Implement Select .... AS OF syntax to support Stale Read

[nolouch]

Background

This is a subtask of #21094.
pingcap/parser#1206 has implemented the syntax of the Select .... FROM xxx AS OF timestamp xxx syntax, so we have to implement the syntax. for example, we can select with:

SELECT * FROM TABLE t AS OF TIMESTAMP '2020-02-02'

Implementation

SelectStmt is different with BeginStmt(#21897.). The read relative executors have a unified entry getSnapshotTS :

tidb/executor/builder.go

Lines 1358 to 1362 in 3eedd40

	// `getSnapshotTS` returns the timestamp of the snapshot that a reader should read.
	func (b *executorBuilder) getSnapshotTS() (uint64, error) {
	// `refreshForUpdateTSForRC` should always be invoked before returning the cached value to
	// ensure the correct value is returned even the `snapshotTS` field is already set by other
	// logics. However for `IndexLookUpMergeJoin` and `IndexLookUpHashJoin`, it requires caching the

It gets TS from the session variable or the txn context build from the Optimize stage. we may need to change the behavior:

tidb/planner/optimize.go

Lines 116 to 124 in 3eedd40

	if !useMaxTS(sctx, fp) {
	sctx.PrepareTSFuture(ctx)
	}
	return fp, fp.OutputNames(), nil
	}
	}
	sctx.PrepareTSFuture(ctx)

Tasks:

• Make executor builder can set the specify ts with the as of Clause
• Check multiple AS OF and make they are using the same TS. Our current framework is not very good to support one statement to specify multiple TS.