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常量表达式化简
常量表达式可以进行化简，可降低执行器计算表达式的代价。在逻辑优化阶段，会判断是否可以进行常量表达式化简，如果可以，则在执行器执行之前就预先对常量表达式树进行计算，计算出常量后，以新计算出的常量表达式代替原有的表达式。当进入执行器时，此时表达式已被替换为常量，避免了在执行器中频繁的计算表达式。
化简示例
我们以下面的SQL为例，条件表达式为a > 1 + 1，是常量表达式，可以进行化简。
-- 常量表达式可以进行化简
postgres=# explain select * from t1 where a > 1 + 1; QUERY PLAN
-----------------------------------------------------Seq Scan on t1 (cost=0.00..17.50 rows=998 width=8)Filter: (a > 2) -- 进入执行器中， 表达式已被提前替换为常量，每扫描一个元组，不用再进行1+1的表达式计算了
(2 rows)
-- 不满足常量表达式化简的条件
postgres=# explain select * from t1 where a > 1 + random(); QUERY PLAN
------------------------------------------------------------------------Seq Scan on t1 (cost=0.00..25.00 rows=333 width=8)Filter: ((a)::double precision > ('1'::double precision + random())) -- 执行器中每扫描一个元组，都要进行一次表达式计算
(2 rows)
除了常量表达式，常量函数也可以在逻辑优化阶段提前执行，计算得到常量，用常量替换原有的函数表达式。
源码分析
我们以select * from t1 where a = 1 + 1;这条语句为例，分析一下在PostgreSQL中是如何进行化简的。主流程如下
exec_simple_query
--> pg_parse_query--> raw_parser--> base_yyparse
--> pg_analyze_and_rewrite--> transformStmt--> transformSelectStmt--> transformWhereClause--> transformExpr
--> pg_plan_queries--> pg_plan_query--> planner--> standard_planner--> subquery_planner--> preprocess_qual_conditions--> preprocess_expression--> eval_const_expressions // 在这里完成常量表达式化简，完成1+1表达式替换为常量2--> eval_const_expressions_mutator--> simplify_function--> evaluate_function--> evaluate_expr // 1+1的表达式被计算为常量2--> create_plan
--> PortalStart
--> PortalRun // 执行器执行
--> PortalDrop
可以看到，在逻辑优化阶段，常量表达式已被化简为常量，进入执行器时，表达式树已被替换为常量。下面为详细实现。
/** preprocess_qual_conditions* Recursively scan the query's jointree and do subquery_planner's* preprocessing work on each qual condition found therein.*/
static void preprocess_qual_conditions(PlannerInfo *root, Node *jtnode)
{if (jtnode == NULL)return;if (IsA(jtnode, RangeTblRef)){/* nothing to do here */}else if (IsA(jtnode, FromExpr)){FromExpr *f = (FromExpr *) jtnode;ListCell *l;foreach(l, f->fromlist)preprocess_qual_conditions(root, lfirst(l));f->quals = preprocess_expression(root, f->quals, EXPRKIND_QUAL);}else if (IsA(jtnode, JoinExpr)){// ...}elseelog(ERROR, "unrecognized node type: %d",(int) nodeTag(jtnode));
}/** preprocess_expression* Do subquery_planner's preprocessing work for an expression,* which can be a targetlist, a WHERE clause (including JOIN/ON* conditions), a HAVING clause, or a few other things.*/
static Node *preprocess_expression(PlannerInfo *root, Node *expr, int kind)
{// .../** Simplify constant expressions. For function RTEs, this was already* done by preprocess_function_rtes. */if (kind != EXPRKIND_RTFUNC)expr = eval_const_expressions(root, expr);/* If it's a qual or havingQual, canonicalize it. */if (kind == EXPRKIND_QUAL)expr = (Node *) canonicalize_qual((Expr *) expr, false);// .../** If it's a qual or havingQual, convert it to implicit-AND format. (We* don't want to do this before eval_const_expressions, since the latter* would be unable to simplify a top-level AND correctly. Also,* SS_process_sublinks expects explicit-AND format.)*/if (kind == EXPRKIND_QUAL)expr = (Node *) make_ands_implicit((Expr *) expr);return expr;
}/*--------------------* eval_const_expressions** Reduce any recognizably constant subexpressions of the given* expression tree, for example "2 + 2" => "4". More interestingly,* we can reduce certain boolean expressions even when they contain* non-constant subexpressions: "x OR true" => "true" no matter what* the subexpression x is. */
