老叶茶馆|ClickHouse和他的朋友们（８）纯手工打造的SQL解析器

_本文原题：ClickHouse和他的朋友们（８）纯手工打造的SQL解析器
原文出处：https://bohutang.me/2020/07/25/clickhouse-and-friends-parser/
现实生活中的物品一旦被标记为“纯手工打造” ，给人的第一感觉就是“上乘之品” ，一个字“贵” ，比如北京老布鞋。
但是在计算机世界里，如果有人告诉你 ClickHouse 的 SQL 解析器是纯手工打造的，是不是很惊讶！这个问题引起了不少网友的关注，所以本篇聊聊 ClickHouse 的纯手工解析器，看看它们的底层工作机制及优缺点。
枯燥先从一个 SQL 开始:
EXPLAIN SELECT a,b FROM t1
token
首先对 SQL 里的字符逐个做判断，然后根据其关联性做 token 分割：

本文插图
比如连续的 WordChar ，那它就是 BareWord ，解析函数在 Lexer::nextTokenImpl ，解析调用栈：
DB::Lexer::nextTokenImpl Lexer.cpp:63
DB::Lexer::nextToken Lexer.cpp:52
DB::Tokens::operator[](unsigned long) TokenIterator.h:36
DB::TokenIterator::get TokenIterator.h:62
DB::TokenIterator::operator-> TokenIterator.h:64
DB::tryParseQuery(DB::IParser&, char const*&, char const*, std::__1::basic_string<char, std::__1::char_traits<char>, std::__1::allocator<char> >&, bool, std::__1::basic_string<char, std::__1::char_traits<char>, std::__1::allocator<char> > const&, bool, unsigned long, unsigned long) parseQuery.cpp:224
DB::parseQueryAndMovePosition(DB::IParser&, char const*&, char const*, std::__1::basic_string<char, std::__1::char_traits<char>, std::__1::allocator<char> > const&, bool, unsigned long, unsigned long) parseQuery.cpp:314
DB::parseQuery(DB::IParser&, char const*, char const*, std::__1::basic_string<char, std::__1::char_traits<char>, std::__1::allocator<char> > const&, unsigned long, unsigned long) parseQuery.cpp:332
DB::executeQueryImpl(const char *, const char *, DB::Context &, bool, DB::QueryProcessingStage::Enum, bool, DB::ReadBuffer *) executeQuery.cpp:272
DB::executeQuery(DB::ReadBuffer&, DB::WriteBuffer&, bool, DB::Context&, std::__1:: function<void (std::__1::basic_string<char, std::__1::char_traits<char>, std::__1::allocator<char> > const&, std::__1::basic_string<char, std::__1::char_traits<char>, std::__1::allocator<char> > const&, std::__1::basic_string<char, std::__1::char_traits<char>, std::__1::allocator<char> > const&, std::__1::basic_string<char, std::__1::char_traits<char>, std::__1::allocator<char> > const&)>) executeQuery.cpp:731
DB::MySQLHandler::comQuery(DB::ReadBuffer&) MySQLHandler.cpp:313
DB::MySQLHandler::run MySQLHandler.cpp:150
ast
token 是最基础的元组，他们之间没有任何关联，只是一堆生冷的词组与符号，所以我们还需对其进行语法解析，让这些 token 之间建立一定的关系，达到一个可描述的活力。
ClickHouse 在解每一个 token 的时候，会根据当前的 token 进行状态空间进行预判(parse 返回 true 则进入子状态空间继续) ，然后决定状态跳转，比如：分页标题
EXPLAIN -- TokenType::BareWord
逻辑首先会进入Parsers/ParserQuery.cpp 的 ParserQuery::parseImpl 方法：
bool res = query_with_output_p.parse(pos, node, expected)
|| insert_p.parse(pos, node, expected)
|| use_p.parse(pos, node, expected)
|| set_role_p.parse(pos, node, expected)
|| set_p.parse(pos, node, expected)
|| system_p.parse(pos, node, expected)
|| create_user_p.parse(pos, node, expected)
|| create_role_p.parse(pos, node, expected)
|| create_quota_p.parse(pos, node, expected)
|| create_row_policy_p.parse(pos, node, expected)
|| create_settings_profile_p.parse(pos, node, expected)
|| drop_access_entity_p.parse(pos, node, expected)
|| grant_p.parse(pos, node, expected);
这里会对所有 query 类型进行 parse 方法的调用，直到有分支返回 true 。
我们来看第一层query_with_output_p.parse Parsers/ParserQueryWithOutput.cpp：
bool parsed =
explain_p.parse(pos, query, expected)
|| select_p.parse(pos, query, expected)
|| show_create_access_entity_p.parse(pos, query, expected)
|| show_tables_p.parse(pos, query, expected)
|| table_p.parse(pos, query, expected)
|| describe_table_p.parse(pos, query, expected)
|| show_processlist_p.parse(pos, query, expected)
|| create_p.parse(pos, query, expected)
|| alter_p.parse(pos, query, expected)
|| rename_p.parse(pos, query, expected)
|| drop_p.parse(pos, query, expected)
|| check_p.parse(pos, query, expected)
|| kill_query_p.parse(pos, query, expected)
|| optimize_p.parse(pos, query, expected)
|| watch_p.parse(pos, query, expected)
|| show_access_p.parse(pos, query, expected)
|| show_access_entities_p.parse(pos, query, expected)
|| show_grants_p.parse(pos, query, expected)
|| show_privileges_p.parse(pos, query, expected
跳进第二层explain_p.parse ParserExplainQuery::parseImpl状态空间：
bool ParserExplainQuery::parseImpl(Pos & pos, ASTPtr & node, Expected & expected)
{
ASTExplainQuery::ExplainKind kind;
bool old_syntax = false;
ParserKeyword s_ast( "AST");
ParserKeyword s_analyze( "ANALYZE");
ParserKeyword s_explain( "EXPLAIN");
ParserKeyword s_syntax( "SYNTAX");
ParserKeyword s_pipeline( "PIPELINE");
ParserKeyword s_plan( "PLAN");
... ...
elseif(s_explain.ignore(pos, expected))
{
... ...
}
... ...
ParserSelectWithUnionQuery select_p;
ASTPtr query;
if(!select_p.parse(pos, query, expected))
returnfalse;
... ...
s_explain.ignore 方法会进行一个 keyword 解析，解析出 ast node:
EXPLAIN -- keyword
跃进第三层select_p.parse ParserSelectWithUnionQuery::parseImpl状态空间：
bool ParserSelectWithUnionQuery::parseImpl(Pos & pos, ASTPtr & node, Expected & expected)
{
ASTPtr list_node;
ParserList parser(std::make_unique<ParserUnionQueryElement>, std::make_unique<ParserKeyword>( "UNION ALL"), false);分页标题
if(!parser.parse(pos, list_node, expected))
returnfalse;
...
parser.parse 里又调用第四层ParserSelectQuery::parseImpl 状态空间：
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);
...
if(!exp_list_for_select_clause.parse(pos, select_expression_list, expected))
returnfalse;
第五层exp_list_for_select_clause.parse ParserExpressionList::parseImpl状态空间继续：
bool ParserExpressionList::parseImpl(Pos & pos, ASTPtr & node, Expected & expected)
{
returnParserList(
std::make_unique<ParserExpressionWithOptionalAlias>(allow_alias_without_as_keyword),
std::make_unique<ParserToken>(TokenType::Comma))
.parse(pos, node, expected);
}
... ... 写不下去个鸟！
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:

