Testing of optimizer hints

Our implementation of optimizer hints is intended to bring compatibility with the corresponding functionality of MySQL, which is described here.

Currently we have implemented a partial subset of the hints:

• hints for join buffering: BNL(), NO_BNL(), BKA(), NO_BKA();
• NO_ICP() hint for disabling index condition pushdown;
• MRR(), MO_MRR() hint for multi-range reads control;
• NO_RANGE_OPTIMIZATION() for disabling range optimization;
• QB_NAME() for assigning names for query blocks;
• SEMIJOIN() and NO_SEMIJOIN() for semijoin strategy control;
• SUBQUERY() for control of subquery execution strategy.

Examples of applying hints to SQL statements can be found at

mysql-test/main/opt_hints.test; mysql-test/main/opt_hint_timeout.test; mysql-test/main/opt_hints_subquery.test