PostgreSQL代码分析，查询优化部分，pull_ands()和pull_ors()

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，阅读的顺序例如以下：

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pull_ands()和pull_ors()的代码比較便于理解，就是把树状结构的AND操作拉平，下图是pull_ands的样例，pull_ors逻辑同样：

/*
 * 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;
}