Node *eval_const_expressions(PlannerInfo *root, Node *node)
{eval_const_expressions_context context;if (root)context.boundParams = root->glob->boundParams; /* bound Params */elsecontext.boundParams = NULL;context.root = root; /* for inlined-function dependencies */context.active_fns = NIL; /* nothing being recursively simplified */context.case_val = NULL; /* no CASE being examined */context.estimate = false; /* safe transformations only */return eval_const_expressions_mutator(node, &context);
}/* Recursive guts of eval_const_expressions/estimate_expression_value */
static Node *eval_const_expressions_mutator(Node *node,eval_const_expressions_context *context)
{if (node == NULL)return NULL;switch (nodeTag(node)){case T_FuncExpr:{// ...}case T_OpExpr:{OpExpr *expr = (OpExpr *) node;List *args = expr->args;Expr *simple;OpExpr *newexpr;/* Need to get OID of underlying function. Okay to scribble on input to this extent. */set_opfuncid(expr);/** Code for op/func reduction is pretty bulky, so split it out* as a separate function. */simple = simplify_function(expr->opfuncid,expr->opresulttype, -1,expr->opcollid,expr->inputcollid,&args,false,true,true,context);if (simple) /* successfully simplified it */return (Node *) simple;/** If the operator is boolean equality or inequality, we know* how to simplify cases involving one constant and one* non-constant argument.*/if (expr->opno == BooleanEqualOperator ||expr->opno == BooleanNotEqualOperator){simple = (Expr *) simplify_boolean_equality(expr->opno,args);if (simple) /* successfully simplified it */return (Node *) simple;}/** The expression cannot be simplified any further, so build* and return a replacement OpExpr node using the* possibly-simplified arguments.*/newexpr = makeNode(OpExpr);newexpr->opno = expr->opno;newexpr->opfuncid = expr->opfuncid;newexpr->opresulttype = expr->opresulttype;newexpr->opretset = expr->opretset;newexpr->opcollid = expr->opcollid;newexpr->inputcollid = expr->inputcollid;newexpr->args = args;newexpr->location = expr->location;return (Node *) newexpr;}// ...default:break;}/** For any node type not handled above, copy the node unchanged but* const-simplify its subexpressions. This is the correct thing for node* types whose behavior might change between planning and execution, such* as CurrentOfExpr. It's also a safe default for new node types not* known to this routine.*/return ece_generic_processing(node);
}/** Subroutine for eval_const_expressions: try to simplify a function call* (which might originally have been an operator; we don't care)*/
static Expr *
simplify_function(Oid funcid, Oid result_type, int32 result_typmod,Oid result_collid, Oid input_collid, List **args_p,bool funcvariadic, bool process_args, bool allow_non_const,eval_const_expressions_context *context)
{// .../* Now attempt simplification of the function call proper. */newexpr = evaluate_function(funcid, result_type, result_typmod,result_collid, input_collid,args, funcvariadic,func_tuple, context);// ...return newexpr;
}/** evaluate_function: try to pre-evaluate a function call*/
static Expr *
evaluate_function(Oid funcid, Oid result_type, int32 result_typmod,Oid result_collid, Oid input_collid, List *args,bool funcvariadic,HeapTuple func_tuple,eval_const_expressions_context *context)
