Restructure SELECT INTO's parsetree representation into CreateTableAsStmt.

Making this operation look like a utility statement seems generally a good
idea, and particularly so in light of the desire to provide command
triggers for utility statements.  The original choice of representing it as
SELECT with an IntoClause appendage had metastasized into rather a lot of
places, unfortunately, so that this patch is a great deal more complicated
than one might at first expect.

In particular, keeping EXPLAIN working for SELECT INTO and CREATE TABLE AS
subcommands required restructuring some EXPLAIN-related APIs.  Add-on code
that calls ExplainOnePlan or ExplainOneUtility, or uses
ExplainOneQuery_hook, will need adjustment.

Also, the cases PREPARE ... SELECT INTO and CREATE RULE ... SELECT INTO,
which formerly were accepted though undocumented, are no longer accepted.
The PREPARE case can be replaced with use of CREATE TABLE AS EXECUTE.
The CREATE RULE case doesn't seem to have much real-world use (since the
rule would work only once before failing with "table already exists"),
so we'll not bother with that one.

Both SELECT INTO and CREATE TABLE AS still return a command tag of
"SELECT nnnn".  There was some discussion of returning "CREATE TABLE nnnn",
but for the moment backwards compatibility wins the day.

Andres Freund and Tom Lane

src/include/executor/executor.h

@@ -30,8 +30,8 @@
  *
  * EXPLAIN_ONLY indicates that the plan tree is being initialized just so
  * EXPLAIN can print it out; it will not be run.  Hence, no side-effects
- * of startup should occur (such as creating a SELECT INTO target table).
- * However, error checks (such as permission checks) should be performed.
+ * of startup should occur.  However, error checks (such as permission checks)
+ * should be performed.
  *
  * REWIND indicates that the plan node should try to efficiently support
  * rescans without parameter changes.  (Nodes must support ExecReScan calls
@@ -49,12 +49,18 @@
  * AfterTriggerBeginQuery/AfterTriggerEndQuery.  This does not necessarily
  * mean that the plan can't queue any AFTER triggers; just that the caller
  * is responsible for there being a trigger context for them to be queued in.
+ *
+ * WITH/WITHOUT_OIDS tell the executor to emit tuples with or without space
+ * for OIDs, respectively.  These are currently used only for CREATE TABLE AS.
+ * If neither is set, the plan may or may not produce tuples including OIDs.
  */
 #define EXEC_FLAG_EXPLAIN_ONLY	0x0001	/* EXPLAIN, no ANALYZE */
 #define EXEC_FLAG_REWIND		0x0002	/* need efficient rescan */
 #define EXEC_FLAG_BACKWARD		0x0004	/* need backward scan */
 #define EXEC_FLAG_MARK			0x0008	/* need mark/restore */
 #define EXEC_FLAG_SKIP_TRIGGERS 0x0010	/* skip AfterTrigger calls */
+#define EXEC_FLAG_WITH_OIDS		0x0020	/* force OIDs in returned tuples */
+#define EXEC_FLAG_WITHOUT_OIDS	0x0040	/* force no OIDs in returned tuples */
 
 
 /*
@@ -204,7 +210,6 @@ extern void EvalPlanQualFetchRowMarks(EPQState *epqstate);
 extern TupleTableSlot *EvalPlanQualNext(EPQState *epqstate);
 extern void EvalPlanQualBegin(EPQState *epqstate, EState *parentestate);
 extern void EvalPlanQualEnd(EPQState *epqstate);
-extern DestReceiver *CreateIntoRelDestReceiver(void);
 
 /*
  * prototypes from functions in execProcnode.c