RFC - distributed SQL, part #1/N - AST

[tsafin]

First I need to update those who was not involved in our internal
discussions - idea to serialize to SQL for sending commands to
data nodes have been rejected, and we go full steam with normal
AST way:

• create AST for SQL query;
• serialize it/deserialize once we are about to transfer it elsewhere;
• There were doubts about complexities of implementing AST for
  statements beyond currently implemented SELECT/VIEW and triggers.
  But more experienced colleagues believe that it will be easy to
  create AST for them also, so we will try to implement it also.

Also, Mons wanted to look into code examples how AST manipulations
will look like, so please see additional appendix created.

Exporting AST as cdata is easy to do, but in this case it will be
inconvenient to manipulate AST nodes, before sending to cluster nodes.
So there is 2nd, more idiomatic approach suggested - to convert AST
to nested Lua object/tables. Which should simplify some massaging
and provide natural way to serialization to msgpack.

Appendix B - programmatic interfaces

SYNOPSIS

   local sql = require `sqlparser`

   -- parse, return ast, pass it back unmodified for execution

   local ast = sql.parse [[ select * from "table" where id > 10 limit 10 ]]
   assert(type(ast) == 'cdata')
   local ok = sql.execute(ast)

   -- free allocated memory, like box.unprepare but for ast
   sql.unparse(ast)

   -- raw access to cdata structures
   local cdata = ast.as_cdata()
   if cdata.ast_type == ffi.C.AST_TYPE_SELECT
      handle_select_stmt(cdata.select)
   end

   -- Lua access to structurs as Lua tables
   local tree = ast.as_table()
   if tree.type == AST_TYPE_SELECT
      handle_columns_list(tree.select.columns)
      handle_where_clause(tree.select.where)
      limit = tree.select.limit
   end
   -- massaging with tree data

   -- serialization
   local msgpack = require 'msgpack'
   local to_wire = msgpack.encode(tree)

   -- networking magics ...
   -- ... deserialization
   local table = msgpack.decode(from_wire)
   ast.set_tree(tree)
   sql.execute(ast)


Regards,
Timur

[tsafin]

Disregard this RFC - the fuller, better, more up-to-date version of it is in pull-request - #5535 a644162?short_path=ce7bd91#diff-ce7bd916acb982bf37b6388d9c0eb1c29e923f1d0ae157d4f26efc9040b7b1c7