{// ...return evaluate_expr((Expr *) newexpr, result_type, result_typmod,result_collid);
}/** evaluate_expr: pre-evaluate a constant expression** We use the executor's routine ExecEvalExpr() to avoid duplication of* code and ensure we get the same result as the executor would get.*/
Expr *
evaluate_expr(Expr *expr, Oid result_type, int32 result_typmod,Oid result_collation)
{EState *estate;ExprState *exprstate;MemoryContext oldcontext;Datum const_val;bool const_is_null;int16 resultTypLen;bool resultTypByVal;/** To use the executor, we need an EState.*/estate = CreateExecutorState();/* We can use the estate's working context to avoid memory leaks. */oldcontext = MemoryContextSwitchTo(estate->es_query_cxt);/* Make sure any opfuncids are filled in. */fix_opfuncids((Node *) expr);/** Prepare expr for execution. (Note: we can't use ExecPrepareExpr* because it'd result in recursively invoking eval_const_expressions.)*/exprstate = ExecInitExpr(expr, NULL);/** And evaluate it.** It is OK to use a default econtext because none of the ExecEvalExpr()* code used in this situation will use econtext. That might seem* fortuitous, but it's not so unreasonable --- a constant expression does* not depend on context, by definition, n'est ce pas?*/const_val = ExecEvalExprSwitchContext(exprstate,GetPerTupleExprContext(estate),&const_is_null); // 计算表达式 1 + 1/* Get info needed about result datatype */get_typlenbyval(result_type, &resultTypLen, &resultTypByVal);/* Get back to outer memory context */MemoryContextSwitchTo(oldcontext);/** Must copy result out of sub-context used by expression eval.** Also, if it's varlena, forcibly detoast it. This protects us against* storing TOAST pointers into plans that might outlive the referenced* data. (makeConst would handle detoasting anyway, but it's worth a few* extra lines here so that we can do the copy and detoast in one step.)*/if (!const_is_null){if (resultTypLen == -1)const_val = PointerGetDatum(PG_DETOAST_DATUM_COPY(const_val));elseconst_val = datumCopy(const_val, resultTypByVal, resultTypLen);}/* Release all the junk we just created */FreeExecutorState(estate);/** Make the constant result node.*/return (Expr *) makeConst(result_type, result_typmod, result_collation,resultTypLen,const_val, const_is_null,resultTypByVal);
}
如果对源码不熟悉，可以补充看一下这里关于解析层的代码，有助于理解PostgreSQL源码中表达式处理相关的逻辑。
补充解析层相关代码
我们看一下在解析层表达式是如何表示的，1+1表示为A_Expr，类型为AEXPR_OP。where a = 1 + 1则是A_Expr，左子树为变量a，右子树为1+1的表达式A_Expr。
where_clause:WHERE a_expr { $$ = $2; }| /*EMPTY*/ { $$ = NULL; };
a_expr: c_expr { $$ = $1; }| a_expr '+' a_expr // 表示 1 + 1{ $$ = (Node *) makeSimpleA_Expr(AEXPR_OP, "+", $1, $3, @2); }| a_expr '=' a_expr{ $$ = (Node *) makeSimpleA_Expr(AEXPR_OP, "=", $1, $3, @2); }
c_expr: columnref { $$ = $1; }| AexprConst { $$ = $1; }
AexprConst: Iconst{$$ = makeIntConst($1, @1);}
Iconst: ICONST { $$ = $1; }; // 1 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;
}typedef struct A_Expr
{NodeTag type;A_Expr_Kind kind; /* see above */List *name; /* possibly-qualified name of operator */Node *lexpr; /* left argument, or NULL if none */Node *rexpr; /* right argument, or NULL if none */int location; /* token location, or -1 if unknown */
} A_Expr;/* A_Const - a literal constant */
typedef struct A_Const
{NodeTag type;Value val; /* value (includes type info, see value.h) */int location; /* token location, or -1 if unknown */
} A_Const;