SELECT Question

[Alex]

Hi,
I have a column with a sequence in a table and want to allocate the
first 200 for special purpose thus starting the sequence from 200. For
the first 200 I have entries that will be added and removed.

Is there an easy way to write a select statement that returns me the
frist free number or any within the range of 200?
For example if 1-30, and 32-50 are occupied then i would like to fill in
the new entry with id 31.
I currently do it with a function but I was just wondering if there is a
way without it..

Thanks
Alex

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[Tom Lane]

Joe Conway <ma**@joeconway .com> writes:

Kris Jurka wrote:

A useful generic function would be one something like range(min,max) that
would return a set of rows so you wouldn't have to actually have a table.

You mean like this? CREATE OR REPLACE FUNCTION test(int,int) RETURNS SETOF int AS '
BEGIN
FOR i IN $1..$2 LOOP
RETURN NEXT i;
END LOOP;
RETURN;
END;
' LANGUAGE 'plpgsql' STRICT IMMUTABLE;

I was thinking of proposing that we provide something just about like
that as a standard function (written in C, not in plpgsql, so that it
would be available whether or not you'd installed plpgsql). There are
some places in the information_sch ema that desperately need it ---
right now, the value of FUNC_MAX_ARGS is effectively hard-wired into
some of the information_sch ema views, which means they are broken if
one changes that #define. We could fix this if we had a function like
the above and exported FUNC_MAX_ARGS as a read-only GUC variable.

regards, tom lane

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[Joe Conway]

Tom Lane wrote:

Joe Conway <ma**@joeconway .com> writes:

CREATE OR REPLACE FUNCTION test(int,int) RETURNS SETOF int AS '
BEGIN
FOR i IN $1..$2 LOOP
RETURN NEXT i;
END LOOP;
RETURN;
END;
' LANGUAGE 'plpgsql' STRICT IMMUTABLE;

I was thinking of proposing that we provide something just about like
that as a standard function (written in C, not in plpgsql, so that it
would be available whether or not you'd installed plpgsql). There are
some places in the information_sch ema that desperately need it ---
right now, the value of FUNC_MAX_ARGS is effectively hard-wired into
some of the information_sch ema views, which means they are broken if
one changes that #define. We could fix this if we had a function like
the above and exported FUNC_MAX_ARGS as a read-only GUC variable.

I've been really busy on other-than-postgres stuff lately, but I'm
planning to carve out time next week to start doing some 7.5
development. I'll take this one if you want.

Joe
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[Alex]

All,
thanks for the many suggestions
Alex

Manfred Koizar wrote:

On Thu, 20 Nov 2003 16:52:37 +0900, Alex <al**@meerkatso ft.com> wrote:

>Is there an easy way to write a select statement that returns me the
>frist free number or any within the range of 200?
>For example if 1-30, and 32-50 are occupied then i would like to fill in
>the new entry with id 31.
>
>

Fortunately this is not the performance mailing list :-)

First free number:
SELECT max(t1.id) + 1
FROM t AS t1 INNER JOIN t AS t2
ON (t1.id < 200 AND t1.id < t2.id AND t2.id <= 200)
GROUP BY t2.id
HAVING max(t1.id) + 1 < t2.id
ORDER BY t2.id
LIMIT 1;

Make sure that there is always a row with id=0 and a row with id=200.

Any free number:
SELECT id - 1
FROM t
WHERE 1 < id AND id <= 200
AND NOT EXISTS (SELECT * FROM t AS t2 WHERE t2.id = t.id - 1)
LIMIT 1;

Always having a row with id=200 helps avoid unwanted corner cases.

One more:
SELECT coalesce(max(id ), 0) + 1
FROM t
WHERE id <= 200
AND id = (SELECT count(*) FROM t AS t2 WHERE t2.id <= t.id);

This should work without any dummy rows. And it will not work, if id
is not unique or there is any row with id < 1.

Servus
Manfred

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