Обсуждение: Alternative Serial

Hi list,<br /><br />I know that serials must be created like this.<br /><br />Create the Serial<br /><br />Create a
fieldtable and reffers to the pre-created serial.<br /><br />Now I noticed that it is possible to use the default
value.I am almost quite sure that this is an alternative way and non polite. <br /><br />I would like to know if I am
correctand what does it means:<br /><br />nextval('mySerial_id'::regclass)<br clear="all" /><br />What is regclass ?<br
/><br/>Regards ,<br />-- <br />Ezequias Rodrigues da Rocha<br /><a href="http://ezequiasrocha.blogspot.com/">
http://ezequiasrocha.blogspot.com/</a><br/>use Mozilla Firefox:<a
href="http://br.mozdev.org/firefox/">http://br.mozdev.org/firefox/</a>

Ezequias Rodrigues da Rocha написа:
> Hi list,
> 
> I know that serials must be created like this.

Actually no.

> Create the Serial

There is no db object 'Serial' you could create independently - you
probably mean 'sequence'.

> 
> Create a field table and reffers to the pre-created serial.

You could and should create a field of (pseudo-)type 'serial' and be
done with it.

> 
> Now I noticed that it is possible to use the default value. I am almost
> quite sure that this is an alternative way and non polite.


The old and still working way to create a field that would function as
surrogate key is to create a sequence, create a field in the table, mark
it integer and set its default value to be the 'nextval' of the created
sequence. That was too much typing for some (I'm included) and later a
syntax sugar was added - the pseudo-type 'serial'.

So the following syntax:

CREATE TABLE example (  example_sid SERIAL NOT NULL PRIMARY KEY,  ...
)

is equivalen to:


CREATE SEQUENCE example_example_sid_seq;

CREATE TABLE example (  example_sid INTEGER PRIMARY KEY DEFAULT
NEXVAL)('example_example_sid_seq'),  ...
)


As you could see - very convenient for the 99.9% of the cases.

> 
> I would like to know if I am correct and what does it means:
> 
> nextval('mySerial_id'::regclass)
> 
> What is regclass ?

Read here about 'regclass' (there is a note about it) -
http://www.postgresql.org/docs/8.2/static/functions-sequence.html, but
that should not be important to you.


-- 
Milen A. Radev

Yes you are right, I means sequences.

The second way is better for me that have Id (PK) fields not  defined  as Sequences. Now it is much more difficult to change the  type of my PK than define a Default value. I am just concerned about the consistency of this sequence. Could you make me more easy ?

And about the cast I have found ?

nextval('mySerial_id'::regclass)

I ask again. What is regclass ? What does it really is.

Ezequias




--
Ezequias Rodrigues da Rocha
http://ezequiasrocha.blogspot.com/
use Mozilla Firefox: http://br.mozdev.org/firefox/

Ezequias Rodrigues da Rocha написа:
> Yes you are right, I means sequences.
> 

Please re-read my reply!

> The second way is better for me that have Id (PK) fields not  defined  as
> Sequences. Now it is much more difficult to change the  type of my PK than

A field could not be defined as 'sequence', but as 'serial', because
'sequence' is a db object and 'serial' is a (pseudo-)type. The actual
type of a field defined as 'serial' is 'integer' (or 'bigint' in case of
'bigserial'). You could change the type of a field (defined as sequence
earlier) as easy as you could any other field. Or you could just change
its default value. Or anything else.

> define a Default value. I am just concerned about the consistency of this
> sequence. Could you make me more easy ?

What do you mean by 'consistency' here? Guaranteed uniqueness of the
numbers generated by the sequence? Pleas read the fine documentation
here - http://www.postgresql.org/docs/8.2/static/sql-createsequence.html.


> 
> And about the cast I have found ?
> 
> nextval('mySerial_id'::regclass)
> 
> I ask again. What is regclass ? What does it really is.

I already answered that one, please re-read my reply and the pages it
mentions.

[...]



-- 
Milen A. Radev

Yes you are right, I means sequences.

The second way is better for me that have Id (PK) fields not  defined  as Sequences. Now it is much more difficult to change the  type of my PK than define a Default value. I am just concerned about the consistency of this sequence. Could you make me more easy ?

And about the cast I have found ?

nextval('mySerial_id'::regclass)

I ask again. What is regclass ? What does it really is.

Ezequias




--
Ezequias Rodrigues da Rocha
http://ezequiasrocha.blogspot.com/
use Mozilla Firefox: http://br.mozdev.org/firefox/