Re: tricky query

John A Meinel wrote:

> Merlin Moncure wrote:
>
>> I need a fast way (sql only preferred) to solve the following problem:
>>
>> 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.
>>
>> I've already worked out a query using generate_series (not scalable) and
>> pl/pgsql. An SQL only solution would be preferred, am I missing
>> something obvious?
>>
>> Merlin
>>
>>
>
> Not so bad. Try something like this:
>
> SELECT min(id+1) as id_new FROM table
> WHERE (id+1) NOT IN (SELECT id FROM table);
>
> Now, this requires probably a sequential scan, but I'm not sure how you
> can get around that.
> Maybe if you got trickier and did some ordering and limits. The above
> seems to give the right answer, though.
>
> I don't know how big you want to scale to.
>
> You might try something like:
> SELECT id+1 as id_new FROM t
> WHERE (id+1) NOT IN (SELECT id FROM t)
> ORDER BY id LIMIT 1;
>
> John
> =:->

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 created a test table which has 90k randomly inserted rows. And this is
what EXPLAIN ANALYZE says:

QUERY PLAN

----------------------------------------------------------------------------------------------------------------------------------------
Limit (cost=0.00..12.10 rows=1 width=4) (actual time=0.000..0.000 rows=1 loops=1)
-> Index Scan using id_test_pkey on id_test t1 (cost=0.00..544423.27 rows=45000 width=4) (actual time=0.000..0.000 rows=1 loops=1)
Filter: (NOT (subplan))
SubPlan
-> Index Scan using id_test_pkey on id_test t2 (cost=0.00..6.01 rows=1 width=4) (actual time=0.000..0.000 rows=1 loops=15)
Index Cond: (id = ($0 + 1))
Total runtime: 0.000 ms
(7 rows)

The only thing I have is a primary key index on id_test(id);

John
=:->