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Java & MySQL 8.0.19

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It’s the in-between term time and we’re all stuck at home. I decided to update the image for my Fedora 30 virtual machine. I had a work around to the update issue that I had encountered last October in Bug #96969 but it was not required with the current version. However, after updating from MySQL 8.0.17 to MySQL 8.0.19, I found that my Java connection example failed.

The $CLASSPATH value was correct:

/usr/share/java/mysql-connector-java.jar:.

The first error that I got was the my reference to MySQL JDBC driver was incorrect. The error message is quite clear:

Loading class `com.mysql.jdbc.Driver'. This is deprecated. The new driver class is `com.mysql.cj.jdbc.Driver'. The driver is automatically registered via the SPI and manual loading of the driver class is generally unnecessary.
Cannot connect to database server:
The server time zone value 'MDT' is unrecognized or represents more than one time zone. You must configure either the server or JDBC driver (via the 'serverTimezone' configuration property) to use a more specifc time zone value if you want to utilize time zone support.

I changed the MySQL Driver reference as instructed by the error message:

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      // Create instance of MySQLDriver.
      Class.forName ("com.mysql.cj.jdbc.Driver").newInstance();
      conn = DriverManager.getConnection (url, username, password);

After the change, I got the following error while retesting my little MySQL Java driver connection test program. Initially, I thought this required a change in the Java environment but that wasn’t it.

Cannot connect to database server:
The server time zone value 'MDT' is unrecognized or represents more than one time zone. You must configure either the server or JDBC driver (via the 'serverTimezone' configuration property) to use a more specifc time zone value if you want to utilize time zone support.

It required me to add the following line to my /etc/my.cnf configuration file, which synchronizes the database’s timezone with the operating system.

# Synchronize the MySQL clock with the computer system clock.
default-time-zone='+00:00'

Then, running my MySQL Driver connection test program worked like a charm. It returns the following:

Database connection established
MySQLDriver Version [8.0.19]
Database connection terminated

Here’s the MySQL Connector Java code if you’d like to use the MySQLDriver.java file:

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// Import classes.
import java.sql.*;
 
/* You can't include the following on Linux without raising an exception. */
// import com.mysql.jdbc.Driver;
 
public class MySQLDriver {
  public MySQLDriver() {
    /* Declare variables that require explicit assignments because
       they're addressed in the finally block. */
    Connection conn = null;
    Statement stmt = null;
    ResultSet rset = null;
 
    /* Declare other variables. */
    String url;
    String username = "student";
    String password = "student";
    String database = "studentdb";
    String hostname = "localhost";
    String port = "3306";
    String sql;
 
    /* Attempt a connection. */
    try {
      // Set URL.
      url = "jdbc:mysql://" + hostname + ":" + port + "/" + database;
 
      // Create instance of MySQLDriver.
      Class.forName ("com.mysql.cj.jdbc.Driver").newInstance();
      conn = DriverManager.getConnection (url, username, password);
 
      // Query the version of the database.
      sql = "SELECT version()";
      stmt = conn.createStatement();
      rset = stmt.executeQuery(sql);
 
      System.out.println ("Database connection established");
 
      // Read row returns for one column.
      while (rset.next()) {
        System.out.println("MySQLDriver Version [" + rset.getString(1) + "]"); }
 
    }
    catch (SQLException e) {
      System.err.println ("Cannot connect to database server:");
      System.out.println(e.getMessage());
    }
    catch (ClassNotFoundException e) {
      System.err.println ("Cannot find MySQL driver class:");
      System.out.println(e.getMessage());
    }
    catch (InstantiationException e) {
      System.err.println ("Cannot instantiate class:");
      System.out.println(e.getMessage());
    }
    catch (IllegalAccessException e) {
      System.err.println ("Illegal access exception:");
      System.out.println(e.getMessage());
    }
    finally {
      if (conn != null) {
        try {
          rset.close();
          stmt.close();
          conn.close();
          System.out.println ("Database connection terminated");
        }
        catch (Exception e) { /* ignore close errors */ }
      }
    }
  }
  /* Unit test. */
  public static void main(String args[]) {
    new MySQLDriver();
  }
}

As always, I hope this helps those who encounter similar problems.

Written by maclochlainn

April 10th, 2020 at 12:42 pm

Posted in DBA,DevOps,Fedora,Java,Java 8,LAMP,Linux,MySQL,MySQL 8,MySQL Connect

Tagged with ,

PostgreSQL Table Function

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This shows how to write a PL/pgSQL function that returns a filtered table result set while writing to a debug log file. The example requires a small character table, like:

DROP TABLE IF EXISTS CHARACTER;
CREATE TABLE CHARACTER
( character_id    SERIAL
, character_name  VARCHAR );

and, a logger table:

DROP TABLE IF EXISTS logger;
CREATE TABLE logger
( logger_id     SERIAL
, message_text  VARCHAR );

Now, let’s insert a couple rows into the character table. The following query inserts one row:

INSERT INTO CHARACTER
( character_name )
VALUES
('Harry Potter');

It was simply too much fun to write this tricky insert statement to the character table. It only submits a value when it doesn’t already exist in the set of character_name column values. While it eliminates the need for a unique constraint on the character_name column, it makes every insert statement more costly in terms of machine resources.

WITH cte AS
( SELECT 'Harry Potter' AS character_name
  UNION ALL
  SELECT 'Hermione Granger' AS character_name
  UNION ALL
  SELECT 'Ronald Weasily' AS character_name )
INSERT INTO CHARACTER
( character_name )
( SELECT character_name
  FROM   cte
  WHERE  NOT EXISTS
          (SELECT NULL
           FROM   CHARACTER c
           WHERE  c.character_name = cte.character_name));

You can verify these insert statements work with the following query:

SELECT * FROM CHARACTER;

It returns:

 character_id |  character_name  
--------------+------------------
            1 | Harry Potter
            2 | Hermione Granger
            3 | Ronald Weasily
(3 rows)

The following character_query PL/pgSQL function filters table results and returns a table of values. The function defines the future query return results, which is a full fledged object-oriented programming adapter pattern.

