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PostgreSQL代码分析，查询优化部分。

这里把规范谓词表达式的部分就整理完了，阅读的顺序如下：

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pull_ands()和pull_ors()的代码比较便于理解，就是把树状结构的AND操作拉平，下图是pull_ands的例子，pull_ors逻辑相同：

/* * pull_ands *	  Recursively flatten nested AND clauses into a single and-clause list. * * Input is the arglist of an AND clause. * Returns the rebuilt arglist (note original list structure is not touched). */static List *pull_ands(List *andlist){	List	   *out_list = NIL;	ListCell   *arg;	foreach(arg, andlist)	{		Node	   *subexpr = (Node *) lfirst(arg);		/*		 * Note: we can destructively concat the subexpression's arglist		 * because we know the recursive invocation of pull_ands will have		 * built a new arglist not shared with any other expr. Otherwise we'd		 * need a list_copy here.		 */		if (and_clause(subexpr))			out_list = list_concat(out_list,								   pull_ands(((BoolExpr *) subexpr)->args));		else			out_list = lappend(out_list, subexpr);	}	return out_list;}/* * pull_ors *	  Recursively flatten nested OR clauses into a single or-clause list. * * Input is the arglist of an OR clause. * Returns the rebuilt arglist (note original list structure is not touched). */static List *pull_ors(List *orlist){	List	   *out_list = NIL;	ListCell   *arg;	foreach(arg, orlist)	{		Node	   *subexpr = (Node *) lfirst(arg);		/*		 * Note: we can destructively concat the subexpression's arglist		 * because we know the recursive invocation of pull_ors will have		 * built a new arglist not shared with any other expr. Otherwise we'd		 * need a list_copy here.		 */		if (or_clause(subexpr))			out_list = list_concat(out_list,								   pull_ors(((BoolExpr *) subexpr)->args));		else			out_list = lappend(out_list, subexpr);	}	return out_list;}