SQL Parse In ClickHouse

原文链接

首先经过token进行切词，然后通过ast node来进行组织

可以发现，ast parser 的时候，预先构造好状态空间，比如 select 的状态空间:

1. expression list
2. from tables
3. where
4. group by
5. with …
6. order by
7. limit

在一个状态空间內，还可以根据 parse 返回的 bool 判断是否继续进入子状态空间，一直递归解析出整个 ast。

bool ParserSelectQuery::parseImpl(Pos & pos, ASTPtr & node, Expected & expected)
{
    auto select_query = std::make_shared<ASTSelectQuery>();
    node = select_query;

    ParserKeyword s_select("SELECT");
    ParserKeyword s_distinct("DISTINCT");
    ParserKeyword s_from("FROM");
    ParserKeyword s_prewhere("PREWHERE");
    ParserKeyword s_where("WHERE");
    ParserKeyword s_group_by("GROUP BY");
    ParserKeyword s_with("WITH");
    ParserKeyword s_totals("TOTALS");
    ParserKeyword s_having("HAVING");
    ParserKeyword s_order_by("ORDER BY");
    ParserKeyword s_limit("LIMIT");
    ParserKeyword s_settings("SETTINGS");
    ParserKeyword s_by("BY");
    ParserKeyword s_rollup("ROLLUP");
    ParserKeyword s_cube("CUBE");
    ParserKeyword s_top("TOP");
    ParserKeyword s_with_ties("WITH TIES");
    ParserKeyword s_offset("OFFSET");

    ParserNotEmptyExpressionList exp_list(false);
    ParserNotEmptyExpressionList exp_list_for_with_clause(false);
    ParserNotEmptyExpressionList exp_list_for_select_clause(true);  
  ...
}