermediate results may be needed. The scans are
performed with usage of the rnd_next() handler function that might
leave record buffers used for the temporary table not in a state that
is good for following writes into the table. For example it happens for
aria engine when the last call of rnd_next() encounters only deleted
records. Thus a cleanup of record buffers is needed after each such scan
of the temporary table.

Approved by Oleksandr Byelkin <sanja@mariadb.com>

mysql-test/main/set_operation.result

@@ -1155,3 +1155,37 @@ count(*)
 319
 drop table t1;
 drop table t2;
+#
+# MDEV-24242: set expression with empty intermediate result
+#             when tmp_memory_table_size is set to 0
+#
+create table t1 (a int, b int) engine=MyISAM;
+insert into t1 values (1,1), (2,2);
+create table t2 (a int, b int) engine=MyISAM;
+insert into t2 values (11,11), (12,12), (13,13);
+select * from t1
+except all
+select * from t1
+except
+select * from t1
+union all
+select * from t2;
+a	b
+12	12
+11	11
+13	13
+set tmp_memory_table_size=0;
+select * from t1
+except all
+select * from t1
+except
+select * from t1
+union all
+select * from t2;
+a	b
+12	12
+11	11
+13	13
+set tmp_memory_table_size=default;
+drop table t1,t2;
+# End of 10.4 tests

mysql-test/main/set_operation.test

@@ -524,3 +524,31 @@ select count(*) from
 
 drop table t1;
 drop table t2;
+
+--echo #
+--echo # MDEV-24242: set expression with empty intermediate result
+--echo #             when tmp_memory_table_size is set to 0
+--echo #
+
+create table t1 (a int, b int) engine=MyISAM;
+insert into t1 values (1,1), (2,2);
+create table t2 (a int, b int) engine=MyISAM;
+insert into t2 values (11,11), (12,12), (13,13);
+
+let $q=
+select * from t1
+except all
+select * from t1
+except
+select * from t1
+union all
+select * from t2;
+
+eval $q;
+set tmp_memory_table_size=0;
+eval $q;
+set tmp_memory_table_size=default;
+
+drop table t1,t2;
+
+--echo # End of 10.4 tests

sql/sql_union.cc

@@ -880,6 +880,10 @@ bool select_unit_ext::send_eof()
     table->file->ha_rnd_end();
   }
 
+  /* Clean up table buffers for the next set operation from pipeline */
+  if (next_sl)
+    restore_record(table,s->default_values);
+
   if (unlikely(error))
     table->file->print_error(error, MYF(0));