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：
一、PostgreSQL代码分析，查询优化部分，canonicalize_qual
二、PostgreSQL代码分析，查询优化部分，pull_ands()和pull_ors()
三、PostgreSQL代码分析，查询优化部分，process_duplicate_ors
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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;
}