Alter a column data type in all tables

We must change the timestamp format for all columns of type timestamp.

The format is like that :

2017-06-14 17:47:23.295343

and it must be like that :

2017-06-14 17:47:23

The SQL command to be done is :

alter table my_table 
   alter column my_timestamp_column type timestamp(0);

And we embed this command into a loop that takes relevant information from the catalog :

create or replace function public.alter_all_timestamps() 
 returns void as $$
declare
  r       record;
  stmt    varchar;
begin
  for r in 
    select 
        c.table_name
       ,c.column_name
    from 
        information_schema.columns as c
    inner join 
        information_schema.tables as t 
    on 
        c.table_name = t.table_name
    where 
        c.table_schema = 'public'
    and 
        c.data_type = 'timestamp without time zone'
    and 
        t.table_type = 'BASE TABLE'
  loop
    
        stmt = 'alter table ' 
                || quote_ident(r.table_name )
                || ' alter column '
                || quote_ident(r.column_name)
                || ' type timestamp(0)';
                
        execute stmt;

  end loop;

end $$ language 'plpgsql' volatile;

That’s simple !

If there are views on those tables that select a column of type timestamp, those views must be dropped before the run of the function and recreated after.

By the way, when I was writing this article, I was listening Nagoya Guitars of Steve Reich.

Calculated columns with a trigger

Some say that calculated columns is bad design because it does not follow the relational model. It is true but performance can be greatly enhanced.

In this simple example, we have a table with 3 columns. The first and the second are given by the user and the third is calculated by a trigger. We add a condition : x * 0 = 0 * x = 1 (This fantasy comes from the requirements of a customer).  It is done with the case expression.

Some say that calculated columns is bad design because it does not follow the relational model. It is true but performance can be greatly enhanced.

In this simple example, we add a condition : x * 0 = 0 * x = 1. It is done with the case construct.

drop table if exists tup;

create table tup (
     col1   numeric(10,2)
    ,col2   numeric(10,2)
    ,col3   numeric(10,2)
);

insert into tup(col1,col2) values
     (2.87      ,3.77)
    ,(4         ,5.11)
    ,(2.12      ,0)
    ,(0.0       ,3);

update tup
    set col3 =     (case col1 when 0 then 1 else col1 end)
                *  (case col2 when 0 then 1 else col2 end);

select * from tup;

will give

col1     col2   col3
---------------------
2.87     3.77   10.82
4.00     5.11   20.44
2.12     0.00    2.12
0.00     3.00    3.00

Let now add the trigger and the trigger function :

create or replace function calculate_columns() returns trigger as $$
begin

  new.col3 =   (case new.col1 when 0 then 1 else new.col1 end)
             * (case new.col2 when 0 then 1 else new.col2 end);

  return new;

end $$ language plpgsql;

create trigger calculated_columns  
    before insert or update 
    on tup
    for each row
    execute procedure calculate_columns();

Note that we do a before insert or update trigger and that we update the columns in the row new.

insert into tup(col1, col2) values
     (6.23      ,2)
    ,(0         ,55.11);

select * from tup;

will give :

col1     col2   col3
---------------------
2.87     3.77   10.82
4.00     5.11   20.44
2.12     0.00    2.12
0.00     3.00    3.00
6.23     2.00   12.46
0.00    55.11   55.11

The deal is in the bag!

To list all the pk’s and fk’s

The system catalogs of PostgreSQL have very well designed. They are not easy to understand at first glance but they allow to write short and efficient queries.

As usual, if the data model is sound, the queries are simple and natural.

Just an example : the comma separated list of columns for composite keys is automatically created.

select
     conrelid::regclass as table_name
    ,conname
    ,pg_get_constraintdef(c.oid)
from
    pg_constraint c
join
    pg_namespace n on n.oid = c.connamespace
where
    contype in ('f', 'p')
and
    n.nspname = 'public'
order by
    conrelid::regclass::text
   ,contype desc;

The output is like that :

...
"SLM_TYPE"           SLM_TYPE_pkey                   PRIMARY KEY ("SLM_TYPE_NR")
"SLM_TYPE_CONTACT"   SLM_TYPE_CONTACT_CONTACT_fkey   FOREIGN KEY ("CONTACT") 
                                                        REFERENCES "CONTACTS"("CONTACT_NR") 
                                                        ON DELETE CASCADE
...

