first working-ish Postgres backend

Subcommits:

implement iterator and remove ResultTree

add Err() to sql

remove redundant and less helpful indices, change fillfactor, and use COPY FROM

graph/sql/lru.go

@@ -0,0 +1,63 @@
+// Copyright 2014 The Cayley Authors. All rights reserved.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package sql
+
+import (
+	"container/list"
+)
+
+// cache implements an LRU cache.
+type cache struct {
+	cache    map[string]*list.Element
+	priority *list.List
+	maxSize  int
+}
+
+type kv struct {
+	key   string
+	value int64
+}
+
+func newCache(size int) *cache {
+	var lru cache
+	lru.maxSize = size
+	lru.priority = list.New()
+	lru.cache = make(map[string]*list.Element)
+	return &lru
+}
+
+func (lru *cache) Put(key string, value int64) {
+	if _, ok := lru.Get(key); ok {
+		return
+	}
+	if len(lru.cache) == lru.maxSize {
+		lru.removeOldest()
+	}
+	lru.priority.PushFront(kv{key: key, value: value})
+	lru.cache[key] = lru.priority.Front()
+}
+
+func (lru *cache) Get(key string) (int64, bool) {
+	if element, ok := lru.cache[key]; ok {
+		lru.priority.MoveToFront(element)
+		return element.Value.(kv).value, true
+	}
+	return 0, false
+}
+
+func (lru *cache) removeOldest() {
+	last := lru.priority.Remove(lru.priority.Back())
+	delete(lru.cache, last.(kv).key)
+}