txn: add document for read-consistency read tso optimization (#32806)

close pingcap/tidb#33159

docs/design/2022-03-03-rc-read-tso-optimization.md

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+# Read-Committed Read With Timestamp Check
+
+- Author(s): cfzjywxk
+- Last updated: March. 3, 2022
+- Discussion at:
+
+## Motivation
+
+For the `read-committed` isolation level, each read request in a single transaction will need to fetch a new `ts` to read the latest committed data.
+If the workload is a read-heavy one whose read QPS is significantly higher than writes or there're few read-write conflicts, fetching ts each time will increase the query lantecy.
+
+The new ts itself is used to ensure the most recent data will be returned, if the data version does not change frequently then it's unnecessary to fetch a new timestamp every time.
+The ts fetching could be processed in an optimistic way that ts fetching happens only when a more recent data version is encoutered, this saves the cost of many unncessary ts fetching.
+
+## Detailed Design
+
+Introduce another system variable `tidb_rc_read_check_ts` to enable the "lazy ts fetch" read mode. It will take effect for all the in-transaction select statements when the transaction mode is `pessimistic` and the isolation level is `read-committed`.
+
+The execution flow is like this:
+
+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).
+2. Do build the plan and executor like before.
+3. If the execution is successful, a `resultSet` will be returned.
+4. The connection layer will drive the response procesure and call `Next` function using the returned `resultSet`.
+5. The read executor will try to fetch data to fill in a data `chunk`. The bottommost executor could be a `pointGet`, `batchPointGet` or a coprocessor task. What's different is a `RcReadCheckTS` flag will be set on all these read requests.
+6. 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.
+7. For data write record, do check if it's newer than the current `read_ts`.
+8. For lock record, return `ErrKeyIsLocked` even though the `lock.ts` is greater than the `read_ts` as the `read_ts` could be a stale one.
+9. 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.
+
+
+## Compatibility
+
+The default behaviours of the `read-committed` isolation level will not change. One thing different is that if the user client uses `COM_STMT_FETCH` like utility to read data from `TiDB`,
+there could be problem if the returned first chunk result is already used by the client but an error is reported processing next result chunk.