Here is a set of functions for working with a PostgreSQL+JSON+PLV8 database.

json_path.sql

-- Function: json_path(json, text)

-- DROP FUNCTION json_path(json, text);

CREATE OR REPLACE FUNCTION json_path(data json, path text)
  RETURNS text AS
$BODY$
/* JSONPath 0.8.0 - XPath for JSON
 *
 * Copyright (c) 2007 Stefan Goessner (goessner.net)
 * Licensed under the MIT (MIT-LICENSE.txt) licence.
 */
function jsonPath(obj, expr, arg) {
   var P = {
      resultType: arg && arg.resultType || "VALUE",
      result: [],
      normalize: function(expr) {
         var subx = [];
         return expr.replace(/[\['](\??\(.*?\))[\]']/g, function($0,$1){return "[#"+(subx.push($1)-1)+"]";})
                    .replace(/'?\.'?|\['?/g, ";")
                    .replace(/;;;|;;/g, ";..;")
                    .replace(/;$|'?\]|'$/g, "")
                    .replace(/#([0-9]+)/g, function($0,$1){return subx[$1];});
      },
      asPath: function(path) {
         var x = path.split(";"), p = "$";
         for (var i=1,n=x.length; i<n; i++)
            p += /^[0-9*]+$/.test(x[i]) ? ("["+x[i]+"]") : ("['"+x[i]+"']");
         return p;
      },
      store: function(p, v) {
         if (p) P.result[P.result.length] = P.resultType == "PATH" ? P.asPath(p) : v;
         return !!p;
      },
      trace: function(expr, val, path) {
         if (expr) {
            var x = expr.split(";"), loc = x.shift();
            x = x.join(";");
            if (val && val.hasOwnProperty(loc))
               P.trace(x, val[loc], path + ";" + loc);
            else if (loc === "*")
               P.walk(loc, x, val, path, function(m,l,x,v,p) { P.trace(m+";"+x,v,p); });
            else if (loc === "..") {
               P.trace(x, val, path);
               P.walk(loc, x, val, path, function(m,l,x,v,p) { typeof v[m] === "object" && P.trace("..;"+x,v[m],p+";"+m); });
            }
            else if (/,/.test(loc)) { // [name1,name2,...]
               for (var s=loc.split(/'?,'?/),i=0,n=s.length; i<n; i++)
                  P.trace(s[i]+";"+x, val, path);
            }
            else if (/^\(.*?\)$/.test(loc)) // [(expr)]
               P.trace(P.eval(loc, val, path.substr(path.lastIndexOf(";")+1))+";"+x, val, path);
            else if (/^\?\(.*?\)$/.test(loc)) // [?(expr)]
               P.walk(loc, x, val, path, function(m,l,x,v,p) { if (P.eval(l.replace(/^\?\((.*?)\)$/,"$1"),v[m],m)) P.trace(m+";"+x,v,p); });
            else if (/^(-?[0-9]*):(-?[0-9]*):?([0-9]*)$/.test(loc)) // [start:end:step]  phyton slice syntax
               P.slice(loc, x, val, path);
         }
         else
            P.store(path, val);
      },
      walk: function(loc, expr, val, path, f) {
         if (val instanceof Array) {
            for (var i=0,n=val.length; i<n; i++)
               if (i in val)
                  f(i,loc,expr,val,path);
         }
         else if (typeof val === "object") {
            for (var m in val)
               if (val.hasOwnProperty(m))
                  f(m,loc,expr,val,path);
         }
      },
      slice: function(loc, expr, val, path) {
         if (val instanceof Array) {
            var len=val.length, start=0, end=len, step=1;
            loc.replace(/^(-?[0-9]*):(-?[0-9]*):?(-?[0-9]*)$/g, function($0,$1,$2,$3){start=parseInt($1||start);end=parseInt($2||end);step=parseInt($3||step);});
            start = (start < 0) ? Math.max(0,start+len) : Math.min(len,start);
            end   = (end < 0)   ? Math.max(0,end+len)   : Math.min(len,end);
            for (var i=start; i<end; i+=step)
               P.trace(i+";"+expr, val, path);
         }
      },
      eval: function(x, _v, _vname) {
         try { return $ && _v && eval(x.replace(/@/g, "_v")); }
         catch(e) { throw new SyntaxError("jsonPath: " + e.message + ": " + x.replace(/@/g, "_v").replace(/\^/g, "_a")); }
      }
   };