By the way, it is always better to use serials for pk’s and fk’s. But I am working with database I migrate from Access to PostgreSQL. I would like to write a script that will add serials for those pk’s and fk’s and that will transform the initial pk’s and fk’s to unique constraints. Not so easy…

To list all the indexes

When we create a pk in PostgreSQL, an index named table_name_pkey is automatically created.

The following script gives all indexes, those created by the pk’s and the others.

Don’t forget that PostgreSQL does not create automatically an index when you create a fk. You have to do it yourself !

By the way, this script works for composite indexes thanks to the handy function array_to_string that creates of comma separated list of the column names. It works because the two first columns in the select clause are given in the order by.

select 
     t.relname as table_name
    ,i.relname as index_name
    ,array_to_string(array_agg(a.attname), ', ') as column_names
from
     pg_class      t
    ,pg_class      i
    ,pg_index      ix
    ,pg_attribute  a
where
    t.oid = ix.indrelid
and 
    i.oid = ix.indexrelid
and 
    a.attrelid = t.oid
and 
    a.attnum = any(ix.indkey)
and 
    t.relkind = 'r' -- takes regular tables
and 
    t.relname not like 'pg_%' -- excludes system catalogs
group by
    t.relname
   ,i.relname
order by
    t.relname
   ,i.relname;

By the way, when I was working on the script, I was listening one of the last masterworks of the Johann Sebastian Bach of our time : Pulse of Steve Reich. The music of Steve Reich is a good music to listen when you write programs because this music is well structured and transmits lofty feelings…

How to safely delete records ?

To collect accurate data is expensive !

To structure data is very expensive !

To record accurate structured data is very very expensive !

Then…

Once data is recorded in a database, it is not a good idea to definitely delete it. It is better to archive the records we don’t want to see anymore in the database.

We will build a very simple framework for a soft delete with the PostgreSQL trigger functions. In a previous post, we developed an audit system that we will use again. We have just to modify the pre-delete trigger.

The components are :

  • a schema called ‘deleted’ having the same tables that the database
  • a trigger pre-delete on each table

and, inherited from the audit system :

  • an additional column on each table called audit_id
  • a table called audit_history

Let’s begin with the creation of the schema ‘deleted’ :

create or replace function public.create_schema_deleted() returns void as $$
declare 
 r	record;
 stmt	varchar;
begin

 execute 'create schema deleted;'
 
 for r in
  select 
   quote_ident(table_name) as table_name
  from 
   information_schema.tables 
  where 
   table_schema = 'public'

  loop

   stmt :=    'create table deleted.' 
     || r.table_name 
     || ' (like public.' 
     || r.table_name 
     || ')';

   execute stmt;

  end loop;
  
  execute 'grant usage on schema deleted to generic user'
  execute 'grant insert on all tables in schema deleted to generic_user;'

end $$ language 'plpgsql' volatile;

select public.create_schema_deleted();

This schema contains all the table of your production database without indexes, without pk/fk constraints, without sequences and without triggers. The role generic_user is granted to actual users.

Just after the creation of the schema, the tables are empty.

Each table has a column audit_id. It is a big serial that is incremented automatically when we insert a record in the production database.

The tables in the schema ‘deleted’ have of course also this column.

