使用两个LEFT JOIN时查询加载速度非常慢

Question

下面是我的查询2 LEFT JOIN

$customers_query_raw = "select 
c.*, c.customers_shopping_points as c_points, 
a.entry_state, a.entry_zone_id, a.entry_country_id, 
ci.customers_info_date_account_created, 
cc.credit from " . TABLE_CUSTOMERS . " c 
left join " . TABLE_ADDRESS_BOOK . " a on c.customers_id = a.customers_id and c.customers_default_address_id = a.address_book_id 
left join ".TABLE_CUSTOMERS_CREDIT." cc on c.customers_id = cc.customer_id
, " . TABLE_CUSTOMERS_INFO . " ci 
where (c.customers_type = 0 or c.customers_type = 1 or c.customers_type = 2) and ci.customers_info_id = c.customers_id 
order by ".$order; 

结果仅在43.53秒后显示（非常慢），但如果我删除此行

left join ".TABLE_CUSTOMERS_CREDIT." cc on c.customers_id = cc.customer_id

结果将在1.61秒后显示（正常），并确定此行是主要问题。

下面是TABLE_CUSTOMERS_CREDIT

的表结构

Column              Type           
customers_credit_id int(11)              
customer_id         int(11)              
credit              decimal(15,2)            
date_added          datetime             
last_modified_by    int(11)              
comment             varchar(255)             
order_id            int(11)              

我的索引是customers_id