Thread: finding (and recycling) holes in sequences

 
If one has a unique-id generating sequence that sometimes is bound to
have holes in it (ie: it could happen that a nextval(seq) happens
without a corresponding INSERT in the table), then how could one
efficiently scan for these holes to recycle them in subsequent INSERTs?

I'm just looking for a "standard" way of doing this if such a thing
exists.

Louis-David Mitterrand - ldm@apartia.org - http://www.apartia.fr
 

If you want to create a sequence of numbers with no holes, you might consider NOT using sequences at all (the are really meant for something different).  Rather, each time you wish to do an insert, query the existing data to see what the maximum value is existing in the database.  Add 1 to that and use this as your new value.  This is guaranteed to not have any holes in the sequence.  To make inserts easier, you can define an sql function that will return the max value.  Here's an example:

create function maxid () returns int4 as '
    declare
        mv int4;
    begin
        select max(id) into mv from mytab;
        if mv is null then return 0; end if;
        return mv;
    end;
' language 'plpgsql';

create table mytab (
    id int4 primary key default (maxid() + 1),
    data text
);

insert into mytab (data) values ('Hi there');
insert into mytab (data) values ('Howdy');
insert into mytab (data) values ('Guten Tag');
insert into mytab (data) values ('Terve');

select * from mytab;
 
 

BTW hackers, this is a common enough request, I wonder if there should be a built-in feature to make it easier for people to put this into the default value in the table definition i.e.:

create table mytab (
    keycolumn int4 primary key default coalesce(max(keycolumn),0)+1,
    data text
);

Wouldn't this eliminate the need for rollback-able sequences?
 

There is a similar discussion going on in the jonas users mailing list..
Your approach below does not appear to be thread safe. There is no
guarantee that the same value will not be selected twice on two concurrent
inserts.. Even more so since it sounds like he is going to be doing the
maxid() and the insert statements separately..

-Erik


At 09:35 AM 6/26/00 -0600, Kyle Bateman wrote:
>     
>If one has a unique-id generating sequence that sometimes is bound to 
>have holes in it (ie: it could happen that a nextval(seq) happens 
>without a corresponding INSERT in the table), then how could one 
>efficiently scan for these holes to recycle them in subsequent INSERTs? I'm
>just looking for a "standard" way of doing this if such a thing 
>exists. Louis-David Mitterrand - ldm@apartia.org - http://www.apartia.fr 
> 
>      Here's an example: create function maxid () returns int4 as ' 
>    declare 
>        mv int4; 
>    begin 
>        select max(id) into mv from mytab; 
>        if mv is null then return 0; end if; 
>        return mv; 
>    end; 
>' language 'plpgsql'; create table mytab ( 
>    id int4 primary key default (maxid() + 1), 
>    data text 
>); insert into mytab (data) values ('Hi there'); 
>insert into mytab (data) values ('Howdy'); 
>insert into mytab (data) values ('Guten Tag'); 
>insert into mytab (data) values ('Terve'); select * from mytab; 
>  
>  BTW hackers, this is a common enough request, I wonder if there should be
>a built-in feature to make it easier for people to put this into the
>default value in the table definition i.e.: create table mytab ( 
>    keycolumn int4 primary key default coalesce(max(keycolumn),0)+1, 
>    data text 
>); Wouldn't this eliminate the need for rollback-able sequences? 
>   Attachment Converted: "c:\program files\eudora lists\attach\kyle13.vcf"