We create now the trigger function that will be executed each time an user deletes a record in the production database :

create or replace function public.pre_delete() returns trigger as $$
declare
 stmt 	varchar;
begin

 insert into public.audit_history
  (table_name, operation, audit_id, user_name, audit_date)
 values 
  (TG_TABLE_NAME, TG_OP, old.audit_id, current_user, now())

  stmt :=    'insert into deleted.'
   || quote_ident(TG_TABLE_NAME)
   || ' select * from public.'
   || quote_ident(TG_TABLE_NAME)
   || ' where audit_id = $1;';

  execute stmt using old.audit_id;
        
  return old;

end; $$ language 'plpgsql'; 

and a trigger pre-delete for each table :

create or replace function public.create_pre_delete_triggers() 
   returns void as $$
declare
 r	record;
 stmt	varchar;
begin
 for r in 
    select table_name 
    from information_schema.tables 
    where table_schema = 'public'
  loop

    stmt :=    'create trigger ' 
     || quote_ident(r.table_name || '_pre_delete')
     || ' after insert on public.' 
     || quote_ident(r.table_name)
     || ' for each row execute procedure public.pre_delete();';

    execute stmt;

  end loop;

end; $$ language 'plpgsql'; 

select public.create_pre_delete_triggers();

If we want to find a deleted record, we have a look in the table audit_history that has the following structure :

 id_audit_history  bigserial unique 
,table_name        text 
,operation         text 
,audit_id          bigint 
,user_name         text 
,audit_date        timestamp

And we do a select in the schema ‘deleted’ for this table and for this audit_id.

That’s simple !

By the way, when I was working on the script, I was listening one of the last masterworks of the Johann Sebastian Bach of our time : Runner of Steve Reich. I can listen this music for hours. This music enhances the intellect and purify the soul.

A simple audit system

Let’s suppose we want to audit all insert, update and delete committed on a database.

We create a table to store relevant data :

drop table if exists audit_history;
create table audit_history (
  id_audit_history       serial
 ,table_name             text
 ,operation              text
 ,audit_id               bigint
 ,user_name              text
 ,insertion_date         timestamp       default now()
);

drop sequence if exists audit_history_seq;
create sequence audit_history_seq;

We create two test tables with a pk constraints and one having a fk constraint to the other. Note that this fk constraint has the clause on delete cascade.

drop table if exists app_table_ref;
drop table if exists app_table;

create table app_table (
  pk             integer         primary key
 ,info           varchar
 ,audit_id       bigserial
);

create table app_table_ref (
  pk             integer         primary key
 ,info           varchar
 ,fk             integer         
          references app_table(pk)        
          on delete cascade
 ,audit_id       bigserial
);

Note the presence of the column audit_id, a bigserial that is not a pk.

We create now the trigger function, a generic fonction for the two triggers : after insert or update and before delete.

create or replace function populate_audit_history() 
   returns trigger as $$
begin

  if (TG_OP = 'INSERT' or TG_OP = 'UPDATE') then

      insert into audit_history
        (table_name, operation, audit_id, user_name)
      values
        (TG_TABLE_NAME, TG_OP, new.audit_id, current_user);

      return new;

  else
  
     insert into audit_history
        (table_name, operation, audit_id, user_name)
      values
        (TG_TABLE_NAME, TG_OP, old.audit_id, current_user);

     return old;

  end if;
end;
$$ language 'plpgsql';


create or replace function create_audit_triggers() 
   returns void as $$
declare
   r       record;
   stmt    varchar;
begin
  for r in select table_name from information_schema.tables 
              where table_name like 'app_table%' loop

     stmt :=    'create trigger '
             || quote_ident(r.table_name || '_audit_after_iu')
             || ' after insert or update on '
             || quote_ident(r.table_name)
             || ' for each row 
                    execute procedure populate_audit_history();';

     execute stmt;
   
     stmt :=    'create trigger '
             || quote_ident(r.table_name || '_audit_before_d')
             || ' before delete on '
             || quote_ident(r.table_name)
             || ' for each row 
                    execute procedure populate_audit_history();';

     execute stmt;

   end loop;
end;
$$ language 'plpgsql';

select create_audit_triggers();

Note the execute instruction that allows us to execute dynamic SQL statements.

