I need to select count of specific rows from table, then insert a new record with one field calculated using that value (something like auto_increment but for groups of records). Following queries doesn’t always work:

T1: BEGIN;
T2: BEGIN;

T1: SELECT COUNT(*) INTO @for_later_insert_t1 FROM table1 WHERE field1 = ‘not_existing_const’ AND field2 like ‘const_prefix_%’ FOR UPDATE; — (table1 has a key – field1, field2) selects 0.

T2: SELECT COUNT(*) INTO @for_later_insert_t2 FROM table1 WHERE field1 = ‘not_existing_const’ AND field2 like ‘const_prefix_%’ FOR UPDATE; — waits.

T1: INSERT INTO table1 SET field1 = ‘not_existing_const’, field2 = ‘const_prefix_1′, field3 = @for_later_insert_t1;
T1: COMMIT;

T2: — unblocks, @for_later_insert_t2 == 1
T2: INSERT INTO table1 SET field1 = ‘not_existing_const’, field2 = ‘const_prefix_1′, field3 = @for_later_insert_t2;
T2: COMMIT;

They doesn’t work:
* if field2 like ‘const_prefix_%’ is replaced with field2 = ‘const_prefix_1′ (SELECT … FOR UPDATE in that case isn’t locked, and insert causes deadlock).
* If the SELECT … FOR UPDATE in one transaction is using different key.

However those queries work if records with field1 = ‘not_existing_const’, field2 = ‘const_prefix_1′ already exist.
Why deadlock occurs if there are no such records?
Is it safe to retry the failed transaction? Or transaction is not meant to solve such problems and better use GET_LOCK()/RELEASE_LOCK()?

$this->db->query(‘SET AUTOCOMMIT=0′);

$this->db->query(“LOCK TABLES hotels READ, location WRITE”);

$this->db->query(“SELECT hotel_id FROM hotels WHERE hotel_id = 1 “);
$query = $this->db->get();

$this->db->query(“UPDATE location SET location_id = 5 WHERE location_id = 1″);

$this->db->query(“UNLOCK TABLES”);

i can’t access to my hotels table..in my SELECT statement..please help to resolve this one..

LOCK TABLES … IN [EXCLUSIVE|SHARE] MODE syntax does not exist in 5.1

the version im using now is 5.1 …what a mess im doing with my code…its not the code is the problem..my version…lol..:)

i’ve tried EXCLUSIVE and SHARED LOCK , and i found out..it wont work..i kinda get this error [#1064 - You have an error in your SQL syntax; check the manual that corresponds to your MySQL server version for the right syntax to use near 'IN SHARE MODE' at line 1]

my syntax is correct i use this one…LOCK TABLE hotels IN SHARE MODE and LOCK TABLE hotels IN EXCLUSIVE MODE…

..i test my code in xampp…try to query it.

and when i replace it with this one ..LOCK TABLE hotels WRITE..it works …also with READ..so strange….

i’ve read one of this article it says

so the transactional locks are converted into
standard READ|WRITE locks with a warning and then work as expected from the conventional
locks (or, if sql_mode is set to STRICT*, an attempt to acquire a transactional lock
returns an error).

heres the link if you want to see it..
http://bugs.mysql.com/bug.php?id=46663

i’ll now continue my coding ..i’ve understand it clearly now…:)…

if i will encounter a problem..i will post again here..:)…

hope it would be a success^_^..

SELECT … FOR UPDATE sets a XLOCK on the accessed record.
SELECT … LOCK IN SHARE MODE sets a SLOCK on the accessed record.

In both cases you will get serializability, but to improve
concurrency, one should only set a XLOCK if the transaction
actually updates/writes to the record.

To summarize: If you transactions are fast and no
user interaction happens in between, it is probably
best to quickly lock the entire table and do the
work. If transactions may be slow or user dialogs
are placed in between, you may be better of using
records locks. If using records locks, than innodb
knows about two kind of locks: Exclusive and shared.
Last time I checked, innodb does not support record
‘update’ locks (which I think is kind of flawed).

okay..i’ll just read more about lock compatibility…so it means to say ill be using shared/exclusive rather than write or read…

is it ok ill put the lock on the query for example this one

SELECT pep_id, pep_name FROM person WHERE id= $id LOCK IN SHARE MODE;

OR ill just do it like this

function InsertData {

$query = “LOCK TABLE person IN SHARE MODE”;

$query = INSERT INTO person(pep_id, pep_name) VALUES (’1′,’John’);
}

Thanks Again for the reply Darseq:)