OMMAND"
>VACUUM</TT
>).  Since these lock types do not conflict, the access
   method is responsible for handling any fine-grained locking it might need.
   An exclusive lock on the index as a whole will be taken only during index
   creation, destruction, or <TT
CLASS="COMMAND"
>REINDEX</TT
>.
  </P
><P
>   Building an index type that supports concurrent updates usually requires
   extensive and subtle analysis of the required behavior.  For the b-tree
   and hash index types, you can read about the design