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