PostgreSQL源码分析 —— 除法运算符

我们分析一下在PostgreSQL数据库中，是如何实现除法操作的。其他的运算符操作与之类似。以下面的语句为例：

select 2/4;

源码分析

主流程如下：

exec_simple_query
--> pg_parse_query
--> pg_analyze_and_rewrite
--> pg_plan_queries
--> PortalStart
--> PortalRun
--> PortalDrop

解析部分

exec_simple_query
--> pg_parse_query   // 构造语法解析树SelectStmt-->ResTarget-->A_Expr--> raw_parser--> base_yyparse

对select 2/4的语法解析部分，比较简单。构造SelectStmt节点，只存在target部分，构造targetList字段， target为ResTarget，在ResTarget中构造val字段，为2/4的表达式节点表示A_Expr。在运算符的语法解析表示上，gram.y中定义的源码如下：

simple_select:SELECT opt_all_clause opt_target_listinto_clause from_clause where_clause group_clause having_clause window_clause{SelectStmt *n = makeNode(SelectStmt);n->targetList = $4;-- ...}
/*******************************************************************************	target list for SELECT******************************************************************************/
opt_target_list: target_list		{ $$ = $1; }| /* EMPTY */	{ $$ = NIL; };target_list:target_el	{ $$ = list_make1($1); }| target_list ',' target_el	{ $$ = lappend($1, $3); };
target_el:	a_expr{$$ = makeNode(ResTarget);$$->name = NULL;$$->indirection = NIL;$$->val = (Node *)$1;$$->location = @1;}
a_expr:		c_expr	{ $$ = $1; }/** These operators must be called out explicitly in order to make use* of bison's automatic operator-precedence handling.  All other* operator names are handled by the generic productions using "Op",* below; and all those operators will have the same precedence.** If you add more explicitly-known operators, be sure to add them* also to b_expr and to the MathOp list below.*/| '+' a_expr		%prec UMINUS{ $$ = (Node *) makeSimpleA_Expr(AEXPR_OP, "+", NULL, $2, @1); }| '-' a_expr		%prec UMINUS{ $$ = doNegate($2, @1); }| a_expr '+' a_expr{ $$ = (Node *) makeSimpleA_Expr(AEXPR_OP, "+", $1, $3, @2); }| a_expr '-' a_expr{ $$ = (Node *) makeSimpleA_Expr(AEXPR_OP, "-", $1, $3, @2); }| a_expr '*' a_expr{ $$ = (Node *) makeSimpleA_Expr(AEXPR_OP, "*", $1, $3, @2); }| a_expr '/' a_expr{ $$ = (Node *) makeSimpleA_Expr(AEXPR_OP, "/", $1, $3, @2); }

构造了一个A_Expr节点：

/** makeSimpleA_Expr -*		As above, given a simple (unqualified) operator name*/
A_Expr *makeSimpleA_Expr(A_Expr_Kind kind, char *name, Node *lexpr, Node *rexpr, int location)
{A_Expr	   *a = makeNode(A_Expr);a->kind = kind;a->name = list_make1(makeString((char *) name));a->lexpr = lexpr;a->rexpr = rexpr;a->location = location;return a;
}

