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, 27 insertions, 3 deletions

diff --git a/src/backend/tcop/utility.c b/src/backend/tcop/utility.c
index 31e9d4cf8b4..59f09dc93af 100644
--- a/src/backend/tcop/utility.c
+++ b/src/backend/tcop/utility.c
@@ -128,14 +128,15 @@ CommandIsReadOnly(Node *parsetree)
 static void
 check_xact_readonly(Node *parsetree)
 {
-	if (!XactReadOnly)
+	/* Only perform the check if we have a reason to do so. */
+	if (!XactReadOnly && !IsInParallelMode())
 		return;
 
 	/*
 	 * Note: Commands that need to do more complicated checking are handled
 	 * elsewhere, in particular COPY and plannable statements do their own
-	 * checking.  However they should all call PreventCommandIfReadOnly to
-	 * actually throw the error.
+	 * checking.  However they should all call PreventCommandIfReadOnly
+	 * or PreventCommandIfParallelMode to actually throw the error.
 	 */
 
 	switch (nodeTag(parsetree))
@@ -208,6 +209,7 @@ check_xact_readonly(Node *parsetree)
 		case T_ImportForeignSchemaStmt:
 		case T_SecLabelStmt:
 			PreventCommandIfReadOnly(CreateCommandTag(parsetree));
+			PreventCommandIfParallelMode(CreateCommandTag(parsetree));
 			break;
 		default:
 			/* do nothing */
@@ -233,6 +235,24 @@ PreventCommandIfReadOnly(const char *cmdname)
 }
 
 /*
+ * PreventCommandIfParallelMode: throw error if current (sub)transaction is
+ * in parallel mode.
+ *
+ * This is useful mainly to ensure consistency of the error message wording;
+ * most callers have checked IsInParallelMode() for themselves.
+ */
+void
+PreventCommandIfParallelMode(const char *cmdname)
+{
+	if (IsInParallelMode())
+		ereport(ERROR,
+				(errcode(ERRCODE_INVALID_TRANSACTION_STATE),
+		/* translator: %s is name of a SQL command, eg CREATE */
+				 errmsg("cannot execute %s during a parallel operation",
+						cmdname)));
+}
+
+/*
  * PreventCommandDuringRecovery: throw error if RecoveryInProgress
  *
  * The majority of operations that are unsafe in a Hot Standby slave
@@ -618,6 +638,7 @@ standard_ProcessUtility(Node *parsetree,
 		case T_ClusterStmt:
 			/* we choose to allow this during "read only" transactions */
 			PreventCommandDuringRecovery("CLUSTER");
+			/* forbidden in parallel mode due to CommandIsReadOnly */
 			cluster((ClusterStmt *) parsetree, isTopLevel);
 			break;
 
@@ -628,6 +649,7 @@ standard_ProcessUtility(Node *parsetree,
 				/* we choose to allow this during "read only" transactions */
 				PreventCommandDuringRecovery((stmt->options & VACOPT_VACUUM) ?
 											 "VACUUM" : "ANALYZE");
+				/* forbidden in parallel mode due to CommandIsReadOnly */
 				ExecVacuum(stmt, isTopLevel);
 			}
 			break;
@@ -704,6 +726,7 @@ standard_ProcessUtility(Node *parsetree,
 			 * outside a transaction block is presumed to be user error.
 			 */
 			RequireTransactionChain(isTopLevel, "LOCK TABLE");
+			/* forbidden in parallel mode due to CommandIsReadOnly */
 			LockTableCommand((LockStmt *) parsetree);
 			break;
 
@@ -735,6 +758,7 @@ standard_ProcessUtility(Node *parsetree,
 
 				/* we choose to allow this during "read only" transactions */
 				PreventCommandDuringRecovery("REINDEX");
+				/* forbidden in parallel mode due to CommandIsReadOnly */
 				switch (stmt->kind)
 				{
 					case REINDEX_OBJECT_INDEX: