Tag Archives: database

The Database Relational Fuckup

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In his book The Database Relational Model on page 25 Date says:

Here I would just comment–here comes the hindsight once again–that if as suggested earlier the database is to be regarded as a (correct!) logical system, it must never be allowed to contain any inconsistencies, at least from the user’s point of view. In other words, “remedying inconsistencies” needs to be done on an individual statement-by-statement basis (not even on a transaction-by-transaction basis). See reference [52] for further elaboration on this point.

Reference [52] is then:

C. J. Date and Hugh Darwen: Foundation for Object/Relational Databases: The Third Manifesto. Reading, Mass.: Addison-Wesley (1998). A second edition of this book, under the revised title Foundation for Future Database Systems: The Third Manifesto, is due to appear concurrently with the present book.

I think Date did a lot of harm with this comment. The classic example of a transaction is taking $100 from my account and depositing it in your account. So there’s one statement to take the money out, and another statement to put the money in, and after the first statement but before the second statement the database will always go through an invalid state. If RDBMS integrity facilities were on a transaction-by-transaction basis we could actually use them, as they’re not we suffer. Either we need to do more work than necessary or we have to under specify our constraints, or some awkward combination of both.

A great MySQL session

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I found this great MySQL session over here. It has a clever approach for creating a big table, and shows how to invoke shell commands from the `mysql` client.

USE test;
SET GLOBAL innodb_file_per_table=1;
SET GLOBAL autocommit=0;

-- Create an uncompressed table with a million or two rows.
CREATE TABLE big_table AS SELECT * FROM information_schema.columns;
INSERT INTO big_table SELECT * FROM big_table;
INSERT INTO big_table SELECT * FROM big_table;
INSERT INTO big_table SELECT * FROM big_table;
INSERT INTO big_table SELECT * FROM big_table;
INSERT INTO big_table SELECT * FROM big_table;
INSERT INTO big_table SELECT * FROM big_table;
INSERT INTO big_table SELECT * FROM big_table;
INSERT INTO big_table SELECT * FROM big_table;
INSERT INTO big_table SELECT * FROM big_table;
INSERT INTO big_table SELECT * FROM big_table;
COMMIT;
ALTER TABLE big_table ADD id int unsigned NOT NULL PRIMARY KEY auto_increment;

SHOW CREATE TABLE big_table\G

select count(id) from big_table;

-- Check how much space is needed for the uncompressed table.
\! ls -l data/test/big_table.ibd

CREATE TABLE key_block_size_4 LIKE big_table;
ALTER TABLE key_block_size_4 key_block_size=4 row_format=compressed;

INSERT INTO key_block_size_4 SELECT * FROM big_table;
commit;

-- Check how much space is needed for a compressed table
-- with particular compression settings.
\! ls -l data/test/key_block_size_4.ibd

Administering PostgreSQL

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So I found this article which said:

 $ sudo -u postgres psql

 postgres=> alter user postgres password 'apassword';
 postgres=> create user your-user createdb createuser password 'passwd';
 postgres=> create database your-db-name owner your-user;
 postgres=> \q

Note: to enable password logins for the ‘postgres’ admin account, edit: /etc/postgresql/9.4/main/pg_hba.conf and after this line:

local   all             postgres                                peer

Add this line:

local   all             postgres                                md5

Creating a user and database in PostgreSQL

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To create a database and user account for use on PostgreSQL:

root@devotion:~# su - postgres

postgres@devotion:~$ psql
psql (8.4.21)
Type "help" for help.

postgres=# create user "www-data" superuser;
CREATE ROLE

postgres=# create database mydb with owner "www-data";
CREATE DATABASE

postgres=# \c mydb
psql (8.4.21)
You are now connected to database "mydb".
                                            ^
mydb=# alter user "www-data" with password 'secret';
ALTER ROLE