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IDS Forum

Re: split update into multiple transactions

Posted By: Marcus Haarmann
Date: Wednesday, 31 May 2017, at 4:35 p.m.

In Response To: split update into multiple transactions (Roderick Schertler)

take a look at tx_split, it is in the iiug code repository.
http://members.iiug.org/software/archive/tx_split.html
ftp://ftp.iiug.org/pub/informix/pub/tx_split.tar.gz

You will need esql/c for this.

Marcus Haarmann

Von: "Roderick Schertler" <roderick@argon.org>
An: "ids" <ids@iiug.org>
Gesendet: Mittwoch, 31. Mai 2017 20:02:38
Betreff: split update into multiple transactions [39306]

Hi folks.

I'm trying to split a simple update into multiple transactions, to
avoid holding arbitrarily many locks. Locking the table (even in
shared mode) is not an option.

I need to be able to skip rows which are currently locked, they'll be
updated by another run later. Using "lock mode wait" also isn't an
option.

Additionally, it would be good if I could do the update "where current
of" a cursor rather than relying on the table's key or rowid. This is
where I run into trouble.

I first tried to do this using SPL. In order to use "where current of"
you have to use a foreach loop. When I use "on exception" to deal with
the locked rows I can't get it to continue to the next iteration of the
foreach loop, instead it exits the foreach. See sample below.

I next tried to do this using 4GL. This fails because an error encountered
doing a fetch of the locked row keeps the cursor on that locked row -- the
next fetch tries to fetch the same row and fails again. I can't figure out
a way to get it to move past the locked row. A sample of this is below
too.

Thanks for any help.

Roderick

PS: Is there any way to post code samples so that they won't be
left-justified by the mailing list?

-------------------------------------------------------------------------------

These samples assume a database "db" with a table "tab" which has a column
"col int", and one of the rows is locked by a different session/process.

I tested this with IDS 11.70.FC8GE, 4GL 7.51.FC1XD, and sqlcmd 88.00 on
RHEL 5.11.

-- SPL sample
-----------------------------------------------------------------

-- This fails because an error caught doing the foreach fetch (the

-- locked row) resumes after the foreach (exiting it) rather than with

-- the next iteration of the foreach.

set debug file to '/tmp/t.debug';

drop procedure if exists update_test;

create procedure update_test();

define i like tab.col;

begin work;

trace 'before block';

begin

on exception

trace 'caught exception outside foreach';

end exception with resume;

trace 'before foreach';

foreach update_c for

select col into i from tab

on exception

trace 'caught exception in foreach';

continue foreach;

end exception with resume;

update tab set col = 23 where current of update_c;

trace 'inside foreach';

end foreach;

trace 'after foreach';

end;

trace 'after block';

rollback work;

end procedure;

execute procedure update_test();

drop procedure update_test;

-- SPL results
----------------------------------------------------------------

If it's the 3rd row in the table which is locked the debug file ends up
like this:

trace expression :before block

trace expression :before foreach

trace expression :inside foreach

trace expression :inside foreach

trace expression :caught exception outside foreach

trace expression :after foreach

trace expression :after block

-- 4GL sample
-----------------------------------------------------------------

database db

main

define s like tab.col

declare select_c cursor for

select col from tab for update

begin work

open select_c

while TRUE

whenever error continue

fetch select_c into s

whenever error stop

case

when sqlca.sqlcode = NOTFOUND exit while

when sqlca.sqlcode != 0 display sqlca.sqlcode

otherwise display "[", s, "]"

end case

end while

rollback work

end main

-- 4GL results
----------------------------------------------------------------

This outputs the rows before the lock then loops outputting "-244"
repeatedly. If you release the lock in the other window it continues
through the rest of the rows in the table.

-------------------------------------------------------------------------------

*******************************************************************************
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