Default optimization modes v15

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You can choose an optimization mode as the default setting for an EDB Postgres Advanced Server database cluster. You can also change this setting on a per-session basis by using the ALTER SESSION command as well as in individual DELETE, SELECT, and UPDATE commands in an optimizer hint. The configuration parameter that controls these default modes is OPTIMIZER_MODE.

The table shows the possible values.

HintDescription
ALL_ROWSOptimizes for retrieving all rows of the result set.
CHOOSEDoes no default optimization based on assumed number of rows to retrieve from the result set. This is the default.
FIRST_ROWSOptimizes for retrieving only the first row of the result set.
FIRST_ROWS_10Optimizes for retrieving the first 10 rows of the results set.
FIRST_ROWS_100Optimizes for retrieving the first 100 rows of the result set.
FIRST_ROWS_1000Optimizes for retrieving the first 1000 rows of the result set.
FIRST_ROWS(n)Optimizes for retrieving the first n rows of the result set. You can't use this form as the object of the ALTER SESSION SET OPTIMIZER_MODE command. You can use it only in the form of a hint in a SQL command.

These optimization modes are based on the assumption that the client submitting the SQL command is interested in viewing only the first n rows of the result set and not the remainder of the result set. Resources allocated to the query are adjusted as such.

Example: Specifying the number of rows to retrieve in the result set

Alter the current session to optimize for retrieval of the first 10 rows of the result set:

ALTER SESSION SET OPTIMIZER_MODE = FIRST_ROWS_10;

Example: Showing the current value of the OPTIMIZER_MODE parameter

You can show the current value of the OPTIMIZER_MODE parameter by using the SHOW command. This command depends on the utility. In PSQL, use the SHOW command as follows:

SHOW OPTIMIZER_MODE;
Output
optimizer_mode
-----------------
 first_rows_10
(1 row)

The SHOW command compatible with Oracle databases has the following syntax:

SHOW PARAMETER OPTIMIZER_MODE;

NAME
--------------------------------------------------
VALUE
--------------------------------------------------
optimizer_mode
first_rows_10

This example shows an optimization mode used in a SELECT command as a hint:

SELECT /*+ FIRST_ROWS(7) */ * FROM emp;
Output
empno| ename |  job     | mgr  |      hiredate      | sal     | comm  | deptno
-----+-------+----------+------+--------------------+---------+-------+-------
7369 | SMITH | CLERK    | 7902 | 17-DEC-80 00:00:00 | 800.00  |       | 20
7499 | ALLEN | SALESMAN | 7698 | 20-FEB-81 00:00:00 | 1600.00 | 300.00| 30
7521 | WARD  | SALESMAN | 7698 | 22-FEB-81 00:00:00 | 1250.00 | 500.00| 30
7566 | JONES | MANAGER  | 7839 | 02-APR-81 00:00:00 | 2975.00 |       | 20
7654 | MARTIN| SALESMAN | 7698 | 28-SEP-81 00:00:00 | 1250.00 |1400.00| 30
7698 | BLAKE | MANAGER  | 7839 | 01-MAY-81 00:00:00 | 2850.00 |       | 30
7782 | CLARK | MANAGER  | 7839 | 09-JUN-81 00:00:00 | 2450.00 |       | 10
7788 | SCOTT | ANALYST  | 7566 | 19-APR-87 00:00:00 | 3000.00 |       | 20
7839 | KING  | PRESIDENT|      | 17-NOV-81 00:00:00 | 5000.00 |       | 10
7844 | TURNER| SALESMAN | 7698 | 08-SEP-81 00:00:00 | 1500.00 | 0.00  | 30
7876 | ADAMS | CLERK    | 7788 | 23-MAY-87 00:00:00 | 1100.00 |       | 20
7900 | JAMES | CLERK    | 7698 | 03-DEC-81 00:00:00 |  950.00 |       | 30
7902 | FORD  | ANALYST  | 7566 | 03-DEC-81 00:00:00 | 3000.00 |       | 20
7934 | MILLER| CLERK    | 7782 | 23-JAN-82 00:00:00 | 1300.00 |       | 10
(14 rows)
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