…) in use but zero tables are locked. Innodb does not need any row locks for conventional selects it will just use MVCC to… happen concurrently. Lets now try same select but add LOCK IN SHARE MODE so it performs locking reads to validate our theory: —TRANSACTION… data select count(*) from sbtest,sbtest x lock in share mode TABLE LOCK table `sbtest`.`sbtest` trx id 12305 lock mode IS RECORD LOCKS space…
Post: How FLUSH TABLES WITH READ LOCK works with Innodb Tables
March 23, 2012 By 20 Comments
… Lock should not conflict with another Read Lock, it does for this statement, and as such it has to wait for any SELECT statement to complete in order to complete locking tables. This means if you have workload which includes some very long SELECT… and either kill FLUSH TABLES WITH READ LOCK and fail backup or kill long running SELECT queries to let backup to proceed…
Post: How much memory Innodb locks really take ?
July 13, 2006 By 11 Comments
… OK, 0 rows affected (0.00 sec) mysql> select count(i) from sample lock in share mode; +———-+ | count(i) | +———-+ | 1638400 | +———-+ 1 row… per locked row; Now lets see how much exclusinve locks take: mysql> begin; Query OK, 0 rows affected (0.00 sec) mysql> select… any locks is going to be faster than any of locking modes, however we can see locking overhead is not that large: mysql> select…
Post: Innodb performance gotcha w Larger queries.
August 28, 2009 By 7 Comments
… (trx->isolation_level select_lock_type != LOCK_NONE && trx->mysql_query_str) { /* Scan the MySQL query string; check if SELECT is the first word…->isolation_level select_lock_type != LOCK_NONE && trx->mysql_thd != NULL && thd_is_select(trx->mysql_thd)) { /* It is a plain locking SELECT and the isolation…
Post: How fast is FLUSH TABLES WITH READ LOCK?
April 24, 2010 By 30 Comments
… lock, anything else that currently holds the lock must finish what it’s doing. That means that every currently running query, including SELECT… | State | Info | +—-+——+———–+——+————+——+——————-+———————————————————————-+ | 4 | root | localhost | test | Query | 80 | Sending data | select count(*) from t t1 join t t2 join t t3…
Post: Index lock and adaptive search - next two biggest InnoDB problems
February 25, 2010 By 12 Comments
… run mixed read / write queries, InnoDB locks index for write operation and thus keeps all selects waiting when update/insert is done… ALL table and ALL indexes), so write query blocks ALL select queries. Usually first action is to disable adaptive_search (it…: I think some hashing algorithms may be applied to not lock select queries. We may look how to implement it. Until that…
Post: Innodb locking and Foreign Keys
December 12, 2006 By 2 Comments
… do not think about foreign keys overhead in terms of locking. The overhead about checking referenced table is usually considered but… do SELECT FOR UPDATE on CHILD table it will not lock associated rows in PARENT table and so you can run into lock… different tables and traditionally would not affect each other with locks now can because of foreign keys. So implementing foreign keys…
Comment: Are PHP persistent connections evil ?
November 14, 2008 By 32 Comments
Please help! in PHP mySQL, the use of SELECT FOR UPDATE and SELECT LOCK IN SHARE MODE, How to use it? They don… a web application, because when you first read the data (SELECT FOR UPDATE)the script ends once it sends the filled… auto mysql_close.(Which would rollback your transaction in the SELECT) With a persistent connection, may be it would keep the…
Comment: INSERT INTO ... SELECT Performance with Innodb tables.
July 7, 2010 By 22 Comments
… SELECT … FROM HighlyContentiousTableInInnoDb; See my stackoverflow question/answer for more info: http://stackoverflow.com/questions/2640898/how-to-improve-insert-into-select-locking…
Comment: SELECT LOCK IN SHARE MODE and FOR UPDATE
January 28, 2010 By 51 Comments
So the practical explaination is: SELECT … FOR UPDATE sets a XLOCK on the accessed record. SELECT … LOCK IN SHARE MODE sets a SLOCK…
