ORA-12518: TNS:listener could not hand off client connection

Database is Shared Server:

Shutdown the dispatcher and add new dispatcher.

SQL> show parameter dispatchers
SQL> select name from v$dispatcher;
SQL> alter system shutdown immediate 'D001';

Add new dispatcher:

SQL> alter system set DISPATCHERS = '(protocol=tcp)(dispatchers=4)';

Database is Dedicated Server:

PGA is fully allocated for all processes.Check PGA memory allocation.

SQL> SELECT * FROM V$PROCESS ORDER BY PGA_ALLOC_MEM DESC


Kill unnecessary Sessions to free PGA memory.

Or, increase PROCESSES parameter so that it can handle the needed number of processes.

SQL>show parameter processes

SQL>alter system set processes=450 scope=spfile;

Rebounce the database.