Physical Optimize

task

// task is a new version of `PhysicalPlanInfo`. It stores cost information for a task.
// A task may be CopTask, RootTask, MPPTask or a ParallelTask.
type task interface {
	count() float64
	addCost(cost float64)
	cost() float64
	copy() task
	plan() PhysicalPlan
	invalid() bool
}

task分两种, roottask在TiDB端执行

• rootTask is the final sink node of a plan graph. It should be a single goroutine on tidb.
• copTask is a task that runs in a distributed kv store.

1. task是怎么执行的呢？
2. coptask和rootTask的执行在哪而体现的呢？

Physical PhysicalProperty

// It contains the orders and the task types.
type PhysicalProperty struct {
	Items []Item

	// TaskTp means the type of task that an operator requires.
	//
	// It needs to be specified because two different tasks can't be compared
	// with cost directly. e.g. If a copTask takes less cost than a rootTask,
	// we can't sure that we must choose the former one. Because the copTask
	// must be finished and increase its cost in sometime, but we can't make
	// sure the finishing time. So the best way to let the comparison fair is
	// to add TaskType to required property.
	TaskTp TaskType

	// ExpectedCnt means this operator may be closed after fetching ExpectedCnt
	// records.
	ExpectedCnt float64

	// hashcode stores the hash code of a PhysicalProperty, will be lazily
	// calculated when function "HashCode()" being called.
	hashcode []byte

	// whether need to enforce property.
	Enforced bool
}

taskType

// TaskType is the type of execution task.
type TaskType int

const (
	// RootTaskType stands for the tasks that executed in the TiDB layer.
	RootTaskType TaskType = iota

	// CopSingleReadTaskType stands for the a TableScan or IndexScan tasks
	// executed in the coprocessor layer.
	CopSingleReadTaskType

	// CopDoubleReadTaskType stands for the a IndexLookup tasks executed in the
	// coprocessor layer.
	CopDoubleReadTaskType

	// CopTiFlashLocalReadTaskType stands for flash coprocessor that read data locally,
	// and only a part of the data is read in one cop task, if the current task type is
	// CopTiFlashLocalReadTaskType, all its children prop's task type is CopTiFlashLocalReadTaskType
	CopTiFlashLocalReadTaskType

	// CopTiFlashGlobalReadTaskType stands for flash coprocessor that read data globally
	// and all the data of given table will be read in one cop task, if the current task
	// type is CopTiFlashGlobalReadTaskType, all its children prop's task type is
	// CopTiFlashGlobalReadTaskType
	CopTiFlashGlobalReadTaskType
)

findBestTask

type LogicalPlan interface {
	// findBestTask converts the logical plan to the physical plan. It's a new interface.
	// It is called recursively from the parent to the children to create the result physical plan.
	// Some logical plans will convert the children to the physical plans in different ways, and return the one
	// With the lowest cost and how many plans are found in this function.
	// planCounter is a counter for planner to force a plan.
	// If planCounter > 0, the clock_th plan generated in this function will be returned.
	// If planCounter = 0, the plan generated in this function will not be considered.
	// If planCounter = -1, then we will not force plan.
	findBestTask(prop *property.PhysicalProperty, planCounter *PlanCounterTp) (task, int64, error)
  //..
}