Create an infrastructure for parallel computation in PostgreSQL.

This does four basic things.  First, it provides convenience routines
to coordinate the startup and shutdown of parallel workers.  Second,
it synchronizes various pieces of state (e.g. GUCs, combo CID
mappings, transaction snapshot) from the parallel group leader to the
worker processes.  Third, it prohibits various operations that would
result in unsafe changes to that state while parallelism is active.
Finally, it propagates events that would result in an ErrorResponse,
NoticeResponse, or NotifyResponse message being sent to the client
from the parallel workers back to the master, from which they can then
be sent on to the client.

Robert Haas, Amit Kapila, Noah Misch, Rushabh Lathia, Jeevan Chalke.
Suggestions and review from Andres Freund, Heikki Linnakangas, Noah
Misch, Simon Riggs, Euler Taveira, and Jim Nasby.

1 files changed, 7 insertions, 0 deletions

diff --git a/src/backend/access/transam/varsup.c b/src/backend/access/transam/varsup.c
index 42ee57fe8d7..cf3e964fc6e 100644
--- a/src/backend/access/transam/varsup.c
+++ b/src/backend/access/transam/varsup.c
@@ -50,6 +50,13 @@ GetNewTransactionId(bool isSubXact)
 	TransactionId xid;
 
 	/*
+	 * Workers synchronize transaction state at the beginning of each parallel
+	 * operation, so we can't account for new XIDs after that point.
+	 */
+	if (IsInParallelMode())
+		elog(ERROR, "cannot assign TransactionIds during a parallel operation");
+
+	/*
 	 * During bootstrap initialization, we return the special bootstrap
 	 * transaction id.
 	 */