Re: Showing progress of a database function

Would it be a solution for your problem to use LIMIT and OFFSET and make X
iterations?

A Dimarts 01 Març 2005 14:55, Tambet Matiisen va escriure:
> Hi everybody!
>
> When designing database-centric applications it is always good idea to
> push as much logic as possible to server side. The logic can be
> implemented as check constraints, triggers and functions. The problem
> is, that for lengthy function calls it will be impossible to see the
> progress and to cancel the operation, unless your client library
> supports asynchronous queries. Qt for example doesn't, like many others.
>
> My idea was to use cursor as progress indicator for the function.
> Simplified example:
>
> create or replace function process_big_table() returns refcursor
> language plpgsql as
> declare
>   cur as refcursor;
> begin;
>   open cur for
>     select process_row(id)
>     from big_table
>     where special_condition
>     order by special_order;
>   return cur;
> end;
>
> As you see, there is another function process_row(), which does the
> actual processing. This function has to be called for certain rows in
> big table, in certain order. In client code I do something like this:
>
> begin;
> select process_big_table(); -- returns the name of the cursor
> loop
>    fetch next from <cursor name>;
>    <update progress counter>
>    <if cancel was pressed do rollback and break>
> end loop
> commit;
>
> I expected it to call process_row() every time I fetch a row. But
> actually it processes all of them on first fetch and all following rows
> are returned in constant time. Is this a normal way of operation? Or
> does it depend on query? Are the query results always precalculated at
> the server side and the save from using cursors is just not to download
> all rows to client?
>
> If anyone has other ideas how to show progress of a database function,
> then you are welcome to share. I have considered using notice
> processing, but this requires access to libpq objects such as PGconn,
> which can be tricky with Qt (altough possible, as Qt uses libpq
> internally). And to be able to cancel requests I still need asynchronous
> query API, which is missing in Qt.
>
>   Tambet
>
> ---------------------------(end of broadcast)---------------------------
> TIP 7: don't forget to increase your free space map settings

--
Albert Cervera Areny
Dept. Informàtica Sedifa, S.L.

Av. Can Bordoll, 149
08202 - Sabadell (Barcelona)
Tel. 93 715 51 11
Fax. 93 715 51 12

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