CREATE OR REPLACE
  FUNCTION character_query (pattern VARCHAR)
  RETURNS TABLE ( character_id    INTEGER
                , character_text  VARCHAR ) AS
$$
BEGIN
  RETURN QUERY
  SELECT c.character_id
  ,      c.character_name
  FROM   CHARACTER c
  WHERE  c.character_name SIMILAR TO '%'||pattern||'%';
END;
$$ LANGUAGE plpgsql;

You can test the character_query function, like this:

SELECT * FROM character_query('Hermione');

It returns:

 character_id |  character_text  
--------------+------------------
            2 | Hermione Granger
(1 row)

Building on what we did, let’s log our query word in the logger table. You add an insert statement after the BEGIN keyword and before the RETURN QUERY phrases, like:

CREATE OR REPLACE
  FUNCTION character_query (pattern VARCHAR)
  RETURNS TABLE ( character_id    INTEGER
                , character_text  VARCHAR ) AS
$$
BEGIN
  INSERT INTO logger
  ( message_text )
  VALUES
  ( pattern );
 
  RETURN QUERY
  SELECT c.character_id
  ,      c.character_name
  FROM   CHARACTER c
  WHERE  c.character_name SIMILAR TO '%'||pattern||'%';
END;
$$ LANGUAGE plpgsql;

Now let’s test the new character_query function with this test case:

SELECT * FROM character_query('Ron');

Then, let’s check the logger table with this query:

SELECT * FROM logger;

It displays:

 logger_id | message_text 
-----------+--------------
         1 | Hermione
(1 row)

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

Written by maclochlainn

February 21st, 2020 at 12:58 am

Posted in Linux,Postgres,PostgreSQL

Tagged with ,

Python List & Dictionaries

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The following two sample programs are used in an Python programming course that I teach. I find them useful in qualifying how to work with loops, couple loops, and queues. The first example uses two lists and coupled loops, while the second example uses a single dictionary and FILO queue approach.

The Twelve Days of Christmas lyrics can be printed like so with coupled loops:

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days = ['first','second','third','fourth'       \
       ,'fifth','sixth','seventh','eighth'      \
       ,'nineth','tenth','eleventh','twelveth']
 
verse = ['partridge in a pear tree.'     \
        ,'Two turtle doves,'             \
        ,'Three French hens,'            \
        ,'Four calling birds,'           \
        ,'Five gold rings,'              \
        ,'Six geese a-laying,'           \
        ,'Seven swans a-swimming,'       \
        ,'Eight maids a-milking,'        \
        ,'Nine ladies dancing,'          \
        ,'Tenth lords a-leaping,'        \
        ,'Eleven pipers piping,'         \
        ,'Twelve drummers drumming,']
 
# Loop forward, couple inner loop, and loop backward through list.
for i in range(0,len(days), 1):
  print("On the",str(days[i]),"day of Christmas my true love sent to me")
  for j in range(i, -1, -1):
    if (j > 0):
      print(" ",verse[j])
    elif (i == j):
      print("  A",verse[j])
    else:
      print("  and a",verse[j])

Recreating the problem into a single dictionary, you can solve by approaching it as a FILO queue. Here’s the approach:

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lyric = {'first':'partridge in a pear tree.'
        ,'second':'Two turtle doves,'
        ,'third':'Three French hens,'
        ,'fourth':'Four calling birds,'
        ,'fifth':'Five gold rings,'
        ,'sixth':'Six geese a-laying,'
        ,'seventh':'Seven swans a-swimming,'
        ,'eighth':'maids a-milking,'
        ,'nineth':'Nine ladies dancing,'
        ,'tenth':'Ten lords a-leaping,'
        ,'eleventh':'Eleven pipers piping,'
        ,'twelfth':'Twelve drummers drumming,'}
 
# Intiate a list for collecting stanza.
stanza = list()
 
# Generate a list of keys.
for i in lyric.keys():
  # Append keys to list of stanza.
  stanza.append(i)
 
  # Print the first line of each stanza.
  print("On the",i,"day of Christmas my true love sent to me")
 
  # Print the progressive stanza.
  for j in reversed(stanza):
    if (j not in ['first','twelveth']):
      print(" ",lyric[j])
    elif (i == j):
      print("  A",lyric[j])
    else:
      print("  and a",lyric[j])

As always, I hope this helps for approaches and solutions.

Written by maclochlainn

February 16th, 2020 at 12:42 am

Posted in Python,Python 3.x

Tagged with

Developing Python Libraries

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I put this together to show my students how to simplify writing and testing Python library files. The trick requires that you learn how to set a relative $PYTHONPATH environment file.

export set PYTHONPATH=./lib

After setting the $PYTHONPATH environment variable, connect to Python’s IDLE environment and run the following code:

import os
print(os.environ['PYTHONPATH'])

It prints the following:

./lib

You can also discover all the standard libraries and your $PYTHONPATH value in your environment with the following command:

for i in sys.path:
  print(i)

It prints the following, which lists the one set by the $PYTHONPATH first:

/home/student/Code/python/path/lib
/usr/lib64/python37.zip
/usr/lib64/python3.7
/usr/lib64/python3.7/lib-dynload
/home/student/.local/lib/python3.7/site-packages
/usr/lib64/python3.7/site-packages
/usr/lib/python3.7/site-packages

You create a test my_module.py library file in the relative ./lib directory, like the following:

# Define a hello function that accept a name and prints a salutation.
def hello(whom):
  return "Hello " + whom + "!"

Next, you can create a testlib.py program:

# Import the hello function into the local namesapce from the my_module.
from my_module import hello
 
# Call the module hello, which returns a formatted string.
print(hello("Suzie Q"))

It imports the hello(whom) function into the local namespace and then calls the hello(whom) function with the string literal "Susie". It prints:

Hello Suzie Q!

