ORDER BY Queries

You can improve the performance of queries that contain only ORDER BY clauses if the columns in a projection's ORDER BY clause are the same as the columns in the query.

If you define the projection sort order in the CREATE PROJECTION statement, the Vertica query optimizer does not have to sort projection data before performing certain ORDER BY queries.

The following table, sortopt, contains the columns a, b, c, and d. Projection sortopt_p specifies to order on columns a, b, and c.

CREATE TABLE sortopt (
    a INT NOT NULL, 
    b INT NOT NULL,
    c INT,
    d INT
);
CREATE PROJECTION sortopt_p (
   a_proj,
   b_proj,
   c_proj,
   d_proj )
AS SELECT * FROM sortopt
ORDER BY a,b,c 
UNSEGMENTED ALL NODES;
INSERT INTO sortopt VALUES(5,2,13,84);
INSERT INTO sortopt VALUES(14,22,8,115);
INSERT INTO sortopt VALUES(79,9,401,33);

Based on this sort order, if a SELECT * FROM sortopt query contains one of the following ORDER BY clauses, the query does not have to resort the projection:

• ORDER BY a
• ORDER BY a, b
• ORDER BY a, b, c

For example, Vertica does not have to resort the projection in the following query because the sort order includes columns specified in the CREATE PROJECTION..ORDER BY a, b, c clause, which mirrors the query's ORDER BY a, b, c clause:

=> SELECT * FROM sortopt ORDER BY a, b, c;
 a  | b  |  c  |  d
----+----+-----+-----
  5 |  2 |  13 |  84
 14 | 22 |   8 | 115
 79 |  9 | 401 |  33
(3 rows)

If you include column d in the query, Vertica must re-sort the projection data because column d was not defined in the CREATE PROJECTION..ORDER BY clause. Therefore, the ORDER BY d query won't benefit from any sort optimization.

You cannot specify an ASC or DESC clause in the CREATE PROJECTION statement's ORDER BY clause. Vertica always uses an ascending sort order in physical storage, so if your query specifies descending order for any of its columns, the query still causes Vertica to re-sort the projection data. For example, the following query requires Vertica to sort the results:

=> SELECT * FROM sortopt ORDER BY a DESC, b, c;
 a  | b  |  c  |  d
----+----+-----+-----
 79 |  9 | 401 |  33
 14 | 22 |   8 | 115
  5 |  2 |  13 |  84
(3 rows)