语义分析阶段

exec_simple_query
--> pg_parse_query
--> pg_analyze_and_rewrite--> parse_analyze--> transformStmt--> transformSelectStmt     // transforms a Select Statement--> transformTargetList /* Turns a list of ResTarget's into a list of TargetEntry's. */--> transformTargetEntry--> transformExpr--> transformAExprOp--> makeTargetEntry--> make_op // 构造操作符表达式Expr节点OpExpr--> oper // search for a binary operator、--> ket_ok = make_oper_cache_key // 先在缓存中找, 找到返回--> DeconstructQualifiedName  // 获取操作符name, namespace--> fetch_search_path_array--> recomputeNamespacePath();--> if (key_ok)--> find_oper_cache_entry   --> binary_oper_exact // 如果缓存中没有找到--> OpernameGetOprid  // 查找pg_operator系统表找--> enforce_generic_type_consistency--> make_fn_arguments
--> pg_plan_queries--> standard_planner--> subquery_planner--> pull_up_subqueries--> preprocess_expression// 化简常量表达式，如果可以的话，immutable才行--> eval_const_expressions--> set_opfuncid--> simplify_function--> expand_function_arguments--> evaluate_function   // try to pre-evaluate a function call--> evaluate_expr   // 预执行一个常量表达式--> CreateExecutorState--> ExecInitExpr--> ExecInitExprRec--> ExecInitFunc--> ExecReadyExpr--> ExecEvalExprSwitchContext // 执行获取一个常量值--> ExecInterpExpr--> int4div  // 调用除法实现函数 计算结果--> makeConst // 将执行常量表达式获得的结果转换为Const常量节点返回--> grouping_planner--> query_planner--> build_simple_rel--> create_empty_pathtarget // 构造PathTarget--> create_group_result_path  // 生成GroupResultPath--> apply_scanjoin_target_to_paths--> create_projection_path--> create_plan--> create_projection_plan--> create_group_result_plan
--> PortalStart--> ExecutorStart--> InitPlan--> ExecInitResult
--> PortalRun--> ExecutorRun--> standard_ExecutorRun--> ExecutePlan--> ExecResult--> ExecProject
--> PortalDrop

在这个阶段，会找到/除法操作符的oid，即查系统表pg_operaotr，在函数oper中，我们下面查看一下系统表，查询一下除法/操作符，这个操作符你一定意义上可以认为是函数，除法函数。需要输入参数，输出结果。

PG文档：10.2. 操作符

postgres=# select * from pg_operator where oprname = '/';oid  | oprname | oprnamespace | oprowner | oprkind | oprcanmerge | oprcanhash | oprleft | oprright | oprresult | oprcom | oprnegate |    oprcode    | oprrest | oprjoin 
------+---------+--------------+----------+---------+-------------+------------+---------+----------+-----------+--------+-----------+---------------+---------+---------527 | /       |           11 |       10 | b       | f           | f          |      21 |       21 |        21 |      0 |         0 | int2div       | -       | -528 | /       |           11 |       10 | b       | f           | f          |      23 |       23 |        23 |      0 |         0 | int4div       | -       | -546 | /       |           11 |       10 | b       | f           | f          |      21 |       23 |        23 |      0 |         0 | int24div      | -       | -547 | /       |           11 |       10 | b       | f           | f          |      23 |       21 |        23 |      0 |         0 | int42div      | -       | -588 | /       |           11 |       10 | b       | f           | f          |     700 |      700 |       700 |      0 |         0 | float4div     | -       | -593 | /       |           11 |       10 | b       | f           | f          |     701 |      701 |       701 |      0 |         0 | float8div     | -       | -687 | /       |           11 |       10 | b       | f           | f          |      20 |       20 |        20 |      0 |         0 | int8div       | -       | -691 | /       |           11 |       10 | b       | f           | f          |      20 |       23 |        20 |      0 |         0 | int84div      | -       | -695 | /       |           11 |       10 | b       | f           | f          |      23 |       20 |        20 |      0 |         0 | int48div      | -       | -821 | /       |           11 |       10 | b       | f           | f          |      20 |       21 |        20 |      0 |         0 | int82div      | -       | -825 | /       |           11 |       10 | b       | f           | f          |      21 |       20 |        20 |      0 |         0 | int28div      | -       | -734 | /       |           11 |       10 | b       | f           | f          |     600 |      600 |       600 |      0 |         0 | point_div     | -       | -739 | /       |           11 |       10 | b       | f           | f          |     602 |      600 |       602 |      0 |         0 | path_div_pt   | -       | -807 | /       |           11 |       10 | b       | f           | f          |     603 |      600 |       603 |      0 |         0 | box_div       | -       | -844 | /       |           11 |       10 | b       | f           | f          |     790 |      700 |       790 |      0 |         0 | cash_div_flt4 | -       | -909 | /       |           11 |       10 | b       | f           | f          |     790 |      701 |       790 |      0 |         0 | cash_div_flt8 | -       | -3347 | /       |           11 |       10 | b       | f           | f          |     790 |       20 |       790 |      0 |         0 | cash_div_int8 | -       | -913 | /       |           11 |       10 | b       | f           | f          |     790 |       23 |       790 |      0 |         0 | cash_div_int4 | -       | -915 | /       |           11 |       10 | b       | f           | f          |     790 |       21 |       790 |      0 |         0 | cash_div_int2 | -       | -3825 | /       |           11 |       10 | b       | f           | f          |     790 |      790 |       701 |      0 |         0 | cash_div_cash | -       | -1118 | /       |           11 |       10 | b       | f           | f          |     700 |      701 |       701 |      0 |         0 | float48div    | -       | -1128 | /       |           11 |       10 | b       | f           | f          |     701 |      700 |       701 |      0 |         0 | float84div    | -       | -1519 | /       |           11 |       10 | b       | f           | f          |     718 |      600 |       718 |      0 |         0 | circle_div_pt | -       | -1585 | /       |           11 |       10 | b       | f           | f          |    1186 |      701 |      1186 |      0 |         0 | interval_div  | -       | -1761 | /       |           11 |       10 | b       | f           | f          |    1700 |     1700 |      1700 |      0 |         0 | numeric_div   | -       | -
(25 rows)

