Thread: Selecting rows with "static" ordering

Hello,

I have a strange problem (or one that I've never had before anyway). I 
am searching for a list of "id's" for a given table (these id values 
are generated at run-time and held statically in an application-local 
variable).
From that application, I want to retrieve all those rows, and I want 
them in the order they are currently stored in that variable. So take 
for example this foreign application variable:
  ids = "3,2,5,1,4"

The application then executes this sql:
  select * from table where id in (3,2,5,1,4)

As-is, of course, the above query will return the 5 records in a 
semi-random (i.e. unpredictable/unreliable) order. And I don't want to 
just "order by id" - I want to "order by id(3,2,5,1,4)" (if you see 
what I mean)

Is there a "neat trick" that anyone knows for pulling this off in a 
single query? Basically right now I'm issuing 5 queries to the backend 
to ensure ordering but this horribly inefficient.

Any input or advice would be appreciated,

Steve

The best I can think of off the top of my head would still be multiple SQL,
but at least it would be in one transaction block:

BEGIN;
SELECT '1' AS ordering, t1.* INTO TEMP TABLE work_table FROM t1 WHERE t1.id
= '3';
SELECT '2' AS ordering, t1.* INTO TEMP TABLE work_table FROM t1 WHERE t1.id
= '2';
SELECT '3' AS ordering, t1.* INTO TEMP TABLE work_table FROM t1 WHERE t1.id
= '5';
SELECT '4' AS ordering, t1.* INTO TEMP TABLE work_table FROM t1 WHERE t1.id
= '1';
SELECT '5' AS ordering, t1.* INTO TEMP TABLE work_table FROM t1 WHERE t1.id
= '4';
SELECT t1.* FROM work_table ORDER BY ordering;
COMMIT;

Something to that effect...? It's at least makes it only 1 connection from
the client to the database.


-----Original Message-----
From: pgsql-sql-owner@postgresql.org [mailto:pgsql-sql-owner@postgresql.org]
On Behalf Of Steve Midgley
Sent: Friday, 27 April 2007 10:01
To: pgsql-sql@postgresql.org
Subject: [SQL] Selecting rows with "static" ordering

Hello,

I have a strange problem (or one that I've never had before anyway). I
am searching for a list of "id's" for a given table (these id values
are generated at run-time and held statically in an application-local
variable).
From that application, I want to retrieve all those rows, and I want
them in the order they are currently stored in that variable. So take
for example this foreign application variable:
  ids = "3,2,5,1,4"

The application then executes this sql:
  select * from table where id in (3,2,5,1,4)

As-is, of course, the above query will return the 5 records in a
semi-random (i.e. unpredictable/unreliable) order. And I don't want to
just "order by id" - I want to "order by id(3,2,5,1,4)" (if you see
what I mean)

Is there a "neat trick" that anyone knows for pulling this off in a
single query? Basically right now I'm issuing 5 queries to the backend
to ensure ordering but this horribly inefficient.

Any input or advice would be appreciated,

Steve



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> them in the order they are currently stored in that variable. So take
> 
> for example this foreign application variable:
> 
>    ids = "3,2,5,1,4"


kludgy, but:

1. store your ids in a pg array
2. select from the array
3. on order by, write a function that takes the row.id and array as
parameters, returning the id offset into the array



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On 4/26/07, Steve Midgley <public@misuse.org> wrote:
select *
from table
where id in (3, 2, 5, 1, 4)
order by
    case when (id = 3) then 1
    when (id = 2) then 2
    when (id = 5) then 3
    when (id = 1) then 4
    when (id = 4) then 5
    else null
    end
;

--
==================================================================
   Aaron Bono
   Aranya Software Technologies, Inc.
   http://www.aranya.com
   http://codeelixir.com
==================================================================

On 27/04/07, Aaron Bono <postgresql@aranya.com> wrote:
> On 4/26/07, Steve Midgley <public@misuse.org> wrote:
> > So take
> > for example this foreign application variable:
> >
> >    ids = "3,2,5,1,4"
> >
> > The application then executes this sql:
> >
> >    select * from table where id in (3,2,5,1,4)
> >
> > As-is, of course, the above query will return the 5 records in a
> > semi-random (i.e. unpredictable/unreliable) order. And I don't want to
> > just "order by id" - I want to "order by id(3,2,5,1,4)" (if you see
> > what I mean)
> >
> >
>
> select *
> from table
> where id in (3, 2, 5, 1, 4)
> order by
>     case when (id = 3) then 1
>     when (id = 2) then 2
>     when (id = 5) then 3
>     when (id = 1) then 4
>     when (id = 4) then 5
>     else null
>      end
> ;
>

could you not use the values statment of psql 8.2 hmmm

select * from table, values ((1,3),(2,2),(3,5),(4,1),(5,4)) as values
where values.id=table.second order by values.first;

never done it but I think it should work with a bit of playing arond...

Peter.

At 8:01p -0400 on 26 Apr 2007, Steve Midgley wrote:
> From that application, I want to retrieve all those rows, and I  
> want them in the order they are currently stored in that variable.  
> So take for example this foreign application variable:
>
>   ids = "3,2,5,1,4"
>
> The application then executes this sql:
>
>   select * from table where id in (3,2,5,1,4)

> Is there a "neat trick" that anyone knows for pulling this off in a  
> single query? Basically right now I'm issuing 5 queries to the  
> backend to ensure ordering but this horribly inefficient.

Why not do this particular ordering on the application side?  With  
this example SQL, and assuming you've PRIMARY KEYed( id ), you're  
guaranteed to get a unique id for each tuple.  Presumably, you've got  
the order you want stored within your application, so just pull it  
out of a hash in the order you need it.

Or is it terribly more complicated than this?

Kevin