5cc8a8f7fd
* SKIP LOCKED clause for SELECT WITH LOCK, UPDATE and DELETE. * Misc. * Change as Dmitry suggested and fix Windows build. * Change order of LOCK to between SKIP and FIRST. * Avoid refetch with READ COMMITTED transaction as suggested by Dmitry. * Revert changes to TRA_wait and use tra_probe in vio/prepare_update when skipping locked. * Add to CHANGELOG.
1.9 KiB
SKIP LOCKED clause (FB 5.0)
SKIP LOCKED clause can be used with SELECT ... WITH LOCK, UPDATE and DELETE statements.
It makes engine skip records locked by others transactions instead of wait on them or raise conflict errors.
This is very useful to implement work queues where one or more processes post work to a table and issue
an event while workers listen for events and read/delete items from the table. Using SKIP LOCKED multiple
workers can get exclusive work items from the table without conflicts.
Syntax
SELECT
[FIRST ...]
[SKIP ...]
FROM <sometable>
[WHERE ...]
[PLAN ...]
[ORDER BY ...]
[{ ROWS ... } | {OFFSET ...} | {FETCH ...}]
[FOR UPDATE [OF ...]]
[WITH LOCK [SKIP LOCKED]]
UPDATE <sometable>
SET ...
[WHERE ...]
[PLAN ...]
[ORDER BY ...]
[ROWS ...]
[SKIP LOCKED]
[RETURNING ...]
DELETE FROM <sometable>
[WHERE ...]
[PLAN ...]
[ORDER BY ...]
[ROWS ...]
[SKIP LOCKED]
[RETURNING ...]
Notes
As it happens with subclauses FIRST/SKIP/ROWS/OFFSET/FETCH record lock
(and "skip locked" check) is done in between of skip (SKIP/ROWS/OFFSET/FETCH) and
limit (FIRST/ROWS/OFFSET/FETCH) checks.
Examples
Prepare metadata
create table emails_queue (
subject varchar(60) not null,
text blob sub_type text not null
);
set term !;
create trigger emails_queue_ins after insert on emails_queue
as
begin
post_event('EMAILS_QUEUE');
end!
set term ;!
Sender application or routine
insert into emails_queue (subject, text)
values ('E-mail subject', 'E-mail text...');
commit;
Client application
Client application can listen to event EMAILS_QUEUE to actually send e-mails using this query:
delete from emails_queue
rows 10
skip locked
returning subject, text;
More than one instance of the application may be running, for example to load balance work.