Provide some rather hokey ways for EXPLAIN to print FieldStore and assignment

ArrayRef expressions that are not in the immediate context of an INSERT or
UPDATE targetlist.  Such cases never arise in stored rules, so ruleutils.c
hadn't tried to handle them.  However, they do occur in the targetlists of
plans derived from such statements, and now that EXPLAIN VERBOSE tries to
print targetlists, we need some way to deal with the case.

I chose to represent an assignment ArrayRef as "array[subscripts] := source",
which is fairly reasonable and doesn't omit any information.  However,
FieldStore is problematic because the planner will fold multiple assignments
to fields of the same composite column into one FieldStore, resulting in a
structure that is hard to understand at all, let alone display comprehensibly.
So in that case I punted and just made it print the source expression(s).

Backpatch to 8.4 --- the lack of functionality exists in older releases,
but doesn't seem to be important for lack of anything that would call it.

src/backend/utils/adt/ruleutils.c

@@ -9,7 +9,7 @@
  *
  *
  * IDENTIFICATION
- *	  $PostgreSQL: pgsql/src/backend/utils/adt/ruleutils.c,v 1.323 2010/02/16 22:34:50 tgl Exp $
+ *	  $PostgreSQL: pgsql/src/backend/utils/adt/ruleutils.c,v 1.324 2010/02/18 22:43:31 tgl Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -4468,6 +4468,22 @@ get_rule_expr(Node *node, deparse_context *context,
 				ArrayRef   *aref = (ArrayRef *) node;
 				bool		need_parens;
 
+				/*
+				 * If the argument is a CaseTestExpr, we must be inside a
+				 * FieldStore, ie, we are assigning to an element of an
+				 * array within a composite column.  Since we already punted
+				 * on displaying the FieldStore's target information, just
+				 * punt here too, and display only the assignment source
+				 * expression.
+				 */
+				if (IsA(aref->refexpr, CaseTestExpr))
+				{
+					Assert(aref->refassgnexpr);
+					get_rule_expr((Node *) aref->refassgnexpr,
+								  context, showimplicit);
+					break;
+				}
+
 				/*
 				 * Parenthesize the argument unless it's a simple Var or a
 				 * FieldSelect.  (In particular, if it's another ArrayRef, we
@@ -4480,14 +4496,35 @@ get_rule_expr(Node *node, deparse_context *context,
 				get_rule_expr((Node *) aref->refexpr, context, showimplicit);
 				if (need_parens)
 					appendStringInfoChar(buf, ')');
-				printSubscripts(aref, context);
 
 				/*
-				 * Array assignment nodes should have been handled in
-				 * processIndirection().
+				 * If there's a refassgnexpr, we want to print the node in
+				 * the format "array[subscripts] := refassgnexpr".  This is
+				 * not legal SQL, so decompilation of INSERT or UPDATE
+				 * statements should always use processIndirection as part
+				 * of the statement-level syntax.  We should only see this
+				 * when EXPLAIN tries to print the targetlist of a plan
+				 * resulting from such a statement.
 				 */
 				if (aref->refassgnexpr)
-					elog(ERROR, "unexpected refassgnexpr");
+				{
+					Node   *refassgnexpr;
+
+					/*
+					 * Use processIndirection to print this node's
+					 * subscripts as well as any additional field selections
+					 * or subscripting in immediate descendants.  It returns
+					 * the RHS expr that is actually being "assigned".
+					 */
+					refassgnexpr = processIndirection(node, context, true);
+					appendStringInfoString(buf, " := ");
+					get_rule_expr(refassgnexpr, context, showimplicit);
+				}
+				else
+				{
+					/* Just an ordinary array fetch, so print subscripts */
+					printSubscripts(aref, context);
+				}
 			}
 			break;
 
@@ -4679,12 +4716,36 @@ get_rule_expr(Node *node, deparse_context *context,
 			break;
 
 		case T_FieldStore:
+			{
+				FieldStore *fstore = (FieldStore *) node;
+				bool		need_parens;
 
 				/*
-			 * We shouldn't see FieldStore here; it should have been stripped
-			 * off by processIndirection().
-			 */
-			elog(ERROR, "unexpected FieldStore");
+				 * There is no good way to represent a FieldStore as real SQL,
+				 * so decompilation of INSERT or UPDATE statements should
+				 * always use processIndirection as part of the
+				 * statement-level syntax.  We should only get here when
+				 * EXPLAIN tries to print the targetlist of a plan resulting
+				 * from such a statement.  The plan case is even harder than
+				 * ordinary rules would be, because the planner tries to
+				 * collapse multiple assignments to the same field or subfield
+				 * into one FieldStore; so we can see a list of target fields
+				 * not just one, and the arguments could be FieldStores
+				 * themselves.  We don't bother to try to print the target
+				 * field names; we just print the source arguments, with a
+				 * ROW() around them if there's more than one.  This isn't
+				 * terribly complete, but it's probably good enough for
+				 * EXPLAIN's purposes; especially since anything more would be
+				 * either hopelessly confusing or an even poorer
+				 * representation of what the plan is actually doing.
+				 */
+				need_parens = (list_length(fstore->newvals) != 1);
+				if (need_parens)
+					appendStringInfoString(buf, "ROW(");
+				get_rule_expr((Node *) fstore->newvals, context, showimplicit);
+				if (need_parens)
+					appendStringInfoChar(buf, ')');
+			}
 			break;
 
 		case T_RelabelType:
@@ -6307,12 +6368,11 @@ processIndirection(Node *node, deparse_context *context, bool printit)
 					 format_type_be(fstore->resulttype));
 
 			/*
-			 * Print the field name.  Note we assume here that there's only
-			 * one field being assigned to.  This is okay in stored rules but
-			 * could be wrong in executable target lists.  Presently no
-			 * problem since explain.c doesn't print plan targetlists, but
-			 * someday may have to think of something ...
+			 * Print the field name.  There should only be one target field
+			 * in stored rules.  There could be more than that in executable
+			 * target lists, but this function cannot be used for that case.
 			 */
+			Assert(list_length(fstore->fieldnums) == 1);
 			fieldname = get_relid_attribute_name(typrelid,
 											linitial_int(fstore->fieldnums));
 			if (printit)