func NewMemPostings() *MemPostings {
	return &MemPostings{
		m:       make(map[labels.Label][]uint64, 512),
		ordered: true,
	}
}
NewMemPostings returns a memPostings that's ready for reads and writes.
package index
import (
	"encoding/binary"
	"runtime"
	"sort"
	"strings"
	"sync"
	"github.com/prometheus/tsdb/labels"
)
var allPostingsKey = labels.Label{}
func AllPostingsKey() (name, value string) {
	return allPostingsKey.Name, allPostingsKey.Value
}
type MemPostings struct {
	mtx     sync.RWMutex
	m       map[labels.Label][]uint64
	ordered bool
}