Fix interaction of parallel query with prepared statements.

Previously, a prepared statement created via a Parse message could get
a parallel plan, but one created with a PREPARE statement could not.
This state of affairs was due to confusion on my (rhaas) part: I
erroneously believed that a CREATE TABLE .. AS EXECUTE statement could
only be performed with a prepared statement by PREPARE, but in fact
one created by a Prepare message works just as well.  Therefore, it
makes no sense to allow parallel query in one case but not the other.

To fix, allow parallel query with all prepared statements, but run
the parallel plan serially (i.e. without workers) in the case of
CREATE TABLE .. AS EXECUTE.  Also, document this.

Amit Kapila and Tobias Bussman, plus an extra sentence of
documentation by me.

doc/src/sgml/parallel.sgml

@@ -227,6 +227,15 @@ EXPLAIN SELECT * FROM pgbench_accounts WHERE filler LIKE '%x%';
       </para>
     </listitem>
 
+    <listitem>
+      <para> 
+        A prepared statement is executed using a <literal>CREATE TABLE .. AS
+        EXECUTE ..</literal> statement.  This construct converts what otherwise
+        would have been a read-only operation into a read-write operation,
+        making it ineligible for parallel query.
+      </para>
+    </listitem>
+
     <listitem>
       <para> 
         The transaction isolation level is serializable.  This situation

src/backend/commands/prepare.c

@@ -159,7 +159,7 @@ PrepareQuery(PrepareStmt *stmt, const char *queryString)
 					   nargs,
 					   NULL,
 					   NULL,
-					   0,		/* default cursor options */
+					   CURSOR_OPT_PARALLEL_OK,	/* allow parallel mode */
 					   true);	/* fixed result */
 
 	/*

src/backend/executor/execMain.c

@@ -1540,10 +1540,11 @@ ExecutePlan(EState *estate,
 	estate->es_direction = direction;
 
 	/*
-	 * If a tuple count was supplied, we must force the plan to run without
-	 * parallelism, because we might exit early.
+	 * If a tuple count was supplied or data is being written to relation, we
+	 * must force the plan to run without parallelism, because we might exit
+	 * early.
 	 */
-	if (numberTuples)
+	if (numberTuples || dest->mydest == DestIntoRel)
 		use_parallel_mode = false;
 
 	/*