根節點至少2顆子樹（若存在子樹的情況下）；
非根節點至少擁有m/2顆子樹，其範圍為m/2 <= childnum(x) <= m；
所有葉子節點都在同一層，且為null；
有k顆子樹的節點，其關鍵字數為k-1，ceil(m/2)-1 <= keynum(x) <= m-1;
m/2+(m/2)^2+...+(m/2)^(h-1) = n
\frac = n
(m/2)^(h-1) = n+1
h = (log_(m/2) (n+1))+1
m+(m)^2+...+(m)^(h-1) = n
\frac = n
(m)^(h-1) = n+1
h = (log_(m) (n+1))+1
有n顆子樹的節點有n個關鍵字；
所有內節點僅存放索引，資料全部儲存在葉子節點上。
紅黑樹高度為35；
show index from pre_sales_rfq其中id為主鍵，(id, name,create_date)聯合索引
explain select * from index_test where id = 1
and name = 'jackshawn'
and create_date = '2017-09-21'
explain select * from pre_sales_rfq where project_id = 1
and item_id = 1
explain select * from pre_sales_project_rfq where item_id = 1
and project_id = 1
explain select * from pre_sales_project_rfq where project_id = 1
(id, name,create_date)聯合索引
explain select * from index_test where id = 1
and create_date = '2017-09-21'
explain select * from pre_sales_project_rfq where item_id = 1
explain select * from index_test where id = 1
and name ='%jack'
explain select * from index_test where id = 1
and name ='jack%'
explain select * from index_test where id > 1
and name ='jackshawn'
explain select * from index_test where id in(1,4) and name ='jackshawn'
explain select * from index_test where id between 1
and4
and name ='jackshawn'
explain select * from index_test where id -1 = 1
and name ='jackshawn'
## 乙個欄位不重複的總量與資料總量的比值，越**擇性越好。
select
count(distinct(key))/count(*) as selectivity from
table;
## 舉例如下擷取last_name前4個字元
alter
table employees.employees
add index `first_name_last_name4` (first_name, last_name(4));
select
count(distinct(concat(first_name, left(last_name, 4))))/count(*) as selectivity from employees.employees;
mysql之count效能比較
1 count 1 count count 列 使用場景 set profiling 1 select sql no cache count from t pay process record select sql no cache count 1 from t pay process record...
唯一索引和普通索引效能比較
前提條件 有乙個資料列不會有重複資料現在需要選擇建立索引，我們應該如何去選擇呢？1.普通索引 查詢時，遇第乙個不符合條件的才返回。更新時，找到位置直接更新 2.唯一性索引 查詢時，遇到第乙個符合條件的直接返回。更新時，先查詢是否唯一，再找到位置然後再更新。我們來通過底層分析一下 select 我們都...
redis memcache 效能比較
from redis和memcache非常像的，都是key,value的方式，將資料存放記憶體中。最近在學習redis，在網上看了一些這方面的資料，有三種觀點 1,redis讀寫記憶體比memcache快 2,memcache讀寫記憶體比redis快 3,memcache讀寫記憶體比redis快，但...