Some small knowledge points

（1） Use mysql Doing set operations

1. Combine union Union and collection

SELECT product_id, product_name

FROM product

UNION

SELECT product_id, product_name

FROM product2;

2. Difference set not in

SELECT *

FROM Product

WHERE product_id NOT IN (SELECT product_id

                        FROM Product2)

3. intersection inner join

SELECT p1.product_id, p1.product_name

FROM Product p1

INNER JOIN Product2 p2

ON p1.product_id=p2.product_id

4. Symmetry difference

SELECT *

FROM Product

WHERE product_id NOT IN (SELECT product_id FROM Product2)

UNION

SELECT *

FROM Product2

WHERE product_id NOT IN (SELECT product_id FROM Product)

5. Cartesian product cross join

The cartesian product ：

SELECT SP.shop_id

   ,SP.shop_name

   ,SP.product_id

   ,P.product_name

   ,P.sale_price

FROM ShopProduct AS SP

CROSS JOIN Product AS P;

（2）mysql Ways to break the deadlock

Set the timeout for transaction waiting

​innodb_lock_wait_timeout

2. Enable active deadlock detection

innodb_deadlock_detect

(3) View the latest deadlock information

show engine innodb status

(4)mysql To perform a sql Statement flow

The first step is actually to query the cache , however mysql8 Query cache canceled . Although query caching improves system performance , But the overhead of maintaining the cache is also relatively large

Parsing statements （ Query analysis ）

select title from order where id=1

① Lexical analysis Extract keywords , The above sentence sql Will be divided into 8 Parts of

stay elasticsearch The engine also has a dictionary to segment sentences

② Syntax analysis According to the result of lexical analysis , Generate syntax tree （ The parse tree ）, Judge sql Whether the statement satisfies the syntax rules .

If there is a syntax error in the statement, it will be displayed at this time ‘You have an error in your SQL syntax’

2. Search and check （ The preprocessor ）

① Semantic analysis

Form a semantic tree transformed from relational algebra

② Symbol name conversion

Bind variables to definitions in symbol table

If the field name and table name do not exist, an error will be reported UnKnow column ‘abc’ in ‘field list’

③ Safety inspection

​-  Autonomous access control 

​-  Forced access control 

④ Preliminary integrity check

​-  Referential integrity 

​-  Entity integrity 

​-  User defined integrity 

3. Query optimization （ Optimizer ）

mysql Use an overhead based optimizer , Choose a plan that costs less

mysql By querying the current session last_query_cost To calculate the cost of the current query

see last_query_cost The order of :show status like ‘last_query_cost’

The costs considered may be ： Number of pages per table or index , The cardinality of the index , Index and data row length . Index distribution, etc .

The optimization strategies are ：

① Redefine the order in which tables are associated , It depends on the size of the watch , Or it is possible to convert an outer connection to an inner connection , The subquery is transformed into a connection

② Optimize to find the maximum and minimum functions

③ Optimization of sorting

…

4. Query execution （ actuator ）

Generate an execution plan for the execution strategy obtained above , Let the code generator execute the query plan , Take the data from the storage engine and return it to the client