Hi Alex! On Sat, 2004-01-17 at 05:50, Zeltser, Alex wrote: > Hi, > > I wanted to take advantage of the InnoDB 'gap' locking to lock 'non-existence' of a > row, the way the > manual recommends. I tried to do this by using 'select ... for update', using the > 'mysql' client > from two separate sessions as shown below: > > Session 1: > > set AUTOCOMMIT=0; > > begin; > > select * from T where A = 'NOT_THERE' for update; > > Session 2: > > set AUTOCOMMIT=0; > > begin; > > select * from T where A = 'NOT_THERE' for update; > > What I'd hoped to see was the 'select' statement in Session 2 block until either a > commit or a > rollback was performed in Session 1. Unfortunately, it didn't work that way. The > 'select's in both > sessions returned right away, and it was only the subsequent 'insert's, 'update's > and 'delete's that > blocked. I can understand the rationale behind this behavior, but unfortunately it > doesn't help me > with my problem. I'd like to be able to reliably check for existence of a record > from two > concurrent sessions and have the 'select' in the 'second' session block until the > first session is > either committed or rolled back. Is there a way to accomplish this somehow? > InnoDB's next-key locking is a bit different to this - it ensures that phantom rows do not appear. This is good for application writers and for MySQL itself, as phantom rows appearing would break MySQL's replication.
Basically, InnoDB will place locks on the various index structures involved in your query around the rows that have been returned bt a SELECT ... FOR UPDATE. As your selects return nothing, InnoDB doesn't find any index sections to place any locks on. Perhaps you should look at using the SERIALIZABLE level of transaction isolation. Regards, Chris > I know I can just try to insert the record and check for duplicates, but is there a > way to > accomplish it with 'select's? > > Thanks in advance, > > Alex Zeltser > > -- > MySQL General Mailing List > For list archives: http://lists.mysql.com/mysql > To unsubscribe: http://lists.mysql.com/[EMAIL PROTECTED] > -- MySQL General Mailing List For list archives: http://lists.mysql.com/mysql To unsubscribe: http://lists.mysql.com/[EMAIL PROTECTED]
