try use max uint64 as tso for table/index reader which only has 1 cop task and in autocommit txn

[crazycs520]

Enhancement

The point-get query can use max uint64 as tso when only has 1 rpc request.

Similarly, try to use max uint64 as tso for table/index reader which only has 1 cop task, and in autocommit transaction.

Here is an example:

create table t (id int key, b int, c int, index idx(b));

For the following query, only read a small range of data, and in most cases, there will be only 1 cop RPC request, then we can use max uint64 as tso too, there will be some performance improvements for small queries.

select * from t where id > 1 and id < 10;

Drawback

This change may break linearizability, so this optimization can only be used when the tidb_guarantee_linearizability is disabled.