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

Author: tglsfdc

contrib/postgres_fdw/deparse.c

@@ -426,23 +426,28 @@ foreign_expr_walker(Node *node,
 					return false;
 
 				/*
-				 * Recurse to remaining subexpressions.  Since the container
-				 * subscripts must yield (noncollatable) integers, they won't
-				 * affect the inner_cxt state.
+				 * Recurse into the remaining subexpressions.  The container
+				 * subscripts will not affect collation of the SubscriptingRef
+				 * result, so do those first and reset inner_cxt afterwards.
 				 */
 				if (!foreign_expr_walker((Node *) sr->refupperindexpr,
 										 glob_cxt, &inner_cxt))
 					return false;
+				inner_cxt.collation = InvalidOid;
+				inner_cxt.state = FDW_COLLATE_NONE;
 				if (!foreign_expr_walker((Node *) sr->reflowerindexpr,
 										 glob_cxt, &inner_cxt))
 					return false;
+				inner_cxt.collation = InvalidOid;
+				inner_cxt.state = FDW_COLLATE_NONE;
 				if (!foreign_expr_walker((Node *) sr->refexpr,
 										 glob_cxt, &inner_cxt))
 					return false;
 
 				/*
-				 * Container subscripting should yield same collation as
-				 * input, but for safety use same logic as for function nodes.
+				 * Container subscripting typically yields same collation as
+				 * refexpr's, but in case it doesn't, use same logic as for
+				 * function nodes.
 				 */
 				collation = sr->refcollid;
 				if (collation == InvalidOid)

doc/src/sgml/catalogs.sgml

@@ -8740,26 +8740,38 @@ SCRAM-SHA-256$<replaceable>&lt;iteration count&gt;</replaceable>:<replaceable>&l
       </para></entry>
      </row>
 
+     <row>
+      <entry role="catalog_table_entry"><para role="column_definition">
+       <structfield>typsubscript</structfield> <type>regproc</type>
+       (references <link linkend="catalog-pg-proc"><structname>pg_proc</structname></link>.<structfield>oid</structfield>)
+      </para>
+      <para>
+       Subscripting handler function's OID, or zero if this type doesn't
+       support subscripting.  Types that are <quote>true</quote> array
+       types have <structfield>typsubscript</structfield>
+       = <function>array_subscript_handler</function>, but other types may
+       have other handler functions to implement specialized subscripting
+       behavior.
+      </para></entry>
+     </row>
+
      <row>
       <entry role="catalog_table_entry"><para role="column_definition">
        <structfield>typelem</structfield> <type>oid</type>
        (references <link linkend="catalog-pg-type"><structname>pg_type</structname></link>.<structfield>oid</structfield>)
       </para>
       <para>
        If <structfield>typelem</structfield> is not 0 then it
-       identifies another row in <structname>pg_type</structname>.
-       The current type can then be subscripted like an array yielding
-       values of type <structfield>typelem</structfield>.  A
-       <quote>true</quote> array type is variable length
-       (<structfield>typlen</structfield> = -1),
-       but some fixed-length (<structfield>typlen</structfield> &gt; 0) types
-       also have nonzero <structfield>typelem</structfield>, for example
-       <type>name</type> and <type>point</type>.
-       If a fixed-length type has a <structfield>typelem</structfield> then
-       its internal representation must be some number of values of the
-       <structfield>typelem</structfield> data type with no other data.
-       Variable-length array types have a header defined by the array
-       subroutines.
+       identifies another row in <structname>pg_type</structname>,
+       defining the type yielded by subscripting.  This should be 0
+       if <structfield>typsubscript</structfield> is 0.  However, it can
+       be 0 when <structfield>typsubscript</structfield> isn't 0, if the
+       handler doesn't need <structfield>typelem</structfield> to
+       determine the subscripting result type.
+       Note that a <structfield>typelem</structfield> dependency is
+       considered to imply physical containment of the element type in
+       this type; so DDL changes on the element type might be restricted
+       by the presence of this type.
       </para></entry>
      </row>