This is what I am doing. mysql> select * from new; +------+------+ | c1 | c2 | +------+------+ | 5 | 5 | | 10 | 10 | | 15 | 15 | | 20 | 20 | | 30 | 30 | +------+------+
Now, Session 1 mysql> begin; Query OK, 0 rows affected (0.00 sec) mysql> select * from new where c1 between 10 and 25 for update; +------+------+ | c1 | c2 | +------+------+ | 10 | 10 | | 15 | 15 | | 20 | 20 | +------+------+ 3 rows in set (0.00 sec) mysql>begin; mysql> insert into new values(29,29); (session is hanging) mysql> insert into new values(31,31); (session is hanging The last value on my table is 30. Still it is not allowing me to insert 31. I tried the scenario without index on column c1 and then with non-unique index on column c1 and then unique index. I am getting the same result. It seems that it is putting up lock on complete table. The scenario is working fine only when I made c1 primary key. After making c1 primary key, I am able to insert value higher than 30. Can you please try the same scenario at your end? Best Regards, Geetanjali Mehra Senior Oracle and MySQL DBA Corporate Trainer and Database Security Specialist On Tue, Sep 2, 2014 at 1:53 PM, Akshay Suryavanshi < akshay.suryavansh...@gmail.com> wrote: > Hello Geetanjali, > > Apologies if I have confused you with the "normal Select" notation. I > meant to write with repeatable-read mode in mind, but looks like that is > not an issue, since you already tested this scenario with that isolation > mode. > > Moving further to the original issue. > > Do you have an index on column c1. Is the query "Select * from new where > c1 between 12 and 17 for update;" using index ? If there is no index on > the particular column, then InnoDB locks out whole table from other > transactions. Which is the case you mentioned. Also this can be dangerous. > Once you have indexed the column checkout the innodb status, you will see > the necessary locking. Also try updating values beyond the boundary values. > > So most important fact to know here is the involvement of secondary > indexes to introduce record locking, gap locking, and how their absence > will affect the transaction. As to "Why" this is happening ? It should be > understood that in InnoDB secondary keys are appended to PRIMARY index, so > if there is no index to search the records PRIMARY index values cannot be > filtered. In absence of secondary indexes a full scan is needed. And > finally Innodb table is one big Index (Clustered table). > > *By default, InnoDB operates in REPEATABLE READ > <http://dev.mysql.com/doc/refman/5.6/en/set-transaction.html#isolevel_repeatable-read> > transaction > isolation level and with theinnodb_locks_unsafe_for_binlog > <http://dev.mysql.com/doc/refman/5.6/en/innodb-parameters.html#sysvar_innodb_locks_unsafe_for_binlog> > system > variable disabled. In this case, InnoDB uses next-key locks for searches > and index scans, which prevents phantom rows.* > > So Index scan above is infact a Full-table-scan (full index scan) > > Please try it out and let me know if you observe any difference. > > Cheers!!! > Akshay > > > On Tue, Sep 2, 2014 at 11:59 AM, geetanjali mehra < > mailtogeetanj...@gmail.com> wrote: > >> Dear Akshay, >> >> ASFIK, normal selects are always non-locking read and they do not put any >> locks. >> Select..., Select..........where......,Select ........where......between >> >> Does above select statement will use next-key locking and/or gap locking? >> I dont think so. >> >> Please correct me if I am wrong. >> >> >> Best Regards, >> Geetanjali Mehra >> Senior Oracle and MySQL DBA Corporate Trainer and Database Security >> Specialist >> >> >> >> On Fri, Aug 29, 2014 at 11:46 AM, Akshay Suryavanshi < >> akshay.suryavansh...@gmail.com> wrote: >> >> > Geetanjali, >> > >> > There is a difference between next-key locking, gap locking and locking >> > reads. >> > >> > Next-key locking and gap-locking are used with normal Selects statement >> in >> > Innodb, whereas locking reads wont release a lock on the whole column >> until >> > transaction completed, and not just selected values. >> > >> > May be you can try your example with SELECT... LOCK IN SHARE MODE; >> > >> > Cheers!!! >> > Akshay Suryawanshi >> > >> > >> > On Fri, Aug 29, 2014 at 11:22 AM, geetanjali mehra < >> > mailtogeetanj...@gmail.com> wrote: >> > >> >> Thanks for your reply. >> >> >> >> I read those docs. Still my doubt is at the same stage. Please clarify >> >> the >> >> same to me. >> >> Should not other sessions be allowed to insert the rows beyond that >> >> range.? >> >> >> >> As far as I understand, Innodb brought the concept of next-key locks >> so as >> >> to prevent phantom problem. So, it is clear to me that issuing the >> below >> >> query >> >> >> >> Select * from new where c1 between 12 and 17 for update; >> >> >> >> will not allow other sessions to insert any value between 12 and 17. >> >> >> >> But if i am trying to insert 20 from other session, it is not allowed. >> Why >> >> this is so? The session is hanging. >> >> >> >> Best Regards, >> >> Geetanjali Mehra >> >> Senior Oracle and MySQL DBA Corporate Trainer and Database Security >> >> Specialist >> >> >> >> >> >> >> >> On Thu, Aug 28, 2014 at 2:26 AM, shawn l.green < >> shawn.l.gr...@oracle.com> >> >> wrote: >> >> >> >> > >> >> > >> >> > On 8/26/2014 1:12 AM, geetanjali mehra wrote: >> >> > >> >> >> Hello to all, >> >> >> In repeatable read isolation level, when we issue: >> >> >> >> >> >> Select * from new where c1 between 12 and 17 for update; >> >> >> >> >> >> this range will be locked by innodb by using next-key locks. >> >> >> >> >> >> But, why is is preventing any other session to insert any value >> beyond >> >> >> that >> >> >> range; any value above the range and any value below the range. I am >> >> >> unable >> >> >> to understand this. >> >> >> >> >> > >> >> > I believe you are confusing gap locking (the space between the >> values) >> >> and >> >> > next-key locking (the space after the range). >> >> > >> >> > >> http://dev.mysql.com/doc/refman/5.6/en/innodb-record-level-locks.html >> >> > >> >> > See also: >> >> > http://dev.mysql.com/doc/refman/5.6/en/innodb-next-key-locking.html >> >> > http://dev.mysql.com/doc/refman/5.6/en/innodb-locks-set.html >> >> > >> >> > >> >> > >> >> >> Best Regards, >> >> >> Geetanjali Mehra >> >> >> Senior Oracle and MySQL DBA Corporate Trainer and Database Security >> >> >> Specialist >> >> >> >> >> >> >> >> > Yours, >> >> > -- >> >> > Shawn Green >> >> > MySQL Senior Principal Technical Support Engineer >> >> > Oracle USA, Inc. - Hardware and Software, Engineered to Work >> Together. >> >> > Office: Blountville, TN >> >> > >> >> > -- >> >> > MySQL General Mailing List >> >> > For list archives: http://lists.mysql.com/mysql >> >> > To unsubscribe: http://lists.mysql.com/mysql >> >> > >> >> > >> >> >> > >> > >> > >
