rc_check_ts question

1. If the query is executed first time, do not fetch a new for_update_ts and just use the last valid one(start_ts for the first time).
Q: what is last valid? the for_update_ts for last SQL?

2.The read executor in the storage layer will check the read results, if a more recent version does exist then a WriteConflict error is reported.
Q: How executor check if there is a new version? if more check will impact performance?

3.If no error is returned then the query is finished. Otherwise if there's no chunk responsed to the client yet, retry the whole query fetching a new global ts to do the read like normal.

Q: if some datas has send to client, client will report error and exit?

_Originally posted by @311ybb in https://github.com/pingcap/tidb/issues/32806#issuecomment-1126682583_