db-control gather-stats command fails with "ORA-00018: maximum number of sessions exceeded"

Issue

• db-control gather-stats command fails with "ORA-00018: maximum number of sessions exceeded"

$ db-control gather-stats 
Gathering statistics...
WARNING: this may be a very slow process.
DBD::Oracle::db do failed: ORA-12801: error signaled in parallel query server P083
ORA-00018: maximum number of sessions exceeded
ORA-06512: at "SYS.DBMS_STATS", line 13591
ORA-06512: at "SYS.DBMS_STATS", line 13937
ORA-06512: at "SYS.DBMS_STATS", line 14015
ORA-06512: at "SYS.DBMS_STATS", line 13974
ORA-06512: at line 1 (DBD ERROR: OCIStmtExecute) [for Statement "begin dbms_stats.gather_schema_stats(NULL, ESTIMATE_PERCENT=> 15, DEGREE=>DBMS_STATS.DEFAULT_DEGREE, CASCADE=>TRUE); end;"] at /usr/lib/perl5/vendor_perl/5.8.8/Dobby/DB.pm line 180.