notes about isolations for database/sql

Author: asmyasnikov

SQL_MIGRATION_v2_v3.md

@@ -8,5 +8,7 @@
 
 In `ydb-go-sdk/v2/ydbsql` was allowed `sql.LevelReadCommitted` and `sql.LevelReadUncommitted` isolation levels for read-only interactive transactions. It implements with fake transaction with true `OnlineReadOnly` transaction control on each query inside transaction.
 
-`ydb-go-sdk/v3` allowed only `sql.LevelSnapshot` for read-only interactive transactions. Currently, snapshot isolation implements over fake transaction with true `OnlineReadOnly` transaction control.
+Transaction controls `OnlineReadOnly` and `StaleReadOnly` will deprecate in the future.
+
+That's why `ydb-go-sdk/v3` allowed only `sql.LevelSnapshot` for read-only interactive transactions. Currently, snapshot isolation implements over `fake` transaction with true `OnlineReadOnly` transaction control on each request inside transaction.
 YDB implements snapshot isolation, but this feature is not deployed on YDB clusters now. After full deploying on each YDB cluster fake transaction will be replaced to true read-only interactive transaction with snapshot isolation level.