Re: Mystery lock being applied
} Dave Voelker wrote:
} >
} > We have jsut released a PowerBuilder application which user Informix
} > 7.01 as the back-end and I have noticed that occassionally, rather
} > infrequently, a table-level lock will get applied by the
application.
} > When we connect our isolation level is set to Committed Read and we
also
} > issue a SET LOCK MODE WAIT 10 immediately after connection. Nowhere
in
} > the application do we alter this locking strategy in code.
} >
} > I am wondering what can cause a table-level lock to get applied
given
} > the previous instructions. If I knew the potential situations under
} > which this could possibly occur it would go a long way to helping us
} > identify when and where this happens.
}
} Table level locks occurr whenever you
}
} + create / drop / alter a table - eXclusive
} + enter: LOCK TABLE IN Share/eXlusive MODE
} + unload your table using "onunload" or "onpload (express mode)"
} ( X locks )
}
} The special "Intent" locks occurr always:
}
} + whenever you lock a row or page - locks like "IS, IX" ( oh, I forgot
} what happens during an Update Lock, I think it was "SIX" or s.th.
} like that.
}
} Because you are using "COMMITTED READ" - a level that does not really
} set Shared Locks, you will rarely see "IS" or "S" locks. Because you
} are wondering about locks, here's just an additional info:
} COMMITTED READ uses BUFFERED CURSORS and whenever your application
} is fetching a row, this will be read from your cursor buffer. Since
} the cursor of the server is running ahead, you might read dirty data.
} But in the past, the data was committed.
}
}
} PS: Any comments, did I forget something ?
}
I believe table locks may also occur when using repeatable read
if there is no index available for key-value locking. Right?
Stephen Campbell
}
} Bye
}
} Stefan
}