Revert SQL/JSON features

The reverts the following and makes some associated cleanups:

    commit f79b803dc: Common SQL/JSON clauses
    commit f4fb45d15: SQL/JSON constructors
    commit 5f0adec25: Make STRING an unreserved_keyword.
    commit 33a377608: IS JSON predicate
    commit 1a36bc9db: SQL/JSON query functions
    commit 606948b05: SQL JSON functions
    commit 49082c2cc: RETURNING clause for JSON() and JSON_SCALAR()
    commit 4e34747c8: JSON_TABLE
    commit fadb48b00: PLAN clauses for JSON_TABLE
    commit 2ef6f11b0: Reduce running time of jsonb_sqljson test
    commit 14d3f24fa: Further improve jsonb_sqljson parallel test
    commit a6baa4bad: Documentation for SQL/JSON features
    commit b46bcf7a4: Improve readability of SQL/JSON documentation.
    commit 112fdb352: Fix finalization for json_objectagg and friends
    commit fcdb35c32: Fix transformJsonBehavior
    commit 4cd8717af: Improve a couple of sql/json error messages
    commit f7a605f63: Small cleanups in SQL/JSON code
    commit 9c3d25e17: Fix JSON_OBJECTAGG uniquefying bug
    commit a79153b7a: Claim SQL standard compliance for SQL/JSON features
    commit a1e7616d6: Rework SQL/JSON documentation
    commit 8d9f9634e: Fix errors in copyfuncs/equalfuncs support for JSON node types.
    commit 3c633f32b: Only allow returning string types or bytea from json_serialize
    commit 67b26703b: expression eval: Fix EEOP_JSON_CONSTRUCTOR and EEOP_JSONEXPR size.

The release notes are also adjusted.

Backpatch to release 15.

Discussion: https://postgr.es/m/40d2c882-bcac-19a9-754d-4299e1d87ac7@postgresql.org

src/backend/optimizer/path/costsize.c

@@ -4923,8 +4923,7 @@ cost_qual_eval_walker(Node *node, cost_qual_eval_context *context)
 			 IsA(node, SQLValueFunction) ||
 			 IsA(node, XmlExpr) ||
 			 IsA(node, CoerceToDomain) ||
-			 IsA(node, NextValueExpr) ||
-			 IsA(node, JsonExpr))
+			 IsA(node, NextValueExpr))
 	{
 		/* Treat all these as having cost 1 */
 		context->total.per_tuple += cpu_operator_cost;

src/backend/optimizer/util/clauses.c

@@ -28,7 +28,6 @@
 #include "catalog/pg_type.h"
 #include "executor/executor.h"
 #include "executor/functions.h"
-#include "executor/execExpr.h"
 #include "funcapi.h"
 #include "miscadmin.h"
 #include "nodes/makefuncs.h"
@@ -51,9 +50,6 @@
 #include "utils/builtins.h"
 #include "utils/datum.h"
 #include "utils/fmgroids.h"
-#include "utils/json.h"
-#include "utils/jsonb.h"
-#include "utils/jsonpath.h"
 #include "utils/lsyscache.h"
 #include "utils/memutils.h"
 #include "utils/syscache.h"
@@ -386,45 +382,6 @@ contain_mutable_functions_walker(Node *node, void *context)
 								context))
 		return true;
 
-	if (IsA(node, JsonConstructorExpr))
-	{
-		const JsonConstructorExpr *ctor = (JsonConstructorExpr *) node;
-		ListCell   *lc;
-		bool		is_jsonb =
-		ctor->returning->format->format_type == JS_FORMAT_JSONB;
-
-		/* Check argument_type => json[b] conversions */
-		foreach(lc, ctor->args)
-		{
-			Oid			typid = exprType(lfirst(lc));
-
-			if (is_jsonb ?
-				!to_jsonb_is_immutable(typid) :
-				!to_json_is_immutable(typid))
-				return true;
-		}
-
-		/* Check all subnodes */
-	}
-
-	if (IsA(node, JsonExpr))
-	{
-		JsonExpr   *jexpr = castNode(JsonExpr, node);
-		Const	   *cnst;
-
-		if (!IsA(jexpr->path_spec, Const))
-			return true;
-
-		cnst = castNode(Const, jexpr->path_spec);
-
-		Assert(cnst->consttype == JSONPATHOID);
-		if (cnst->constisnull)
-			return false;
-
-		return jspIsMutable(DatumGetJsonPathP(cnst->constvalue),
-							jexpr->passing_names, jexpr->passing_values);
-	}
-
 	if (IsA(node, SQLValueFunction))
 	{
 		/* all variants of SQLValueFunction are stable */
@@ -896,18 +853,6 @@ max_parallel_hazard_walker(Node *node, max_parallel_hazard_context *context)
 								 context, 0);
 	}
 
-	/* JsonExpr is parallel-unsafe if subtransactions can be used. */
-	else if (IsA(node, JsonExpr))
-	{
-		JsonExpr   *jsexpr = (JsonExpr *) node;
-
-		if (ExecEvalJsonNeedsSubTransaction(jsexpr, NULL))
-		{
-			context->max_hazard = PROPARALLEL_UNSAFE;
-			return true;
-		}
-	}
-
 	/* Recurse to check arguments */
 	return expression_tree_walker(node,
 								  max_parallel_hazard_walker,
@@ -3567,29 +3512,6 @@ eval_const_expressions_mutator(Node *node,
 					return ece_evaluate_expr((Node *) newcre);
 				return (Node *) newcre;
 			}
-		case T_JsonValueExpr:
-			{
-				JsonValueExpr *jve = (JsonValueExpr *) node;
-				Node	   *raw = eval_const_expressions_mutator((Node *) jve->raw_expr,
-																 context);
-
-				if (raw && IsA(raw, Const))
-				{
-					Node	   *formatted;
-					Node	   *save_case_val = context->case_val;
-
-					context->case_val = raw;
-
-					formatted = eval_const_expressions_mutator((Node *) jve->formatted_expr,
-															   context);
-
-					context->case_val = save_case_val;
-
-					if (formatted && IsA(formatted, Const))
-						return formatted;
-				}
-				break;
-			}
 		default:
 			break;
 	}