Synchronous mutation and classic update/delete statement support

Mutation executes asynchronously in ClickHouse. And unlike other databases following SQL standard, it uses DDL instead of DML. As a result, this may break assumption in certain tools and generate overhead in downstream applications.

In order to follow standards, I hope we can:

1. introduce a new connection setting mutationsSync(same as mutations_sync in ClickHouse) defaults to 1, meaning synchronous mutation
2. <no longer needed as it’s going to be implemented on server-side> take classic update/delete and transform them into ALTER TABLE... for mutation in ClickHouse So if you give UPDATE tbl SET col1=1, col2=2 WHERE col3=3, it will be translated into ALTER TABLE tbl UPDATE col1=1, col2=2 WHERE col3=3.

[filimonov]

2 is a bad idea. Mutations were made this way (with special syntax) intentionally - current ALTER UPDATE / DELETE are very expensive operations, which should be used with care, not too often.

Drivers should NOT introduce any magic to pretend that clickhouse has updates or deletes (hopefully they will be added in 2021).

[zhicwu]

Before light-weight mutation in place, since multi-statement SQL is now supported, we can run SQL like below in one run - session will be enabled automatically if the SQL contains multiple statements:

set mutations_sync = 1;

ALTER TABLE tbl UPDATE col1=1, col2=2 WHERE col3=3;