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.