WL#6707: Umbrella: Refactor MySQL server parser to build the AST in a natural "bottom-up" way

Affects: Parser-Prototype Only   —   Status: In-Design   —   Priority: Medium

MySQL Server uses a Bison-generated parser to process the input SQL language.
Bison is a LALR(1) parser generator that makes bottom-up parsers.
However, by some historical reasons our parser grammar is designed to process
the input language is the unnatural top-down way.
This makes the grammar ambiguous (huge number of conflicts) and overcomplicated
by empty and duplicated rules. Semantic actions depend on many states and hard
to maintain. The parser grammar is complex to extend, parser bugs are hard to fix.

The suggestion of this WL is a transformation of the the current parser into the
pure stateless bottom-up parser step by step.

The resulting parser will output a self-contained parse tree that has a minimal
dependency on the rest of the Server.

Current data flow:

SQL input -> lex. scanner -> parser -> AST (SELECT_LEX, Items etc) -> executor

After this WL:

SQL input -> lex. scanner -> parser -> parse tree -> AST -> executor