ORA-12801: error signaled in parallel query server Pnnn

What does it mean?

In a query running in parallel, one of the parallel execution servers (parallel query slaves) hit an error, and this error was passed to the query coordinator. ORA-12801 is a wrapper; the actual error is on the second line following it.

Why does it happen?

A slave process hit a real error during a parallel operation: insufficient TEMP tablespace (ORA-01652), PGA memory exhaustion (ORA-04030), data corruption, parallel process limits or another underlying SQL error. ORA-12801 only reports that it occurred in a parallel context.

How to fix it

Focus on the actual error code immediately following ORA-12801 and resolve it (e.g. enlarging TEMP, PGA tuning, repairing a corrupt block). If needed, isolate the actual error by running the query serially, removing PARALLEL hints or lowering the degree of parallelism. Review PARALLEL_MAX_SERVERS and resource usage.