Re: UPDATE Query problem

On Thu, 17 Jan 2002, Josh Berkus wrote:

> Folks,
>
> I have a database that contains a chronological journal of activity.  For
>  various reasons, this journal contains both complete and incomplete records,
>  and while all records are timestamped, the primary key is not strictly ordered
>  by timestamp.
>
> What I want to do is update each incomplete record with the contents of the
>  last previous complete record.  As a simple-minded test case:
>
> CREATE TABLE history AS (
> history_id SERIAL PRIMARY KEY,
> period_date TIMESTAMP,
> fieldA VARCHAR(30),
> fieldB INT4 );
>
> CREATE VIEW complete_history_records AS
> SELECT history.*
> FROM history WHERE fieldA IS NOT NULL
>   and fieldB IS NOT NULL
>
> UPDATE history SET fieldA = chr.fieldA
>                    fieldB = chr.fieldB
> FROM (SELECT complete_history_records.*
>       WHERE ??? ) chr
> WHERE (history.fieldA IS NULL or
>           history.fieldB IS NULL);
>
> The problem is that I cannot figure out a subselect that will allow me to
>  select the last complete history record prior to the one being updated.  It
>  seems like I need to reference a field in the main query in the subselect,
>  which can't be done.
>
> To further hamper things, for portability reasons, I can use neither SELECT
>  DISTINCT ON nor custom functions.

Isn't update...from already an extension?

In any case, is performance really important? I think something like:
update history set fieldA=chr.fieldA, fieldB=chr.fieldB from
complete_history_records chr where (history.fieldA is null or
history.fieldB is null) and chr.period_date=(select max(period_date)
from complete_history_records where period_date<history.period_date);

might work if really slow.