If you import the my_module module, you must refer to the hello(whom) function by prefacing it with my_module., like the following example:

# Import the hello function into the local namesapce from the my_module.
import my_module
 
# Call the module hello, which returns a formatted string.
print(my_module.hello("Suzie Q"))

A direct import doesn’t add the method to the local namespace. It remains in the my_module‘s namespace.

It’s probably important to note where my_module.pyc files are written for the those migrating from Python 2.7 to Python 3. In Python 2.7 they would be written to the ./lib directory, but in Python 3 they’re written to the ./lib/__pycache__ directory.

As always, I hope this helps those who find it and read it.

Written by maclochlainn

February 13th, 2020 at 12:23 am

Posted in Python,Python 2.7,Python 3.x

Tagged with

Quick C How-to add .h

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Somebody wanted a quick example of using a user-defined header file for a constant value. While I think there a lot of examples on the Internet already, here’s quick example.

You can define a header (or global.h) file in the local directory with the value of pi, like this:

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// A global header file for constants.
double pi = 3.1415926535;

Then, you can write a little circle.c program like so:

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#include <stdio.h>
#include "global.h"
 
int main() {
  int r;
  double area;
 
  // Get the radius.
  printf("Enter a radius in inches: ");
  scanf("%i",&r);
 
  // Calculate the area of a circle.
  area = pi * (r * r);
 
  // Print the radius and circle area.
  printf("Area of a %d inch circle is %lf square inches!\n",r,area);
  return(0); }

The local global.h file is enclosed in double quotes rather than less than and great than symbols. You can compile circle.c like this on Linux, provided both files are in the same directory:

gcc -o circle circle.c

According to the gcc-documentation, the priority for include <> is, on a “standard Unix system”, as follows:

/usr/local/include
libdir/gcc/target/version/include
/usr/target/include
/usr/include

You should chmod the circle file as executable and then you can run it like so:

./circle

You’ll see more or less the following:

Enter a radius in inches: 3
Area of a 3 inch circle is 28.274334 square inches!

As always, I hope the example helps those looking for a starting point.

Written by maclochlainn

January 26th, 2020 at 12:49 am

Posted in C,C/C++ Programming,Linux,Unix

Tagged with

PostgreSQL Write JSON File

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In the process of working through issues to support ETL transformations from JSON with Python. The first step was extracting a the data from a PostgreSQL table into a JSON file. The syntax wasn’t exactly clear and the PostgreSQL Tutorial was a bit misleading but I worked it out. My initial sample deals with writing the data from the item table of my videodb database to a item.json file in the /tmp directory.

The query retrieves the data but casts the creation_date and last_update_date timestamp data types into date data types, as follows:

SELECT row_to_json(
 (SELECT ROW
  FROM (SELECT item_id
        ,      item_barcode
        ,      item_type
        ,      item_title
        ,      item_subtitle
        ,      item_rating
        ,      item_release_date
        ,      created_by
        ,      creation_date::DATE
        ,      last_updated_by
        ,      last_update_date::DATE) ROW)) AS DATA
        FROM   item;

The internal SELECT-list creates the name element of name:value pairs in the JSON structure. Naturally, it does exclude the type casting ::date portion of the query from the name elements. The row_to_json() maps the name list to the list of value results from in each row, and returns a result set.

The query returns the following 21 JSON objects:

