  Creating an index on a large table can take a long time.  By default,
   <SPAN
CLASS="PRODUCTNAME"
>PostgreSQL</SPAN
> allows reads (<TT
CLASS="COMMAND"
>SELECT</TT
> statements) to occur
   on the table in parallel with index creation, but writes (<TT
CLASS="COMMAND"
>INSERT</TT
>,
   <TT
CLASS="COMMAND"
>UPDATE</TT
>, <TT
CLASS="COMMAND"
>DELETE</TT
>) are blocked until the index build is finished.
   In production environments this is often unacceptable.
   It is possible to allow writes to occur in pa