"Go" MySQL in statement optimization

MySQL will optimize the SQL statement, and the index will still be used after a certain number of conditions.
MySQL determines whether the index is used based on the index length and the number of conditions in the back.


In addition, if the in is followed by a subquery, the index is not used.


An article library with two tables: category and article. There are 10 categorical data in category. There are 200,000 article inside. There is a "article_category" field in article that corresponds to the "category_id" field in the category. The article table has article_category meanings in order to index. The database size is 1.3G.
Problem Description:
To execute a very common query:

Select *  from Where article_category=oneorDERbyDESC5

The execution time is about 5 seconds or so.

Solution:
Build an index:

Create Index  on article (article_category,article_id);

Select *  from Where article_category=oneorDERbyDESC5

Reduced to 0.0027 seconds

To continue the question:

Select *  from Where inch (2,3OrDERbyDESC5

It takes 11.285 seconds to execute.
Use or:

Select *  from article where article_category=2or article_category=3Order  bydesc 5

Execution Time: 11.0777
Solution: Avoid using in or or (or cause a sweep), use UNION ALL

Use UNION ALL:

(Select *  fromArticlewhereArticle_category=2 Order  byarticle_iddescLimit5)UNION  All(Select *  fromArticlewhereArticle_category=3 Order  byarticle_iddescLimit5)OrDER  byarticle_iddescLimit5

Execution Time: 0.0261

"Go" MySQL in statement optimization