{"item_id":1001,"item_barcode":"9736-05640-4","item_type":1016,"item_title":"The Hunt for Red October","item_subtitle":"Special Collector's Edition","item_rating":"PG","item_release_date":"1990-03-02","created_by":1001,"creation_date":"2019-12-13","last_updated_by":1001,"last_update_date":"2019-12-13"}
{"item_id":1002,"item_barcode":"24543-02392","item_type":1016,"item_title":"Star Wars I","item_subtitle":"Phantom Menace","item_rating":"PG","item_release_date":"1999-05-04","created_by":1001,"creation_date":"2019-12-13","last_updated_by":1001,"last_update_date":"2019-12-13"}
{"item_id":1003,"item_barcode":"24543-5615","item_type":1015,"item_title":"Star Wars II","item_subtitle":"Attack of the Clones","item_rating":"PG","item_release_date":"2002-05-16","created_by":1001,"creation_date":"2019-12-13","last_updated_by":1001,"last_update_date":"2019-12-13"}
{"item_id":1004,"item_barcode":"24543-05539","item_type":1016,"item_title":"Star Wars II","item_subtitle":"Attack of the Clones","item_rating":"PG","item_release_date":"2002-05-16","created_by":1001,"creation_date":"2019-12-13","last_updated_by":1001,"last_update_date":"2019-12-13"}
{"item_id":1005,"item_barcode":"24543-20309","item_type":1016,"item_title":"Star Wars III","item_subtitle":"Revenge of the Sith","item_rating":"PG13","item_release_date":"2005-05-19","created_by":1001,"creation_date":"2019-12-13","last_updated_by":1001,"last_update_date":"2019-12-13"}
{"item_id":1006,"item_barcode":"86936-70380","item_type":1016,"item_title":"The Chronicles of Narnia","item_subtitle":"The Lion, the Witch and the Wardrobe","item_rating":"PG","item_release_date":"2002-05-16","created_by":1001,"creation_date":"2019-12-13","last_updated_by":1001,"last_update_date":"2019-12-13"}
{"item_id":1007,"item_barcode":"91493-06475","item_type":1019,"item_title":"RoboCop","item_subtitle":"","item_rating":"Mature","item_release_date":"2003-07-24","created_by":1001,"creation_date":"2019-12-13","last_updated_by":1001,"last_update_date":"2019-12-13"}
{"item_id":1008,"item_barcode":"93155-11810","item_type":1019,"item_title":"Pirates of the Caribbean","item_subtitle":"","item_rating":"Teen","item_release_date":"2003-06-30","created_by":1001,"creation_date":"2019-12-13","last_updated_by":1001,"last_update_date":"2019-12-13"}
{"item_id":1009,"item_barcode":"12725-00173","item_type":1019,"item_title":"The Chronicles of Narnia","item_subtitle":"The Lion, the Witch and the Wardrobe","item_rating":"Everyone","item_release_date":"2003-06-30","created_by":1001,"creation_date":"2019-12-13","last_updated_by":1001,"last_update_date":"2019-12-13"}
{"item_id":1010,"item_barcode":"45496-96128","item_type":1017,"item_title":"MarioKart","item_subtitle":"Double Dash","item_rating":"Everyone","item_release_date":"2003-11-17","created_by":1001,"creation_date":"2019-12-13","last_updated_by":1001,"last_update_date":"2019-12-13"}
{"item_id":1011,"item_barcode":"08888-32214","item_type":1018,"item_title":"Splinter Cell","item_subtitle":"Chaos Theory","item_rating":"Teen","item_release_date":"2003-04-08","created_by":1001,"creation_date":"2019-12-13","last_updated_by":1001,"last_update_date":"2019-12-13"}
{"item_id":1012,"item_barcode":"14633-14821","item_type":1018,"item_title":"Need for Speed","item_subtitle":"Most Wanted","item_rating":"Everyone","item_release_date":"2004-11-15","created_by":1001,"creation_date":"2019-12-13","last_updated_by":1001,"last_update_date":"2019-12-13"}
{"item_id":1013,"item_barcode":"10425-29944","item_type":1019,"item_title":"The DaVinci Code","item_subtitle":"","item_rating":"Teen","item_release_date":"2006-05-19","created_by":1001,"creation_date":"2019-12-13","last_updated_by":1001,"last_update_date":"2019-12-13"}
{"item_id":1014,"item_barcode":"52919-52057","item_type":1019,"item_title":"Cars","item_subtitle":"","item_rating":"Everyone","item_release_date":"2006-04-28","created_by":1001,"creation_date":"2019-12-13","last_updated_by":1001,"last_update_date":"2019-12-13"}
{"item_id":1015,"item_barcode":"9689-80547-3","item_type":1020,"item_title":"Beau Geste","item_subtitle":"","item_rating":"PG","item_release_date":"1992-03-01","created_by":1001,"creation_date":"2019-12-13","last_updated_by":1001,"last_update_date":"2019-12-13"}
{"item_id":1016,"item_barcode":"53939-64103","item_type":1020,"item_title":"I Remember Mama","item_subtitle":"","item_rating":"NR","item_release_date":"1998-01-05","created_by":1001,"creation_date":"2019-12-13","last_updated_by":1001,"last_update_date":"2019-12-13"}
{"item_id":1017,"item_barcode":"24543-01292","item_type":1020,"item_title":"Tora! Tora! Tora!","item_subtitle":"The Attack on Pearl Harbor","item_rating":"G","item_release_date":"1999-11-02","created_by":1001,"creation_date":"2019-12-13","last_updated_by":1001,"last_update_date":"2019-12-13"}
{"item_id":1018,"item_barcode":"43396-60047","item_type":1020,"item_title":"A Man for All Seasons","item_subtitle":"","item_rating":"G","item_release_date":"1994-06-28","created_by":1001,"creation_date":"2019-12-13","last_updated_by":1001,"last_update_date":"2019-12-13"}
{"item_id":1019,"item_barcode":"43396-70603","item_type":1020,"item_title":"Hook","item_subtitle":"","item_rating":"PG","item_release_date":"1991-12-11","created_by":1001,"creation_date":"2019-12-13","last_updated_by":1001,"last_update_date":"2019-12-13"}
{"item_id":1020,"item_barcode":"85391-13213","item_type":1020,"item_title":"Around the World in 80 Days","item_subtitle":"","item_rating":"G","item_release_date":"1992-12-04","created_by":1001,"creation_date":"2019-12-13","last_updated_by":1001,"last_update_date":"2019-12-13"}
{"item_id":1021,"item_barcode":"85391-10843","item_type":1020,"item_title":"Camelot","item_subtitle":"","item_rating":"G","item_release_date":"1998-05-15","created_by":1001,"creation_date":"2019-12-13","last_updated_by":1001,"last_update_date":"2019-12-13"}

Moving on to the next step you enclose the query in the copy command syntax, like:

COPY
(SELECT row_to_json(
  (SELECT ROW
   FROM (SELECT item_id
         ,      item_barcode
         ,      item_type
         ,      item_title
         ,      item_subtitle
         ,      item_rating
         ,      item_release_date
         ,      created_by
         ,      creation_date::DATE
         ,      last_updated_by
         ,      last_update_date::DATE) ROW)) AS DATA
         FROM   item) TO '/tmp/sample.json';

When you run it, it will fail with the following error message:

ERROR:  must be superuser or a member of the pg_write_server_files role to COPY to a file
HINT:  Anyone can COPY to stdout or from stdin. psql's \copy command also works for anyone.

You can fix this problem by connecting as the postgres user and granting the privilege to the student user:

GRANT pg_write_server_files TO student;

It then runs successfully as the student user and creates the sample.json file in the /tmp directory. As always, I hope this helps those looking for a solution.

