Oracle: Latch

The purpose of a latch is to protect shared resources that are needed, accessed and modified by multiple sessions and/or processes from corruption which might occur if one session/process writes to a memory structure while another session/process reads it and thus obtains incosistent (garbage) data.

Such corruption can be prevented by allowing at most one process to «possess» a latch.

When a process wants to read or write a shared resource, it first tries to aquire the associated latch. If another process already has that latch, the process has to wait until the latch becomes available.

Latches don't have a queue: an available latch is assigned to the first process that tries to acquire it, irrespective how many times that process already tried to acquire this latch.

This behavior is different from enqueue latches (such as row latches) which do have such a queue.

Statistics about latches (such as number of tried acquisitions, time held etc.) are recorded in v$latch, v$latch_children and v$latch_parent.

select
   l.name,
   case when exists (
      select 1 from v$latch_children c
      where
         c.latch# = l.latch#
   ) then 'X' end chld
from
   v$latch l
order by
   name;