cost.go

// Copyright 2018 The Cockroach Authors.
//
// Use of this software is governed by the Business Source License
// included in the file licenses/BSL.txt.
//
// As of the Change Date specified in that file, in accordance with
// the Business Source License, use of this software will be governed
// by the Apache License, Version 2.0, included in the file
// licenses/APL.txt.

package memo

import "math"

// Cost is the best-effort approximation of the actual cost of executing a
// particular operator tree.
// TODO: Need more details about what one "unit" of cost means.
type Cost float64

// MaxCost is the maximum possible estimated cost. It's used to suppress memo
// group members during testing, by setting their cost so high that any other
// member will have a lower cost.
var MaxCost = Cost(math.Inf(+1))

// Less returns true if this cost is lower than the given cost.
func (c Cost) Less(other Cost) bool {
	// Two plans with the same cost can have slightly different floating point
	// results (e.g. same subcosts being added up in a different order). So we
	// treat plans with very similar cost as equal.
	//
	// We use "units of least precision" for similarity: this is the number of
	// representable floating point numbers in-between the two values. This is
	// better than a fixed epsilon because the allowed error is proportional to
	// the magnitude of the numbers. Because the mantissa is in the low bits, we
	// can just use the bit representations as integers.
	const ulpTolerance = 1000
	return math.Float64bits(float64(c))+ulpTolerance <= math.Float64bits(float64(other))
}

// Sub subtracts the other cost from this cost and returns the result.
func (c Cost) Sub(other Cost) Cost {
	return c - other
}