Support subscripting of arbitrary types, not only arrays.

This patch generalizes the subscripting infrastructure so that any
data type can be subscripted, if it provides a handler function to
define what that means.  Traditional variable-length (varlena) arrays
all use array_subscript_handler(), while the existing fixed-length
types that support subscripting use raw_array_subscript_handler().
It's expected that other types that want to use subscripting notation
will define their own handlers.  (This patch provides no such new
features, though; it only lays the foundation for them.)

To do this, move the parser's semantic processing of subscripts
(including coercion to whatever data type is required) into a
method callback supplied by the handler.  On the execution side,
replace the ExecEvalSubscriptingRef* layer of functions with direct
calls to callback-supplied execution routines.  (Thus, essentially
no new run-time overhead should be caused by this patch.  Indeed,
there is room to remove some overhead by supplying specialized
execution routines.  This patch does a little bit in that line,
but more could be done.)

Additional work is required here and there to remove formerly
hard-wired assumptions about the result type, collation, etc
of a SubscriptingRef expression node; and to remove assumptions
that the subscript values must be integers.

One useful side-effect of this is that we now have a less squishy
mechanism for identifying whether a data type is a "true" array:
instead of wiring in weird rules about typlen, we can look to see
if pg_type.typsubscript == F_ARRAY_SUBSCRIPT_HANDLER.  For this
to be bulletproof, we have to forbid user-defined types from using
that handler directly; but there seems no good reason for them to
do so.

This patch also removes assumptions that the number of subscripts
is limited to MAXDIM (6), or indeed has any hard-wired limit.
That limit still applies to types handled by array_subscript_handler
or raw_array_subscript_handler, but to discourage other dependencies
on this constant, I've moved it from c.h to utils/array.h.

Dmitry Dolgov, reviewed at various times by Tom Lane, Arthur Zakirov,
Peter Eisentraut, Pavel Stehule

Discussion: https://postgr.es/m/CA+q6zcVDuGBv=M0FqBYX8DPebS3F_0KQ6OVFobGJPM507_SZ_w@mail.gmail.com
Discussion: https://postgr.es/m/CA+q6zcVovR+XY4mfk-7oNk-rF91gH0PebnNfuUjuuDsyHjOcVA@mail.gmail.com

1 files changed, 25 insertions, 17 deletions

diff --git a/src/include/nodes/primnodes.h b/src/include/nodes/primnodes.h
index cdbe781c734..dd85908fe2f 100644
--- a/src/include/nodes/primnodes.h
+++ b/src/include/nodes/primnodes.h
@@ -390,14 +390,14 @@ typedef struct WindowFunc
 	int			location;		/* token location, or -1 if unknown */
 } WindowFunc;
 
-/* ----------------
- *	SubscriptingRef: describes a subscripting operation over a container
- *			(array, etc).
+/*
+ * SubscriptingRef: describes a subscripting operation over a container
+ * (array, etc).
  *
  * A SubscriptingRef can describe fetching a single element from a container,
- * fetching a part of container (e.g. array slice), storing a single element into
- * a container, or storing a slice.  The "store" cases work with an
- * initial container value and a source value that is inserted into the
+ * fetching a part of a container (e.g. an array slice), storing a single
+ * element into a container, or storing a slice.  The "store" cases work with
+ * an initial container value and a source value that is inserted into the
  * appropriate part of the container; the result of the operation is an
  * entire new modified container value.
  *
@@ -410,23 +410,32 @@ typedef struct WindowFunc
  *
  * In the slice case, individual expressions in the subscript lists can be
  * NULL, meaning "substitute the array's current lower or upper bound".
- *
- * Note: the result datatype is the element type when fetching a single
- * element; but it is the array type when doing subarray fetch or either
- * type of store.
+ * (Non-array containers may or may not support this.)
+ *
+ * refcontainertype is the actual container type that determines the
+ * subscripting semantics.  (This will generally be either the exposed type of
+ * refexpr, or the base type if that is a domain.)  refelemtype is the type of
+ * the container's elements; this is saved for the use of the subscripting
+ * functions, but is not used by the core code.  refrestype, reftypmod, and
+ * refcollid describe the type of the SubscriptingRef's result.  In a store
+ * expression, refrestype will always match refcontainertype; in a fetch,
+ * it could be refelemtype for an element fetch, or refcontainertype for a
+ * slice fetch, or possibly something else as determined by type-specific
+ * subscripting logic.  Likewise, reftypmod and refcollid will match the
+ * container's properties in a store, but could be different in a fetch.
  *
  * Note: for the cases where a container is returned, if refexpr yields a R/W
- * expanded container, then the implementation is allowed to modify that object
- * in-place and return the same object.)
- * ----------------
+ * expanded container, then the implementation is allowed to modify that
+ * object in-place and return the same object.
  */
 typedef struct SubscriptingRef
 {
 	Expr		xpr;
 	Oid			refcontainertype;	/* type of the container proper */
-	Oid			refelemtype;	/* type of the container elements */
-	int32		reftypmod;		/* typmod of the container (and elements too) */
-	Oid			refcollid;		/* OID of collation, or InvalidOid if none */
+	Oid			refelemtype;	/* the container type's pg_type.typelem */
+	Oid			refrestype;		/* type of the SubscriptingRef's result */
+	int32		reftypmod;		/* typmod of the result */
+	Oid			refcollid;		/* collation of result, or InvalidOid if none */
 	List	   *refupperindexpr;	/* expressions that evaluate to upper
 									 * container indexes */
 	List	   *reflowerindexpr;	/* expressions that evaluate to lower
@@ -434,7 +443,6 @@ typedef struct SubscriptingRef
 									 * container element */
 	Expr	   *refexpr;		/* the expression that evaluates to a
 								 * container value */
-
 	Expr	   *refassgnexpr;	/* expression for the source value, or NULL if
 								 * fetch */
 } SubscriptingRef;