docs: RFC for better staleness read syntax.

docs/design/2021-01-24-staleness-read-syntax.md

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+# Proposal: Change timestamp bounds read syntax for read only transaction
+
+- Author(s):     [Xiaoguang Sun](https://github.com/sunxiaoguang)
+- Last updated:  2021-01-24
+- Discussion at: https://github.com/pingcap/tidb/issues/22505
+
+## Abstract
+
+This proposal proposes new syntax to staleness read and deprecates existing timestamp bounds read SQL syntax which is over complicate.
+
+## Background
+
+Existing timestamp bound read SQL syntax is verbose and complicate. 
+
+```sql
+START TRANSACTION READ ONLY WITH TIMESTAMP BOUND
+START TRANSACTION READ ONLY WITH TIMESTAMP BOUND STRONG
+START TRANSACTION READ ONLY WITH TIMESTAMP BOUND MAX STALENESS '00:00:10'
+START TRANSACTION READ ONLY WITH TIMESTAMP BOUND EXACT STALENESS '00:00:05'
+START TRANSACTION READ ONLY WITH TIMESTAMP BOUND READ TIMESTAMP '2019-11-04 00:00:00'
+START TRANSACTION READ ONLY WITH TIMESTAMP BOUND MIN READ TIMESTAMP '2019-11-04 00:00:00'
+START TRANSACTION READ ONLY WITH TIMESTAMP BOUND MIN TIMESTAMP '2019-11-04 00:00:00'
+```
+
+Since timestamp bounds read feature is never implemented on TiDB, it is a good time to fix it with a better design and make it more ergonomics.
+
+## Proposal
+
+Make it clear that staleness read can be used only when transaction is read-only. If the transaction contains any writes an error will be returned. Therefore ```READ ONLY``` and ```TIMESTAMP BOUND``` are unnecessary and can be removed. The new statements will become shorter:
+
+```sql
+START TRANSACTION WITH MAX STALENESS '00:00:10'
+START TRANSACTION WITH EXACT STALENESS '00:00:05'
+START TRANSACTION WITH TIMESTAMP '2019-11-04 00:00:00'
+```
+
+
+## Rationale
+
+```sql
+START TRANSACTION READ ONLY WITH TIMESTAMP BOUND
+START TRANSACTION READ ONLY WITH TIMESTAMP BOUND STRONG
+```
+
+These statements are equivalent to START TRANSACTION READ ONLY, therefore can be removed.
+
+```sql
+START TRANSACTION READ ONLY WITH TIMESTAMP BOUND MAX STALENESS '00:00:10'
+START TRANSACTION READ ONLY WITH TIMESTAMP BOUND EXACT STALENESS '00:00:05'
+START TRANSACTION READ ONLY WITH TIMESTAMP BOUND READ TIMESTAMP '2019-11-04 00:00:00'
+```
+
+The redundant words ```READ ONLY``` and ```TIMESTAMP BOUND``` as well as ```READ``` are removed for clarity.
+
+```sql
+START TRANSACTION READ ONLY WITH TIMESTAMP BOUND MIN READ TIMESTAMP '2019-11-04 00:00:00'
+START TRANSACTION READ ONLY WITH TIMESTAMP BOUND MIN TIMESTAMP '2019-11-04 00:00:00'
+```
+
+These statements are removed as they are obscured and useless.
+
+## Compatibility and Mirgration Plan
+Since existing syntax is never implemented on TiDB, it is unlikely to be used by anyone. We can simply remove current syntax and only support the new syntax.