Sort

Description

Sorts the tuples returned by the bottom-layer node. The sort operator is used to sort query results based on specified sorting rules and return ordered result sets.

Typical Scenarios

• If a query statement contains the ORDER BY clause, GaussDB selects the sort operator in the execution plan to perform sorting.
• Merge join is used for join operations.

Examples

Example: The query statement contains the ORDER BY clause.

-- Prepare data.
gaussdb=# CREATE TABLE student(id integer, class_id integer, grade number); 
CREATE TABLE 
gaussdb=# INSERT INTO student VALUES(generate_series(1,50), 1, floor(100 * random())); 
INSERT 0 50 
gaussdb=# INSERT INTO student VALUES(generate_series(51,100), 2, floor(100 * random())); 
INSERT 0 50 
gaussdb=# INSERT INTO student VALUES(generate_series(101,150), 3, floor(100 * random())); 
INSERT 0 50 
gaussdb=# INSERT INTO student VALUES(generate_series(151,200), 3, floor(100 * random())); 
INSERT 0 50

-- Execution result.
gaussdb=# EXPLAIN SELECT * FROM student ORDER BY grade; 
                           QUERY PLAN                            
---------------------------------------------------------------- 
 Sort  (cost=10.64..11.14 rows=200 width=12) 
   Sort Key: grade 
   ->  Seq Scan on student  (cost=0.00..3.00 rows=200 width=12) 
(3 rows)

-- Drop.
gaussdb=# DROP TABLE student;

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

Example: Merge join is used for join operations.

-- Prepare data.
gaussdb=# DROP TABLE IF EXISTS t1;
gaussdb=# DROP TABLE IF EXISTS t2;
gaussdb=# CREATE TABLE t1(id int,info text); 
CREATE TABLE 
gaussdb=# CREATE TABLE t2(id int,info text); 
CREATE TABLE 
gaussdb=# CREATE INDEX t1_k on t1(id);
CREATE INDEX
gaussdb=# INSERT INTO t2 SELECT generate_series(1,100000),'bill'||generate_series(1,100000); 
INSERT 0 100000 
gaussdb=# INSERT INTO t1 SELECT generate_series(1,100000),'bill'||generate_series(1,100000); 
INSERT 0 100000

-- Collect statistics.
gaussdb=# ANALYZE t1;
gaussdb=# ANALYZE t2;

-- Execution result.
gaussdb=# EXPLAIN SELECT /*+ MERGEJOIN(t2 t1) */ * FROM t2 JOIN t1 ON (t1.id=t2.id);
                                  QUERY PLAN
------------------------------------------------------------------------------
 Merge Join  (cost=9710.87..14918.02 rows=100000 width=26)
   Merge Cond: (t1.id = t2.id)
   ->  Index Scan using t1_k on t1  (cost=0.00..3457.25 rows=100000 width=13)
   ->  Sort  (cost=9710.82..9960.82 rows=100000 width=13)
         Sort Key: t2.id
         ->  Seq Scan on t2  (cost=0.00..1406.00 rows=100000 width=13)
(6 rows)


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