expression list
from tables
where
group by
with ...
order by
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 。
总结
手工 parser 的好处是代码清晰简洁，每个细节可防可控，以及友好的错误处理，改动起来不会一发动全身。缺点是手工成本太高，需要大量的测试来保证其正确性，还需要一些fuzz来保证可靠性。好在ClickHouse 已经实现的比较全面，即使有新的需求，在现有基础上修修补补即可。
文内链接

https://github.com/ClickHouse/ClickHouse/blob/558f9c76306ffc4e6add8fd34c2071b64e914103/src/Parsers/ParserQuery.cpp#L26
https://github.com/ClickHouse/ClickHouse/blob/558f9c76306ffc4e6add8fd34c2071b64e914103/src/Parsers/ParserQueryWithOutput.cpp#L31
https://github.com/ClickHouse/ClickHouse/blob/558f9c76306ffc4e6add8fd34c2071b64e914103/src/Parsers/ParserExplainQuery.cpp#L10
https://github.com/ClickHouse/ClickHouse/blob/558f9c76306ffc4e6add8fd34c2071b64e914103/src/Parsers/ParserSelectWithUnionQuery.cpp#L26分页标题
https://github.com/ClickHouse/ClickHouse/blob/558f9c76306ffc4e6add8fd34c2071b64e914103/src/Parsers/ParserSelectQuery.cpp#L24
https://github.com/ClickHouse/ClickHouse/blob/558f9c76306ffc4e6add8fd34c2071b64e914103/src/Parsers/ExpressionListParsers.cpp#L520

【老叶茶馆|ClickHouse和他的朋友们（８）纯手工打造的SQL解析器】Enjoy ClickHouse :)