Re: Re: Using Random Sequence as Key

First of all, let me apologise for stepping so far back into the archives, 
but I wanted to get this in the archives in case anyone else wanted to use 
this.

There's a slight addendum here...as written, it's fairly likely that people 
will get duplicate ID's using this system.

A small change gets rid of the problem: multiply the serialportion by the 
number of digits you'll have in timeportion.  So, if timeportion is three 
digits, you'd multiply serialportion by 1000 and *then* add the timeportion. 
Below, I use the mod operator '%' to make sure it's 3 digits or less.

Caveat: this will only work as long as you have less than 40 million users.  
;^)

-Ken

create FUNCTION new_request_id()
RETURNS INT4
as' DECLARE        timeportion INT4;        serialportion INT4; BEGIN        timeportion := cast 
(date_part(''milliseconds'',timeofday()::timestamp) as integer);        serialportion :=
nextval(''request_idfake_seq'')*1000;        RETURN (timeportion % 1000) + serialportion; END;'
 
LANGUAGE 'plpgsql'
;


On Tuesday 17 April 2001  3:08 pm, Josh Berkus wrote:
> Bernardo,
>
> > I needed the random field because if I use serial and the user gets a
> > 34203
> > he's sure that 34202 exists, and that (probably, there where 34202
> > inserts
> > before him (or at least an offset + some)). Using a random just makes
> > the
> > user totally blind.
> > As I said I could use a serial for indexing the table but I NEED the
> > random
> > field and I need to to be unique since all the queries will be using
> > it as a
> > search parameter.
> > If inserting this way is slow it's not such a big deal since it's a
> > small db
> > and inserts are seldom made.
> > Thanks in advance for any help.
>
> Here's another suggestion for you then:
>
> 1. Add a sequence "Sales_sq"
>
> 1. write a custom function for new id numbers:
> CREATE FUNCTION new_sales_id() RETURNS INT4 AS '
> DECLARE
>     timeportion VARCHAR;
>     serialportion INT4;
> BEGIN
>     timeportion := to_char(current_timestamp, ''ms'');
> -- (or whatever the abbreviation for 2-digit milliseconds is)
>     serialportion := 100*(nextval(''sales_seq''));
>     RETURN CAST(to_number(timeportion) AS INT4) + serialportion;
> END;
>
> 3. Then set the id column to default to this new function.
>
> This would give you (after you correct my mistakes) a number, the first
> X digits of are Serial, and the last 2 digits based on the server's
> internal clock.  Thus, the numbers would *not* be sequential, and would
> appear fairly random, but would be unique *without* and expensive check
> for that value anywhere in the table for each insert.
>
> -Josh Berkus
>
>
>
>
>
>
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-- 
Ken Corey, CTO    Atomic Interactive, Ltd.   ken.corey@atomic-interactive.com