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Re: tricky query

From: Cosimo Streppone <cosimo(at)streppone(dot)it>
To: John A Meinel <john(at)arbash-meinel(dot)com>
Cc: Postgresql Performance list <pgsql-performance(at)postgresql(dot)org>
Subject: Re: tricky query
Date: 2005-06-28 19:33:03
Message-ID: 42C1A5EF.7070505@streppone.it (view raw or flat)
Thread:
Lists: pgsql-performance
John A Meinel wrote:
> John A Meinel wrote:
>> Merlin Moncure wrote:
>>
>>> I need the smallest integer that is greater than zero that is not in the
>>> column of a table.  In other words, if an 'id' column has values
>>> 1,2,3,4,6 and 7, I need a query that returns the value of 5.
>>
 >> [...]
 >
> Well, I was able to improve it to using appropriate index scans.
> Here is the query:
> 
> SELECT t1.id+1 as id_new FROM id_test t1
>    WHERE NOT EXISTS
>        (SELECT t2.id FROM id_test t2 WHERE t2.id = t1.id+1)
>    ORDER BY t1.id LIMIT 1;

I'm very interested in this "tricky query".
Sorry John, but if I populate the `id_test' relation
with only 4 tuples with id values (10, 11, 12, 13),
the result of this query is:

   cosimo=> create table id_test (id integer primary key);
   NOTICE:  CREATE TABLE / PRIMARY KEY will create implicit index 'id_test_pkey' 
for table 'id_test'
   CREATE TABLE
   cosimo=> insert into id_test values (10); -- and 11, 12, 13, 14
   INSERT 7457570 1
   INSERT 7457571 1
   INSERT 7457572 1
   INSERT 7457573 1
   INSERT 7457574 1
   cosimo=> SELECT t1.id+1 as id_new FROM id_test t1 WHERE NOT EXISTS (SELECT 
t2.id FROM id_test t2 WHERE t2.id = t1.id+1) ORDER BY t1.id LIMIT 1;
    id_new
   --------
        15
   (1 row)

which if I understand correctly, is the wrong answer to the problem.
At this point, I'm starting to think I need some sleep... :-)

-- 
Cosimo


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