Re: [HACKERS] Predicate Locks for writes?
| От | Alexander Korotkov |
|---|---|
| Тема | Re: [HACKERS] Predicate Locks for writes? |
| Дата | |
| Msg-id | CAPpHfds_hu1KY3m+8Az7oyECU8FETxSuk-FQByZ7LxWMsu71tg@mail.gmail.com обсуждение исходный текст |
| Ответ на | [HACKERS] Predicate Locks for writes? (Simon Riggs <simon@2ndquadrant.com>) |
| Список | pgsql-hackers |
On Sat, Oct 7, 2017 at 2:26 PM, Simon Riggs <simon@2ndquadrant.com> wrote:
SERIALIZABLE looks for chains of rw cases.
When we perform UPDATEs and DELETEs we search for rows and then modify
them. The current implementation views that as a read followed by a
write because we issue PredicateLockTuple() during the index fetch.
Is it correct that a statement that only changes data will add
predicate locks for the rows that it modifies?
I'm not very aware of how this SI code exactly works. But it seems that once we update row, read SIReadLock on that tuple is released, because we're already holding stronger lock. I made simple experiment.
# begin;
BEGIN
Time: 0,456 ms
smagen@postgres=*# select * from test where id = 1;
id │ val
────┼─────
1 │ xyz
(1 row)
*# select locktype, database, relation, page, tuple, mode, granted from pg_locks where pid = pg_backend_pid();
locktype │ database │ relation │ page │ tuple │ mode │ granted
────────────┼──────────┼──────────┼──────┼───────┼─────────────────┼─────────
relation │ 12558 │ 11577 │ NULL │ NULL │ AccessShareLock │ t
relation │ 12558 │ 65545 │ NULL │ NULL │ AccessShareLock │ t
relation │ 12558 │ 65538 │ NULL │ NULL │ AccessShareLock │ t
virtualxid │ NULL │ NULL │ NULL │ NULL │ ExclusiveLock │ t
page │ 12558 │ 65545 │ 1 │ NULL │ SIReadLock │ t
tuple │ 12558 │ 65538 │ 0 │ 3 │ SIReadLock │ t
(6 rows)
*# update test set val = 'xyz' where id = 1;
*# select locktype, database, relation, page, tuple, mode, granted from pg_locks where pid = pg_backend_pid();
locktype │ database │ relation │ page │ tuple │ mode │ granted
───────────────┼──────────┼──────────┼──────┼───────┼──────────────────┼─────────
relation │ 12558 │ 11577 │ NULL │ NULL │ AccessShareLock │ t
relation │ 12558 │ 65545 │ NULL │ NULL │ AccessShareLock │ t
relation │ 12558 │ 65545 │ NULL │ NULL │ RowExclusiveLock │ t
relation │ 12558 │ 65538 │ NULL │ NULL │ AccessShareLock │ t
relation │ 12558 │ 65538 │ NULL │ NULL │ RowExclusiveLock │ t
virtualxid │ NULL │ NULL │ NULL │ NULL │ ExclusiveLock │ t
transactionid │ NULL │ NULL │ NULL │ NULL │ ExclusiveLock │ t
page │ 12558 │ 65545 │ 1 │ NULL │ SIReadLock │ t
(8 rows)
Once we update the same tuple that we read before, SIReadLock on that tuple disappears.
PredicateLockTuple() specifically avoids adding an SIRead lock if the
tuple already has a write lock on it, so surely it must also be
correct to skip the SIRead lock if we are just about to update the
row?
I am suggesting that we ignore PredicateLockTuple() for cases where we
are about to update or delete the found tuple.
If my thoughts above are correct, than it would be a reasonable optimization. We would skip cycle of getting SIReadLock on tuple and then releasing it, without any change of behavior.
ISTM that a Before Row Trigger would need to add an SIRead lock since
that is clearly a read.
Yes, because depending on result of "Before Row Trigger" update might not really happen. That SIReadLock on tuple is necessary.
However, ISTM that we could place SIReadLock on tuple after Before Row Trigger and only in the case when trigger has cancelled an update.
Alexander Korotkov
Postgres Professional: http://www.postgrespro.com
The Russian Postgres Company
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