Postgres-XC 1.1 Documentation
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# 17.12. Lock Management

`deadlock_timeout` (`integer`)

  This is the amount of time, in milliseconds, to wait on a lock before checking to see if there is a deadlock condition. The check for deadlock is relatively expensive, so the server doesn't run it every time it waits for a lock. We optimistically assume that deadlocks are not common in production applications and just wait on the lock for a while before checking for a deadlock. Increasing this value reduces the amount of time wasted in needless deadlock checks, but slows down reporting of real deadlock errors. The default is one second (`1s`), which is probably about the smallest value you would want in practice. On a heavily loaded server you might want to raise it. Ideally the setting should exceed your typical transaction time, so as to improve the odds that a lock will be released before the waiter decides to check for deadlock. Only superusers can change this setting.

  When log_lock_waits is set, this parameter also determines the length of time to wait before a log message is issued about the lock wait. If you are trying to investigate locking delays you might want to set a shorter than normal `deadlock_timeout`.

  > Note: The following description applies only to Postgres-XC

  Postgres-XC does not detect global deadlocks where multiple node (Coordinators and/or Datanodes) are involved.

`max_locks_per_transaction` (`integer`)

  The shared lock table tracks locks on `max_locks_per_transaction` * (max_connections + max_prepared_transactions) objects (e.g., tables); hence, no more than this many distinct objects can be locked at any one time. This parameter controls the average number of object locks allocated for each transaction; individual transactions can lock more objects as long as the locks of all transactions fit in the lock table. This is not the number of rows that can be locked; that value is unlimited. The default, 64, has historically proven sufficient, but you might need to raise this value if you have clients that touch many different tables in a single transaction. This parameter can only be set at server start.

  Increasing this parameter might cause Postgres-XC to request more System V shared memory than your operating system's default configuration allows. See Section 16.4.1 for information on how to adjust those parameters, if necessary.

  When running a standby server, you must set this parameter to the same or higher value than on the master server. Otherwise, queries will not be allowed in the standby server.

  Even though your application does not issue `PREPARE TRANSACTION` explicitly, Coordinator may generate this command when an updating transaction involves more than one Coordinators and Datanodes. For Datanodes, you should specify this value as the same value as `max_connections`.

`max_pred_locks_per_transaction` (`integer`)

  The shared predicate lock table tracks locks on `max_pred_locks_per_transaction` * (max_connections + max_prepared_transactions) objects (e.g., tables); hence, no more than this many distinct objects can be locked at any one time. This parameter controls the average number of object locks allocated for each transaction; individual transactions can lock more objects as long as the locks of all transactions fit in the lock table. This is not the number of rows that can be locked; that value is unlimited. The default, 64, has generally been sufficient in testing, but you might need to raise this value if you have clients that touch many different tables in a single serializable transaction. This parameter can only be set at server start.

  Increasing this parameter might cause Postgres-XC to request more System V shared memory than your operating system's default configuration allows. See Section 16.4.1 for information on how to adjust those parameters, if necessary.

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