Written by maclochlainn

January 5th, 2020 at 12:24 am

Posted in DBA,DevOps,Fedora,JSON,Linux,Postgres,PostgreSQL,psql,sql

Tagged with ,

PostgreSQL Creating Schema

without comments

The process of creating a schema requires you grant the CREATE ON DATABASE privilege to the user as the postgres user. You use the following syntax:

GRANT CREATE ON DATABASE videodb TO student;

As the student user, you create the app schema with the following syntax:

CREATE SCHEMA app;

Then, you can query the result as follows:

SELECT   * 
FROM     pg_catalog.pg_namespace
ORDER BY nspname;

You should see the following:

      nspname       | nspowner |               nspacl                
--------------------+----------+-------------------------------------
 app                |    16390 | 
 information_schema |       10 | {postgres=UC/postgres,=U/postgres}
 pg_catalog         |       10 | {postgres=UC/postgres,=U/postgres}
 pg_temp_1          |       10 | 
 pg_toast           |       10 | 
 pg_toast_temp_1    |       10 | 
 public             |       10 | {postgres=UC/postgres,=UC/postgres}
(7 rows)

If you create a revision_history table without a schema name, it is automatically placed in the public schema, which means an attempt to describe the table will return an error. For example, you create a revision_history table with the following command:

CREATE TABLE app.revision_history
( revision_history_id serial
, session_id          VARCHAR
, TABLE_NAME          VARCHAR
, revision_id         INTEGER );

You describe the revision_history table with the following command:

\d revision_history

It will show the following because there is no revision_history table in the public schema and the default search path only include a schema that shares the name with the student user and the public schema.

Did not find any relation named "revision_history".

You can show the search path with the following:

show search_path;

It should return the following, which is a schema that shares the user’s name and public.

   search_path   
-----------------
 "$user", public
(1 row)

You set the search path as follows:

SET search_path TO app, "$user", public;

After you set the search_path, a standard attempt to describe the table will find the table whether it is in the app or public schema. That means the following command:

\d revision_history

Shows:

                                                  Table "app.revision_history"
       Column        |       Type        | Collation | Nullable |                            Default                            
---------------------+-------------------+-----------+----------+---------------------------------------------------------------
 revision_history_id | integer           |           | not null | nextval('revision_history_revision_history_id_seq'::regclass)
 session_id          | character varying |           |          | 
 table_name          | character varying |           |          | 
 revision_id         | integer           |           |          |

As always, I hope this helps somebody looking for an answer.

Written by maclochlainn

December 10th, 2019 at 9:37 pm

Posted in LAMP,Linux,Postgres,PostgreSQL,psql,sql

Tagged with ,

PostgreSQL WITH to INSERT

without comments

This demonstrates how you insert results from a common table expression (CTE) in a leading WITH clause. I thought it would be a nice add since the existing tutorials didn’t have an example.

Create the message table, like this:

CREATE TABLE message
( message_id    SERIAL
, message_text  VARCHAR );

Now, here’s a CTE with a two fabricated rows:

WITH cte AS
( SELECT 'x-ray' AS msg
  UNION ALL
  SELECT 'MRI' AS msg )
INSERT INTO message
( message_text )
( SELECT msg FROM cte );

Then, you can query the results from the message table, like this:

SELECT * FROM message;

You should get the following results:

 message_id | message_text 
------------+--------------
          1 | x-ray
          2 | MRI
(2 rows)

Unfortunately, if you re-ran that it would insert a duplicate set of rows. You can avoid that behavior by using correlation between the subquery in the WITH clause and target table of the INSERT statement.

The following only allows you to INSERT new rows in the table.

WITH cte AS
( SELECT 'x-ray' AS msg
  UNION ALL
  SELECT 'MRI' AS msg )
INSERT INTO message
( message_text )
( SELECT msg
  FROM   cte 
  WHERE  NOT EXISTS
          (SELECT NULL
           FROM   message 
           WHERE  message.message_text = cte.msg ));

You can add revision control column to the message table to verify there’s no change with subsequent attempts. The first step requires you create an independent message_rev_id_seq sequence, like this:

CREATE SEQUENCE message_rev_id_seq
  INCREMENT 1
  MINVALUE -9223372036854775807
  MAXVALUE  9223372036854775806
  START    -9223372036854765807;

You can alter the table by using the standard rev_id column name:

ALTER TABLE message
ADD COLUMN rev_id BIGINT NOT NULL DEFAULT NEXTVAL('public.message_rev_id_seq');

The ALTER statement will automatically add the first two values of the message_rev_id_seq to the existing rows. The ALTER statement is an immediate action because all Data Definition Language (DDL) commands are 1-Phase Commit operations.

The next step is to create an update_message_rev_id function that return a trigger. You do this using PL/pgSQL but you must specify the language value explicitly when returning a trigger data type, like:

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CREATE OR REPLACE FUNCTION update_message_rev_id()
RETURNS TRIGGER AS
$$
BEGIN
  NEW.rev_id = NEXTVAL(‘public.message_rev_id_seq’);
  RETURN NEW;
END;
$$ LANGUAGE plpgsql;

The last step requires you create an update_message_revision trigger that calls the update_message_rev_id function, like:

CREATE TRIGGER update_message_revision BEFORE UPDATE
  ON message FOR EACH ROW
  EXECUTE PROCEDURE update_message_rev_id();

You can test it by running the correlated INSERT statement again. You’ll find that no rows are touched by the INSERT statement because the revision numbers are unchanged. You should see these two rows:

 message_id | message_text |        rev_id        
------------+--------------+----------------------
          1 | x-ray        | -9223372036854765807
          2 | MRI          | -9223372036854765806
(2 rows)

As always, I hope this helps those looking for a example of how to solve a problem.

Written by maclochlainn

December 3rd, 2019 at 10:15 pm

Posted in Fedora,Linux,Postgres,PostgreSQL,sql

Tagged with ,

PostgreSQL Upsert Advanced

without comments

Nine years after writing how to use the MERGE statement in Oracle, I am writing how you implement an UPSERT statement in PostgreSQL. I wrote an initial post going over the basics of PostgreSQL’s upsert implementation of the INSERT statement with an DO UPDATE clause and a DO NOTHING clause.

I thought it was interesting that the PostgreSQL Upsert Using INSERT ON CONFLICT Statement web page didn’t cover using a subquery as the source for an INSERT statement.