可以查到，select 2/4;对应的是oid=528,实现该操作符的函数是int4div，oprcode表示实现该操作符的函数，可通过查看系统表pg_proc查看该函数的详细信息：

postgres@postgres=# select * from pg_proc where proname='int4div';
-[ RECORD 1 ]---+--------
oid             | 154
proname         | int4div              -- 函数名
pronamespace    | 11
proowner        | 10
prolang         | 12
procost         | 1
prorows         | 0
provariadic     | 0
prosupport      | -
prokind         | f
prosecdef       | f
proleakproof    | f
proisstrict     | t
proretset       | f
provolatile     | i
proparallel     | s
pronargs        | 2
pronargdefaults | 0
prorettype      | 23                    -- 返回参数
proargtypes     | 23 23                 -- 入参
proallargtypes  | 
proargmodes     | 
proargnames     | 
proargdefaults  | 
protrftypes     | 
prosrc          | int4div               -- 函数source 
probin          | 
proconfig       | 
proacl          |

其中除法表达式A_Expr经过语义分析后，通过OpExpr进行表示：

/** OpExpr - expression node for an operator invocation** Semantically, this is essentially the same as a function call.* 语义上，等同于一个函数调用* Note that opfuncid is not necessarily filled in immediately on creation* of the node.  The planner makes sure it is valid before passing the node* tree to the executor, but during parsing/planning opfuncid can be 0.*/
typedef struct OpExpr
{Expr	xpr;Oid	opno;		/* PG_OPERATOR OID of the operator */   //对应pg_operator.oidOid	opfuncid;	/* PG_PROC OID of underlying function */ // 实现该操作符的函数oidOid	opresulttype;	/* PG_TYPE OID of result value */   // 返回值类型bool	opretset;	/* true if operator returns set */Oid	opcollid;	/* OID of collation of result */Oid	inputcollid;	/* OID of collation that operator should use */List	*args;		/* arguments to the operator (1 or 2) */  // 参数int	location;	/* token location, or -1 if unknown */
} OpExpr;

除法的实现函数：

Datum int4div(PG_FUNCTION_ARGS)
{int32		arg1 = PG_GETARG_INT32(0);int32		arg2 = PG_GETARG_INT32(1);int32		result;if (arg2 == 0){ereport(ERROR,(errcode(ERRCODE_DIVISION_BY_ZERO),errmsg("division by zero")));/* ensure compiler realizes we mustn't reach the division (gcc bug) */PG_RETURN_NULL();}/** INT_MIN / -1 is problematic, since the result can't be represented on a* two's-complement machine.  Some machines produce INT_MIN, some produce* zero, some throw an exception.  We can dodge the problem by recognizing* that division by -1 is the same as negation.*/if (arg2 == -1){if (unlikely(arg1 == PG_INT32_MIN))ereport(ERROR,(errcode(ERRCODE_NUMERIC_VALUE_OUT_OF_RANGE), errmsg("integer out of range")));result = -arg1;PG_RETURN_INT32(result);}/* No overflow is possible */result = arg1 / arg2;PG_RETURN_INT32(result);
}