this value is specified without units, it is taken as kilobytes.
        The default value is four megabytes (<code class="literal">4MB</code>).
        Note that a complex query might perform several sort and hash
        operations at the same time, with each operation generally being
        allowed to use as much memory as this value specifies before
        it starts
        to write data into temporary files.  Also, several running
        sessions could be doing such operations concurrently.
        