Demonstration

Here are the steps to accomplish an import/upload with the COPY statement and the INSERT statement with DO UPDATE and DO NOTHING clauses.

Step #1 : Position your CSV file in the physical directory

The example uses the following upload directory:

/u01/app/postgres/upload

Put the following text in to the kingdom_import.csv file.

Narnia,77600,Peter the Magnificent,1272-03-20,1292-06-19
Narnia,77600,Edmund the Just,1272-03-20,1292-06-19
Narnia,77600,Susan the Gentle,1272-03-20,1292-06-19
Narnia,77600,Lucy the Valiant,1272-03-20,1292-06-19
Narnia,42100,Peter the Magnificent,1531-04-12,1531-05-31
Narnia,42100,Edmund the Just,1531-04-12,1531-05-31
Narnia,42100,Susan the Gentle,1531-04-12,1531-05-31
Narnia,42100,Lucy the Valiant,1531-04-12,1531-05-31
Camelot,15200,King Arthur,0631-03-10,0686-12-12
Camelot,15200,Sir Lionel,0631-03-10,0686-12-12
Camelot,15200,Sir Bors,0631-03-10,0635-12-12
Camelot,15200,Sir Bors,0640-03-10,0686-12-12
Camelot,15200,Sir Galahad,0631-03-10,0686-12-12
Camelot,15200,Sir Gawain,0631-03-10,0686-12-12
Camelot,15200,Sir Tristram,0631-03-10,0686-12-12
Camelot,15200,Sir Percival,0631-03-10,0686-12-12
Camelot,15200,Sir Lancelot,0670-09-30,0682-12-12
Camelot,15200,Modred,0681-09-30,0682-12-12

Step #2 : Run the script that creates tables and sequences

Copy the following code into a create_kingdom_knight_tables.sql file within a directory of your choice. Then, you run it as the student user. Assuming you put the code in the create_kingdom_knight_tables.sql script, you can call it like so

\i create_kingdom_knight_tables.sql

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-- Conditionally drop three tables and sequences.
DO $$
DECLARE
  /* Declare an indefinite length string and record variable. */
  SQL  VARCHAR;
  ROW  RECORD;
 
  /* Declare a cursor. */
  table_cursor CURSOR FOR
    SELECT TABLE_NAME
    FROM   information_schema.tables
    WHERE  table_catalog = 'videodb'
    AND    table_schema = 'public'
    AND    TABLE_NAME IN ('kingdom','knight','kingdom_knight_import');
BEGIN
  /* Open the cursor. */
  OPEN table_cursor;
  LOOP
    /* Fetch table names. */
    FETCH table_cursor INTO ROW;
 
    /* Exit when no more records are found. */
    EXIT WHEN NOT FOUND;
 
    /* Concatenate together a DDL to drop the table with prejudice. */
    SQL := 'DROP TABLE IF EXISTS '||ROW.table_name||' CASCADE';
 
    /* Execute the DDL statement. */
    EXECUTE SQL;
  END LOOP;
 
  /* Close the cursor. */
  CLOSE table_cursor;
END;
$$;
 
-- Create normalized kingdom table.
CREATE TABLE kingdom
( kingdom_id    SERIAL
, kingdom_name  VARCHAR(20)
, population    INTEGER
, CONSTRAINT kingdom_uq_key
  UNIQUE ( kingdom_name
         , population ));
 
-- Create normalized knight table.
CREATE TABLE knight
( knight_id             SERIAL
, knight_name           VARCHAR(24)
, kingdom_allegiance_id INTEGER
, allegiance_start_date DATE
, allegiance_end_date   DATE
, CONSTRAINT knight_uq_key 
  UNIQUE ( knight_name
         , kingdom_allegiance_id
         , allegiance_start_date
         , allegiance_end_date ));
 
-- Create external import table.
CREATE TABLE kingdom_knight_import
( kingdom_name          VARCHAR(20)
, population            INTEGER
, knight_name           VARCHAR(24)
, allegiance_start_date DATE
, allegiance_end_date   DATE);

Step #3 : Run the COPY command.

Run the COPY command to move the data from the Comma Separated Values (CSV) file to the kingdom_knight_import table. Then, run it as the student account.

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COPY kingdom_knight_import
FROM '/u01/app/postgres/upload/kingdom_import1.csv' DELIMITERS ',' CSV;

Step #4 : Create the upload_kingdom procedure

Copy the following code into a create_kingdom_knight_procedure.sql file within a directory of your choice. Assuming you put the code in the create_kingdom_knight_procedure.sql script, you can call it like so

\i create_kingdom_knight_procedure.sql

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CREATE PROCEDURE upload_kingdom() AS 
$$
DECLARE
 
  /* Declare error handling variables. */
  err_num      TEXT;
  err_msg      INTEGER;
 
BEGIN
 
  /* Insert only unique rows. The DISTINCT operator compresses the
     result set to a unique set and avoids consuming sequence values
     for non-unique result sets. */
  INSERT INTO kingdom
  ( kingdom_name
  , population )
  ( SELECT   DISTINCT
             kki.kingdom_name
    ,        kki.population
    FROM     kingdom_knight_import kki LEFT JOIN kingdom k
    ON       kki.kingdom_name = k.kingdom_name
    AND      kki.population = k.population)
  ON CONFLICT ON CONSTRAINT kingdom_uq_key
  DO NOTHING;
 
  /* Insert only unique rows. */
  INSERT INTO knight
  ( knight_name
  , kingdom_allegiance_id
  , allegiance_start_date
  , allegiance_end_date )
  (SELECT kki.knight_name
   ,      k.kingdom_id
   ,      kki.allegiance_start_date AS start_date
   ,      kki.allegiance_end_date AS end_date
   FROM   kingdom_knight_import kki INNER JOIN kingdom k
   ON     kki.kingdom_name = k.kingdom_name
   AND    kki.population = k.population LEFT JOIN knight kn 
   ON     k.kingdom_id = kn.kingdom_allegiance_id
   AND    kki.knight_name = kn.knight_name
   AND    kki.allegiance_start_date = kn.allegiance_start_date
   AND    kki.allegiance_end_date = kn.allegiance_end_date)
  ON CONFLICT ON CONSTRAINT knight_uq_key
  DO NOTHING;
 
