Re: Updating a specific number of rows in pl/pgsql

2009/8/11 Peter Headland <pheadland(at)actuate(dot)com>:
>> there are one fast trick
>> http://www.postgres.cz/index.php/PostgreSQL_SQL_Tricks#Fast_first_n_rows_removing
>
> Thanks - that's a very useful page!
>
> Unfortunately, there is no single column that provides a unique id, and I am reluctant to add one (for example, using a sequence and a new index) for performance reasons.

ctid is unique system column in every table.

postgres=# create table x(a int);
CREATE TABLE
Time: 655,062 ms
postgres=# insert into x values(10);
INSERT 0 1
Time: 49,237 ms
postgres=# insert into x values(10);
INSERT 0 1
Time: 1,740 ms
postgres=# select ctid, a from x;
ctid | a
-------+----
(0,1) | 10
(0,2) | 10
(2 rows)

>
> Given that additional constraint, is my original plan using a loop to iterate over a cursor reasonable? I don't anticipate p_number_of_items being more than 20.

why not? for small number of iteration is loop over cursor good solution.

Pavel Stehule

>
> --
> Peter Headland
> Architect
> Actuate Corporation
>
>
> -----Original Message-----
> From: Pavel Stehule [mailto:pavel(dot)stehule(at)gmail(dot)com]
> Sent: Tuesday, August 11, 2009 03:55
> To: D'Arcy J.M. Cain
> Cc: Peter Headland; pgsql-sql(at)postgresql(dot)org
> Subject: Re: [SQL] Updating a specific number of rows in pl/pgsql
>
> 2009/8/11 D'Arcy J.M. Cain <darcy(at)druid(dot)net>:
>> On Mon, 10 Aug 2009 17:52:36 -0700
>> "Peter Headland" <pheadland(at)actuate(dot)com> wrote:
>>> I can get the rows I want to update like this:
>>>
>>>   SELECT *
>>>    FROM queue
>>>    WHERE id = p_queue_id
>>>    ORDER BY rank
>>>    LIMIT p_number_of_items;
>>>
>>> Of course, there may not be p_number_of_items available in the queue.
>>>
>>> I want to update all the rows in the cursor in the same way:
>>>
>>>   UPDATE queue SET assigned = TRUE;
>>
>> Assuming that there is a unique identifier on queue, let's call it
>> queue_id, you should be able to do something like this:
>>
>>  UPDATE queue SET assigned = TRUE
>>  WHERE queue_id IN (SELECT queue_id
>>    FROM queue
>>    WHERE id = p_queue_id
>>    ORDER BY rank
>>    LIMIT p_number_of_items);
>>
>
> there are one fast trick
> http://www.postgres.cz/index.php/PostgreSQL_SQL_Tricks#Fast_first_n_rows_removing
>
> p.s. replace DELETE by UPDATE
> regards
> Pavel Stehule
>
>> --
>> D'Arcy J.M. Cain <darcy(at)druid(dot)net>         |  Democracy is three wolves
>> http://www.druid.net/darcy/                |  and a sheep voting on
>> +1 416 425 1212     (DoD#0082)    (eNTP)   |  what's for dinner.
>>
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>