[mysql] mysql optimization strategy

The case comes from "The Way of MySQL Management"

not in subquery optimization

not in followed by a subquery

select 
  SQL_NO_CACHE count(*) 
from 
  test1 
where 
  id not in(
             select 
               id 
             from 
               test2
           )

Related queries with left join

select 
  SQL_NO_CACHE 
from 
  test1 
left join 
  test2 
on 
  test1.id=test2.id 
where test2.id is null

Using relational query is better than subquery

Match patterns like '%xxx%' optimized

'xxx%' can use index, '%xxx%' cannot use index, so try to use 'xxx%'.
If you must use index and fuzzy query, you can use the following example:

select 
  count(*) 
from 
  artist a 
join (
       select 
         artist_id 
       from 
         artist 
       where 
         name like '%xxx%'
      ) b
on
  a.artist_id=b.artist_id

Among them, artist_id is the index of the artist table

limit paging optimization

raw query

select
  SQL_NO_CACHE *
from
  test1
order by 
  id
limit 99999, 10

Although the above sql uses an index, the first row is located at row 99999, and the next 10 rows are scanned, which is equivalent to a full table scan.
The optimization is as follows:

select
  SQL_NO_CACHE *
from
  test1
where
  id >= 100000
order by id
limit 10

In this way, the efficiency has been significantly improved.
There a way to optimize through the connection in the table.

select
  SQL_NO_CACHE *
from
  test1 a
join (
       select
         SQL_NO_CACHE *
       from
         test1 b
       order by id
       limit 99999,1
     )b
on
  a.id >= b.id
limit 10

First take out the one after 99999, and then take out the next 10, which can also improve the efficiency

How to optimize count(*)

count(auxiliary index) is faster than count(*)
count(distinct) works better
- or Conditional Optimization
  Ordinary use of or will not use the index, such as:

select
  *
from
  test1
where
  name='a'
or
  age > 17

You can use union all optimizations:

select
  *
from 
  test1
where 
  name='a'
union all
select
  *
from 
  test1
where 
  age > 17

Such a query can use the index to speed up the query

ON DUPLICATE KEY UPDATE 用法

This is a very efficient primary key conflict processing judgment in mysql. If there is a conflict, execute update, and if there is no conflict, execute insert, such as:

insert into
  gg
values
  (
    3,
    'd'
  )
ON DUPLICATE KEY UPDATE
  id = id+1

This sql means that if the value of the primary key id 3 already conflicts when inserting data, then the 3 in the database will be updated to 4