We do now some transactions on those two tables :

insert into app_table values(23,'record 23');
insert into app_table values(56,'record 56');
insert into app_table values(71,'record 71');
insert into app_table values(82,'record 82');
insert into app_table values(85,'record 85');
insert into app_table values(91,'record 91');
insert into app_table values(94,'record 94');
insert into app_table values(97,'record 97');
insert into app_table values(99,'record 99');
update app_table set info = 'modified' where pk = 23;
update app_table set info = 'modified' where pk = 56;
update app_table set info = 'modified' where pk = 97;
delete from app_table where pk = 71;
insert into app_table values(101,'record 101');
insert into app_table values(121,'record 121');
insert into app_table values(167,'record 167');
update app_table set info = 'modified' where pk = 101;
delete from app_table where pk = 121;
insert into app_table_ref values(1, 'ref1', 23);
insert into app_table_ref values(2, 'ref2', 23);
insert into app_table_ref values(3, 'ref3', 82);
delete from app_table where pk = 23;

Here is the table audit_history :

1  | app_table     | INSERT |  1 | mchl | 2017-02-08 15:01:10.28164
2  | app_table     | INSERT |  2 | mchl | 2017-02-08 15:01:10.292839
3  | app_table     | INSERT |  3 | mchl | 2017-02-08 15:01:10.304088
4  | app_table     | INSERT |  4 | mchl | 2017-02-08 15:01:10.315051
5  | app_table     | INSERT |  5 | mchl | 2017-02-08 15:01:10.32618
6  | app_table     | INSERT |  6 | mchl | 2017-02-08 15:01:10.337284
7  | app_table     | INSERT |  7 | mchl | 2017-02-08 15:01:10.348366
8  | app_table     | INSERT |  8 | mchl | 2017-02-08 15:01:10.35954
9  | app_table     | INSERT |  9 | mchl | 2017-02-08 15:01:10.370595
10 | app_table     | UPDATE |  1 | mchl | 2017-02-08 15:01:10.381727
11 | app_table     | UPDATE |  2 | mchl | 2017-02-08 15:01:10.392824
12 | app_table     | UPDATE |  8 | mchl | 2017-02-08 15:01:10.403822
13 | app_table     | DELETE |  3 | mchl | 2017-02-08 15:01:10.414956
14 | app_table     | INSERT | 10 | mchl | 2017-02-08 15:01:10.4261
15 | app_table     | INSERT | 11 | mchl | 2017-02-08 15:01:10.437168
16 | app_table     | INSERT | 12 | mchl | 2017-02-08 15:01:10.448195
17 | app_table     | UPDATE | 10 | mchl | 2017-02-08 15:01:10.459344
18 | app_table     | DELETE | 11 | mchl | 2017-02-08 15:01:10.470463
19 | app_table_ref | INSERT |  1 | mchl | 2017-02-08 15:01:10.481536
20 | app_table_ref | INSERT |  2 | mchl | 2017-02-08 15:01:10.49259
21 | app_table_ref | INSERT |  3 | mchl | 2017-02-08 15:01:10.503579
22 | app_table     | DELETE |  1 | mchl | 2017-02-08 15:01:10.514913
23 | app_table_ref | DELETE |  1 | mchl | 2017-02-08 15:01:10.514913
24 | app_table_ref | DELETE |  2 | mchl | 2017-02-08 15:01:10.514913

And here the table app_table :

  56 | modified   |        2
  82 | record 82  |        4
  85 | record 85  |        5
  91 | record 91  |        6
  94 | record 94  |        7
  97 | modified   |        8
  99 | record 99  |        9
 101 | modified   |       10
 167 | record 167 |       12

and the table app_table_ref :

  3 | ref3 | 82 |        3

We note that the delete in app_table of the record having pk = 23 has deleted also the two records of app_table_ref having fk = 23.

This design is very handy because you can add auditing on an existing database by adding a to each table : 1/  a column serial (that is not a pk) and 2/  the two triggers (after the creation of the generic function).

The applications are not touched and the referential integrity of the database is preserved.

Having for each modifications, the name of the table and the value of the audit_id, we can find, for insert and update, the rows that have changed.

The Book of the Honeycomb’s Flow

The נופת צופים, Nofet Tsoufim, of יהודה בן יחיאל הרופה Yehuda ben Yechiel Harofe, also called Messer Leon of Mantua, is just arrived in Antwerp! The text in the Hebrew language is provided with an English translation entitled The Book of the Honeycomb’s Flow due to Isaac Rabinowitz.  The title comes from Tehilim 19,10-11 and can be translated as “the honey that drips from the rays of the hive”.