   var $ = obj;
   if (expr && obj && (P.resultType == "VALUE" || P.resultType == "PATH")) {
      P.trace(P.normalize(expr).replace(/^\$;/,""), obj, "$");
      return P.result.length ? P.result : false;
   }
} 

var obj = JSON.parse(data);
var results = jsonPath(obj, path);

if(results.length === 1) {
  return JSON.stringify(results[0]);
} else {
  return JSON.stringify(results);
}
$BODY$
  LANGUAGE plv8 IMMUTABLE
  COST 100;
ALTER FUNCTION json_path(json, text)
  OWNER TO ccowan;

json_path_float.sql

-- Function: json_path_float(json, text)

-- DROP FUNCTION json_path_float(json, text);

CREATE OR REPLACE FUNCTION json_path_float(data json, path text)
  RETURNS double precision AS
$BODY$

var results = plv8.execute('SELECT json_path($1::json, $2) as value', [data, path]);
return parseFloat(JSON.parse(results[0]['value']));

$BODY$
  LANGUAGE plv8 IMMUTABLE
  COST 100;
ALTER FUNCTION json_path_float(json, text)
  OWNER TO ccowan;

json_path_intarray.sql

-- Function: json_path_intarray(json, text)

-- DROP FUNCTION json_path_intarray(json, text);

CREATE OR REPLACE FUNCTION json_path_intarray(data json, path text)
  RETURNS integer[] AS
$BODY$

var results = plv8.execute('SELECT json_path($1::json, $2) as value', [data, path]);
return JSON.parse(results[0]['value']);

$BODY$
  LANGUAGE plv8 IMMUTABLE
  COST 100;
ALTER FUNCTION json_path_intarray(json, text)
  OWNER TO ccowan;

json_path_integer.sql

-- Function: json_path_integer(json, text)

-- DROP FUNCTION json_path_integer(json, text);

CREATE OR REPLACE FUNCTION json_path_integer(data json, path text)
  RETURNS integer AS
$BODY$

var results = plv8.execute('SELECT json_path($1::json, $2) as value', [data, path]);
return parseInt(JSON.parse(results[0]['value']),10);

$BODY$
  LANGUAGE plv8 IMMUTABLE
  COST 100;
ALTER FUNCTION json_path_integer(json, text)
  OWNER TO ccowan;

json_path_textarray.sql

-- Function: json_path_textarray(json, text)

-- DROP FUNCTION json_path_textarray(json, text);

CREATE OR REPLACE FUNCTION json_path_textarray(data json, path text)
  RETURNS text[] AS
$BODY$

var results = plv8.execute('SELECT json_path($1::json, $2) as value', [data, path]);
return JSON.parse(results[0]['value']);

$BODY$
  LANGUAGE plv8 IMMUTABLE
  COST 100;
ALTER FUNCTION json_path_textarray(json, text)
  OWNER TO ccowan;

json_path_timestamp.sql

-- Function: json_path_timestamp(json, text)

-- DROP FUNCTION json_path_timestamp(json, text);

CREATE OR REPLACE FUNCTION json_path_timestamp(data json, path text)
  RETURNS timestamp without time zone AS
$BODY$

var results = plv8.execute('SELECT json_path($1::json, $2) as value', [data, path]);
return results[0]['value'];

$BODY$
  LANGUAGE plv8 IMMUTABLE
  COST 100;
ALTER FUNCTION json_path_timestamp(json, text)
  OWNER TO ccowan;

json_path_uuid.sql

-- Function: json_path_uuid(json, text)

-- DROP FUNCTION json_path_uuid(json, text);

CREATE OR REPLACE FUNCTION json_path_uuid(data json, path text)
  RETURNS uuid AS
$BODY$

var results = plv8.execute('SELECT json_path($1::json, $2) as value', [data, path]);
return JSON.parse(results[0]['value']);

$BODY$
  LANGUAGE plv8 IMMUTABLE
  COST 100;
ALTER FUNCTION json_path_uuid(json, text)
  OWNER TO ccowan;