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/nodes/parsenodes.h

@@ -84,7 +84,7 @@ typedef uint32 AclMode;			/* a bitmask of privilege bits */
 
 /*
  * Query -
- *	  Parse analysis turns all statements into a Query tree (via transformStmt)
+ *	  Parse analysis turns all statements into a Query tree
  *	  for further processing by the rewriter and planner.
  *
  *	  Utility statements (i.e. non-optimizable statements) have the
@@ -111,8 +111,6 @@ typedef struct Query
 	int			resultRelation; /* rtable index of target relation for
 								 * INSERT/UPDATE/DELETE; 0 for SELECT */
 
-	IntoClause *intoClause;		/* target for SELECT INTO / CREATE TABLE AS */
-
 	bool		hasAggs;		/* has aggregates in tlist or havingQual */
 	bool		hasWindowFuncs; /* has window functions in tlist */
 	bool		hasSubLinks;	/* has subquery SubLink */
@@ -1009,7 +1007,7 @@ typedef struct SelectStmt
 	 */
 	List	   *distinctClause; /* NULL, list of DISTINCT ON exprs, or
 								 * lcons(NIL,NIL) for all (SELECT DISTINCT) */
-	IntoClause *intoClause;		/* target for SELECT INTO / CREATE TABLE AS */
+	IntoClause *intoClause;		/* target for SELECT INTO */
 	List	   *targetList;		/* the target list (of ResTarget) */
 	List	   *fromClause;		/* the FROM clause */
 	Node	   *whereClause;	/* WHERE qualification */
@@ -2395,6 +2393,25 @@ typedef struct ExplainStmt
 	List	   *options;		/* list of DefElem nodes */
 } ExplainStmt;
 
+/* ----------------------
+ *		CREATE TABLE AS Statement (a/k/a SELECT INTO)
+ *
+ * A query written as CREATE TABLE AS will produce this node type natively.
+ * A query written as SELECT ... INTO will be transformed to this form during
+ * parse analysis.
+ *
+ * The "query" field is handled similarly to EXPLAIN, though note that it
+ * can be a SELECT or an EXECUTE, but not other DML statements.
+ * ----------------------
+ */
+typedef struct CreateTableAsStmt
+{
+	NodeTag		type;
+	Node	   *query;			/* the query (see comments above) */
+	IntoClause *into;			/* destination table */
+	bool		is_select_into;	/* it was written as SELECT INTO */
+} CreateTableAsStmt;
+
 /* ----------------------
  * Checkpoint Statement
  * ----------------------
@@ -2509,7 +2526,6 @@ typedef struct ExecuteStmt
 {
 	NodeTag		type;
 	char	   *name;			/* The name of the plan to execute */
-	IntoClause *into;			/* Optional table to store results in */
 	List	   *params;			/* Values to assign to parameters */
 } ExecuteStmt;