EXCEPTION
  WHEN OTHERS THEN
    err_num := SQLSTATE;
    err_msg := SUBSTR(SQLERRM,1,100);
    RAISE NOTICE 'Trapped Error: %', err_msg;
END
$$ LANGUAGE plpgsql;

Step #5 : Run the upload_kingdom procedure and query the results

You run the upload_kingdom procedure with the CALL statement and then query the results. Assuming you put the code in the call_kingdom_knight_procedure.sql script, you can call it like so

\i call_kingdom_knight_procedure.sql

/* Call the upload_kingdom procedure. */
CALL upload_kingdom();
 
/* Query the kingdom_knight_import table. */
SELECT   kingdom_name
,        population
,        knight_name
,        date_trunc('second',allegiance_start_date AT TIME ZONE 'MST') AS allegiance_start_date
,        date_trunc('second',allegiance_end_date AT TIME ZONE 'MST') AS allegiance_end_date
FROM     kingdom_knight_import;
 
/* Query the kingdom table. */
SELECT   *
FROM     kingdom;
 
/* Query the knight table. */
SELECT   kn.knight_id
,        kki.knight_name
,        k.kingdom_id
,        date_trunc('second',kki.allegiance_start_date AT TIME ZONE 'MST') AS start_date
,        date_trunc('second',kki.allegiance_end_date AT TIME ZONE 'MST') AS end_date
FROM     kingdom_knight_import kki INNER JOIN kingdom k
ON       kki.kingdom_name = k.kingdom_name
AND      kki.population = k.population LEFT JOIN knight kn
ON       k.kingdom_id = kn.kingdom_allegiance_id
AND      kki.knight_name = kn.knight_name
AND      kki.allegiance_start_date = kn.allegiance_start_date
AND      kki.allegiance_end_date = kn.allegiance_end_date;

It prints the following results:

CALL
 kingdom_name | population |      knight_name      | allegiance_start_date | allegiance_end_date 
--------------+------------+-----------------------+-----------------------+---------------------
 Narnia       |      77600 | Peter the Magnificent | 1272-03-19 23:59:56   | 1292-06-18 23:59:56
 Narnia       |      77600 | Edmund the Just       | 1272-03-19 23:59:56   | 1292-06-18 23:59:56
 Narnia       |      77600 | Susan the Gentle      | 1272-03-19 23:59:56   | 1292-06-18 23:59:56
 Narnia       |      77600 | Lucy the Valiant      | 1272-03-19 23:59:56   | 1292-06-18 23:59:56
 Narnia       |      42100 | Peter the Magnificent | 1531-04-11 23:59:56   | 1531-05-30 23:59:56
 Narnia       |      42100 | Edmund the Just       | 1531-04-11 23:59:56   | 1531-05-30 23:59:56
 Narnia       |      42100 | Susan the Gentle      | 1531-04-11 23:59:56   | 1531-05-30 23:59:56
 Narnia       |      42100 | Lucy the Valiant      | 1531-04-11 23:59:56   | 1531-05-30 23:59:56
 Camelot      |      15200 | King Arthur           | 0631-03-09 23:59:56   | 0686-12-11 23:59:56
 Camelot      |      15200 | Sir Lionel            | 0631-03-09 23:59:56   | 0686-12-11 23:59:56
 Camelot      |      15200 | Sir Bors              | 0631-03-09 23:59:56   | 0635-12-11 23:59:56
 Camelot      |      15200 | Sir Bors              | 0640-03-09 23:59:56   | 0686-12-11 23:59:56
 Camelot      |      15200 | Sir Galahad           | 0631-03-09 23:59:56   | 0686-12-11 23:59:56
 Camelot      |      15200 | Sir Gawain            | 0631-03-09 23:59:56   | 0686-12-11 23:59:56
 Camelot      |      15200 | Sir Tristram          | 0631-03-09 23:59:56   | 0686-12-11 23:59:56
 Camelot      |      15200 | Sir Percival          | 0631-03-09 23:59:56   | 0686-12-11 23:59:56
 Camelot      |      15200 | Sir Lancelot          | 0670-09-29 23:59:56   | 0682-12-11 23:59:56
(18 rows)
 
 kingdom_id | kingdom_name | population 
------------+--------------+------------
          1 | Narnia       |      42100
          2 | Narnia       |      77600
          3 | Camelot      |      15200
(3 rows)
 
