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PostgreSQL Trigger 1

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This entry covers how to write a statement logging trigger for PostgreSQL. It creates two tables: avenger and avenger_log; one avenger_t1 trigger, and a testing INSERT statement.

It was written to help newbies know how and what to return from a function written for a statement-level trigger. They often get stuck on the following when they try to return true. The term non-composite is another way to describe the tuple inserted.

psql:basics_postgres.sql:
59: ERROR:  cannot return non-composite value from function returning composite type
CONTEXT:  PL/pgSQL function write_avenger_t1() line 15 at RETURN

The avenger table:

/* Conditionally drop table. */
DROP TABLE IF EXISTS avenger;
 
/* Create table. */
CREATE TABLE avenger
( avenger_id    SERIAL
, avenger_name  VARCHAR(30)
, first_name    VARCHAR(20)
, last_name     VARCHAR(20));

Seed the avenger table:

/* Seed the avenger table with data. */
INSERT INTO avenger
( first_name, last_name, avenger_name )
VALUES
 ('Anthony', 'Stark', 'Iron Man')
,('Thor', 'Odinson', 'God of Thunder')
,('Steven', 'Rogers', 'Captain America')
,('Bruce', 'Banner', 'Hulk')
,('Clinton', 'Barton', 'Hawkeye')
,('Natasha', 'Romanoff', 'Black Widow')
,('Peter', 'Parker', 'Spiderman')
,('Steven', 'Strange', 'Dr. Strange')
,('Scott', 'Lange', 'Ant-man');

The avenger_log table:

/* Conditionally drop table. */
DROP TABLE IF EXISTS avenger_log;
 
/* Create table. */
CREATE TABLE avenger_log
( avenger_log_id  SERIAL
, trigger_name    VARCHAR(30)
, trigger_timing  VARCHAR(6)
, trigger_event   VARCHAR(6)
, trigger_type    VARCHAR(12));

The INSERT statement that tests the trigger:

DROP FUNCTION IF EXISTS avenger_t1_function; 
CREATE FUNCTION avenger_t1_function()
  RETURNS TRIGGER AS
$$
BEGIN
  /* Insert a row into the avenger_log table.
   * Also, see PostrgreSQL 39.9 Trigger Procedures. */
  INSERT INTO avenger_log
  ( trigger_name
  , trigger_timing
  , trigger_event
  , trigger_type )
  VALUES
  ( UPPER(TG_NAME)
  , TG_WHEN
  , TG_OP
  , TG_LEVEL );
 
  /* A statement trigger doesn't use a composite type or tuple,
   * it should simply return an empty composite type or void. */
  RETURN NULL;
END;
$$ LANGUAGE plpgsql;

The avenger_t1 statement trigger:

CREATE TRIGGER avenger_t1
  BEFORE INSERT ON avenger
  EXECUTE FUNCTION avenger_t1_function();

The INSERT statement:

INSERT INTO avenger
( first_name, last_name, avenger_name )
VALUES
 ('Hope', 'van Dyne', 'Wasp');

The results logged to the avenger_log table from a query:

 avenger_log_id | trigger_name | trigger_timing | trigger_event | trigger_type
----------------+--------------+----------------+---------------+--------------
              1 | AVENGER_T1   | BEFORE         | INSERT        | STATEMENT
(1 row)

As always, I hope this helps those looking for a solution.

Written by maclochlainn

July 11th, 2022 at 3:12 pm

Posted in Database,Database Design,DBA,DevOps,PL/pgSQL,Postgres,PostgreSQL 14,psql,sql,Uncategorized

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Setup PostgreSQL

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After you have installed PostgeSQL on Fedora 27 and initialized the database, you have several steps to configure a new instance. This blog post shows you how to create all the implementation pieces for a student database.

Connect as the postgres user from a sudoer user. It requires you to connect as the root user before you connect as the postgres user.

sudo sh

As the root user, you don’t need a password to connect as the postgres user:

su - postgres

As the postgres user, you own the PostgreSQL database and can connect to the database like the following:

[postgres@localhost ~]$ psql
psql (9.6.8)
Type "help" for help.
 
postgres=#

After you connect as the privileged postgres user, you can check the default data location with the following command:

postgres=# show data_directory;

It will return:

   data_directory    
---------------------
 /var/lib/pgsql/data
(1 row)

After you’ve initialized the PostgreSQL database, you may need to restart your database manually unless you configure the server to start it when you boot your server. The syntax to start the PostgreSQL database at the command-line as the postgres privileged user is:

postgres -D /var/lib/pgsql/data &

You can find detailed instructions in Chapter 18.3 Starting the Database Server web page. There are several options available to you to automate the starting process.

The instructions to build a postgresql.service actually require modification for Fedora 27. You should create the following file in the /etc/systemd/system directory:

[Unit]
Description=PostgreSQL database server
Documentation=man:postgres(1)
 
[Service]
Type=notify
User=postgres
ExecStart=/usr/bin/postgres -D /var/lib/pgsql/data
ExecReload=/bin/kill -HUP $MAINPID
KillMode=mixed
KillSignal=SIGINT
TimeoutSec=0
 
[Install]
WantedBy=multi-user.target

You can start the PostgreSQL service with the following command:

systemctl start postgresql.service >/dev/null

The following steps create tablespace, database, role, and user:

  1. Create Tablespace

You can create a video_db tablespace with the following syntax:

CREATE TABLESPACE video_db
  OWNER postgres
  LOCATION '/var/lib/pgsql/data';

This will return the following:

WARNING:  tablespace location should not be inside the data directory
CREATE TABLESPACE

The warning only lets you know that you really shouldn’t create table spaces in the default data directory. You can query whether you successfully create the video_db tablespace with the following:

SELECT * FROM pg_tablespace;

It should return the following:

  spcname   | spcowner | spcacl | spcoptions 
------------+----------+--------+------------
 pg_default |       10 |        | 
 pg_global  |       10 |        | 
 video_db   |       10 |        | 
(3 rows)
  1. Create a Database

You can create a videodb database with the following syntax:

CREATE DATABASE videodb
  WITH OWNER = postgres
       ENCODING = 'UTF8'
       TABLESPACE = video_db
       LC_COLLATE = 'en_US.UTF-8'
       LC_CTYPE = 'en_US.UTF-8'
       CONNECTION LIMIT = -1;

Then, you can assign comment to the database with the following syntax:

COMMENT ON DATABASE videodb IS 'Video Database';
  1. Create a Role, Grant, and User

In this section you create a dba role, grant privileges on a videodb database to a role, and create a user with the role that you created previously with the following three statements. There are three steps in this sections.

  • The first step creates a dba role: 

    CREATE ROLE dba WITH SUPERUSER;
  • The second step grants all privileges on a videodb database to a dba role: 

    GRANT ALL PRIVILEGES ON DATABASE videodb TO dba;
  • The third step creates a student user with the dba role: 

    CREATE USER student
  WITH ROLE dba
       PASSWORD 'student';
  1. Connect to the videodb as the student user

You connect to the videodb database as the student user with the following command:

sql -d videodb -U student;

After connecting to the videodb database, you can query the current database, like

SELECT current_database();

It should return the following:

 current_database 
------------------
 videodb
(1 row)

This has shown you how to create a videodb database, dba role, and student user.

Written by maclochlainn

May 13th, 2018 at 1:01 am

Posted in Database Design,Fedora,LAMP,Linux

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