GetResultForTest gets result, only for test.

InitTopNExecForTest initializes TopN executors, only for test.

NewDataCursor creates a new dataCursor.

SetSmallSpillChunkSizeForTest set spill chunk size for test.

TestKillSignalInTopN is for test.

SignalCheckpointForSort indicates the times of row comparation that a signal detection will be triggered.

SortExec represents sorting executor.

TopNExec implements a Top-N algorithm and it is built from a SELECT statement with ORDER BY and LIMIT.