 knight_id |      knight_name      | kingdom_id |     start_date      |      end_date       
-----------+-----------------------+------------+---------------------+---------------------
         1 | Peter the Magnificent |          2 | 1272-03-19 23:59:56 | 1292-06-18 23:59:56
         2 | Edmund the Just       |          2 | 1272-03-19 23:59:56 | 1292-06-18 23:59:56
         3 | Susan the Gentle      |          2 | 1272-03-19 23:59:56 | 1292-06-18 23:59:56
         4 | Lucy the Valiant      |          2 | 1272-03-19 23:59:56 | 1292-06-18 23:59:56
         5 | Peter the Magnificent |          1 | 1531-04-11 23:59:56 | 1531-05-30 23:59:56
         6 | Edmund the Just       |          1 | 1531-04-11 23:59:56 | 1531-05-30 23:59:56
         7 | Susan the Gentle      |          1 | 1531-04-11 23:59:56 | 1531-05-30 23:59:56
         8 | Lucy the Valiant      |          1 | 1531-04-11 23:59:56 | 1531-05-30 23:59:56
         9 | King Arthur           |          3 | 0631-03-09 23:59:56 | 0686-12-11 23:59:56
        10 | Sir Lionel            |          3 | 0631-03-09 23:59:56 | 0686-12-11 23:59:56
        11 | Sir Bors              |          3 | 0631-03-09 23:59:56 | 0635-12-11 23:59:56
        12 | Sir Bors              |          3 | 0640-03-09 23:59:56 | 0686-12-11 23:59:56
        13 | Sir Galahad           |          3 | 0631-03-09 23:59:56 | 0686-12-11 23:59:56
        14 | Sir Gawain            |          3 | 0631-03-09 23:59:56 | 0686-12-11 23:59:56
        15 | Sir Tristram          |          3 | 0631-03-09 23:59:56 | 0686-12-11 23:59:56
        16 | Sir Percival          |          3 | 0631-03-09 23:59:56 | 0686-12-11 23:59:56
        17 | Sir Lancelot          |          3 | 0670-09-29 23:59:56 | 0682-12-11 23:59:56
        69 | Modred                |          3 | 0681-09-29 23:59:56 | 0682-12-11 23:59:56
(18 rows)

As always, I hope this works to help those trying to solve a similar problem.

Written by maclochlainn

November 25th, 2019 at 1:24 am

Posted in DBA,Linux,PL/pgSQL,Postgres,PostgreSQL

Tagged with ,

PostgreSQL Upsert Intro

with one comment

Oracle and SQL Server use the MERGE statement, MySQL uses the REPLACE INTO statement or ON DUPLICATE KEY, but PostgreSQL uses an upsert. The upsert isn’t a statement per se. It is like MySQL’s INSERT statement with the ON DUPLICATE KEY clause. PostgreSQL uses an ON CONFLICT clause in the INSERT statement and there anonymous block without the $$ delimiters.

The general behaviors of upserts is covered in the PostgreSQL Tutorial. It has the following prototype:

INSERT INTO TABLE_NAME(column_list) VALUES(value_list)
ON CONFLICT target action;

The target can be a column name, an ON CONSTRAINT constraint name, or a WHERE predicate, while the action can be DO NOTHING (or ignore) or a DO UPDATE statement. I wrote the following example to show how to leverage a unique constraint with a DO NOTHING and DO UPDATE behavior.

My example conditionally drops a table, creates a table with a unique constraint, inserts a few rows, updates with a DO UPDATE clause, updates with DO NOTHING clause, and queries the results with a bit of formatting.

  1. Conditionally drop the test table. 

    /* Suppress warnings from the log file. */
SET client_min_messages = 'error';
 
/* Conditionally drop table. */
DROP TABLE IF EXISTS test;

  2. Create the test table. 

    /* Create a test table. */
CREATE TABLE test
( test_id      SERIAL
, first_name   VARCHAR(20)
, middle_name  VARCHAR(20)
, last_name    VARCHAR(20)
, updated      INTEGER DEFAULT 0
, update_time  TIMESTAMP WITH TIME ZONE NOT NULL DEFAULT CURRENT_TIMESTAMP
, CONSTRAINT test_uq_key UNIQUE (first_name,middle_name,last_name));

  3. Insert six rows into the test table. 

    /* Insert six rows. */
INSERT INTO test
( first_name, middle_name, last_name )
VALUES
 ('Harry','James','Potter')
,('Ginerva','Molly','Potter')
,('Lily','Luna','Potter')
,('Albus','Severus','Potter')
,('James',NULL,'Potter')
,('Lily',NULL,'Potter');

  4. Create a five second delay. 

    /* Sleep for 5 seconds. */
DO $$
BEGIN
  PERFORM pg_sleep(5);
END;
$$;

  5. Use the INSERT statement with a DO UPDATE clause that increments the updated column of the test table. 

    /* Upsert on unique key constraint conflict. */
INSERT INTO test
( first_name
, middle_name
, last_name )
VALUES
('Harry'
,'James'
,'Potter')
ON CONFLICT ON CONSTRAINT test_uq_key
DO
  UPDATE
  SET    updated = excluded.updated + 1
  ,      update_time = CURRENT_TIMESTAMP;

  6. Use the INSERT statement with a DO NOTHING clause. 

    /* Upsert on unique key constraint ignore update. */
INSERT INTO test
( first_name
, middle_name
, last_name )
VALUES
('Harry'
,'James'
,'Potter')
ON CONFLICT ON CONSTRAINT test_uq_key
DO NOTHING;

  7. Query the test table. 

    /* Formatted query to demonstrate result of UPSERT statement. */
SELECT   test_id
,        last_name || ', '
||       CASE
           WHEN middle_name IS NOT NULL THEN first_name || ' ' || middle_name
           ELSE first_name
         END AS full_name
,        updated
,        date_trunc('second',update_time AT TIME ZONE 'MST') AS "timestamp"
FROM     test
ORDER BY last_name
,        first_name
,        CASE
           WHEN middle_name IS NOT NULL THEN middle_name
           ELSE 'A'
         END;

    Display results:

     test_id |       full_name       | updated |      timestamp      
---------+-----------------------+---------+---------------------
       4 | Potter, Albus Severus |       0 | 2019-11-24 19:23:10
       2 | Potter, Ginerva Molly |       0 | 2019-11-24 19:23:10
       1 | Potter, Harry James   |       1 | 2019-11-24 19:23:15
       5 | Potter, James         |       0 | 2019-11-24 19:23:10
       6 | Potter, Lily          |       0 | 2019-11-24 19:23:10
       3 | Potter, Lily Luna     |       0 | 2019-11-24 19:23:10
(6 rows)

As always, I hope this helps those looking for clear examples to solve problems.

Written by maclochlainn

November 24th, 2019 at 7:26 pm

Posted in Fedora,LAMP,Linux,Postgres,PostgreSQL,Upsert

Tagged with , , ,

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