The Rav Yehuda ben Yechiel harofé was born in Mantua in 1425 and died in Naples in 1497. He was a doctor as his name suggests. He was also rosh yeshiva and possek. He lived in several Italian cities and wrote many books.

To fully understand the nature and scope of Messer Leo’s work, it is necessary to make a brief history of what is today called Hebrew scholasticism.

Latin medieval philosophy is referred to as scholasticism. Although philosophy was only ancilla theologiae, the servant of theology, scholasticism, essentially Aristotelian, is a model of logical rigor.

When the Jews want to learn something that lies outside the Torah, they will naturally seek out the world in which they live. When the Rambam was learning medicine, he learned it in Arabic. When the Spanish and Italian Jews want to learn logic or physics, they will study the scholastic treatises since they know Latin. This movement is called Hebrew scholasticism.

The later Latins will build an educational system that will be transmitted to the medieval world: the seven Liberal Arts. These arts (sciences or techniques) are liberal in the sense that they are studied for themselves and not for practical application. The Artes Liberales are divided into two groups:

  • The Trivium : grammar, logic and rhetoric
  • The Quadrivium : algebra, geometry, astronomy and music

At the time of the Renaissance, the Italian Jews became enthusiastic for the Artes Liberales.

The Rav Yehuda ben Yechiel composed a Trivium in the Hebrew language :

  • Livnat hasappir : Sapphire stone, on the Hebrew grammar of which you can find the manuscript by clicking here.
  • Mikhlal Yofi : Perfect beauty, on logic, from which you can find the manuscript by clicking there and about which Isaac Huzik wrote a book.
  • Nofet Tzoufim : Honey dripping from the rays of the hive, on the rhetoric of which here is the text in modern characters and of which here is the original.

It is this last work which has just arrived at Antwerp. Is not that wonderful?

Hebrew rhetoric, melitza in Hebrew, pursues purposes other than Greek or Latin rhetoric. It is not a question of teaching eloquence to ambitious young people who run for high positions but to learn to read the Torah books and, if necessary, to write them. Keep in mind that the Pirqe Avos teaches that it is better to be at the tail of the fox than to be at the head of lions so Julius Caesar said it is better to be first among the barbarians second among The Romans.

The Ramchal gives us a beautiful proof of the fundamental moral dimension of Hebrew rhetoric by titling his first treatise on rhetoric by a title that comes from Isaiah (50: 4) :

ה׳ נָתַן לִי לְשׁוֹן לִמּוּדִים לָדַעַת לָעוּת אֶת-יָעֵף

Hashem has given me the tongue of the disciples
so that I may know to support by the word the one who is exhausted.

The Ramchal, who is today one of the keystones of Judaism, is a representative of this school.

During the last year of his stay in Amsterdam, he also composed a Trivium in the Hebrew language :

  • Sefer hadiqduq: Grammar Book
  • Sefer hahigayone: Book of Logic
  • Sefer hamelitza: Book of Rhetoric

Some may wonder if the Artes Liberales are kosher.

There are neutral. There are just methods. Furthermore, halacha tells us that when we see a non-Jewish scholar, we must say: “Blessed are You, Hashem, who gives your wisdom beings of flesh and blood.” The Mishna Beroura tells us that he is a scholar versed in the Shevii chochmot, the Seven Sciences, but he does not give us a list of these seven sciences. Looking that list for several months, but right now, I still do not know what the Chofetz Chaim had in mind but it is likely that he thought the Seven Liberal Arts. Although for reasons dependent on my personal journey, I studied for years the interactions between Latin thought and Hebrew thought, I ask myself a lot of questions about the relevance of such studies.

Several hypotheses are to be considered :

  • This is a time gone forever. These studies have only a historical interest.
  • This culture has disappeared only because of the vicissitudes which the Jewish people have known in the Christian countries. It could be reborn from its ashes.
  • This research on language analysis must be continued using the methods of linguistics, mathematics and computer science. References to older authors are obsolete.

I would like to hear your opinion.