Append

Description

Append is used to append multiple relationship sets and process the linked list that contains one or more subplans.

Typical Scenarios

• SQL statements with UNION.
• SQL statements with UNION ALL.

Examples

Example 1: SQL statement with UNION.

-- Prepare data.
gaussdb=# CREATE TABLE t1(c1 number, c2 number, c3 number); 
CREATE TABLE 
gaussdb=# INSERT INTO t1 VALUES(generate_series(1, 100), 2, 3); 
INSERT 0 100 
gaussdb=# CREATE TABLE t2(c1 number, c2 number, c3 number); 
CREATE TABLE 
gaussdb=# INSERT INTO t2 VALUES(generate_series(1, 100), 2, 3); 
INSERT 0 100

-- Execution result.
gaussdb=# EXPLAIN SELECT * FROM t1 UNION SELECT * FROM t2; 
                            QUERY PLAN                             
------------------------------------------------------------------ 
 HashAggregate  (cost=31.57..39.05 rows=748 width=85) 
   Group By Key: t1.c1, t1.c2, t1.c3 
   ->  Append  (cost=0.00..25.96 rows=748 width=85) 
         ->  Seq Scan on t1  (cost=0.00..2.00 rows=100 width=15) 
         ->  Seq Scan on t2  (cost=0.00..16.48 rows=648 width=96) 
(5 rows)

In the preceding example, the output of the append operator is as follows.

Example 2: SQL statement with UNION ALL.

gaussdb=# EXPLAIN SELECT * FROM t1 UNION ALL SELECT * FROM t2; 
                           QUERY PLAN                             
----------------------------------------------------------------- 
 Result  (cost=0.00..4.00 rows=200 width=15) 
   ->  Append  (cost=0.00..4.00 rows=200 width=15) 
         ->  Seq Scan on t1  (cost=0.00..2.00 rows=100 width=15) 
         ->  Seq Scan on t2  (cost=0.00..2.00 rows=100 width=15) 
(4 rows)

-- Drop.
gaussdb=# DROP TABLE t1,t2;