Execution plan of mysql

1. What is an execution plan?

Use the EXPLAIN keyword to simulate the optimizer executing SQL query statements, so as to know how MYSQL processes your SQL statements.

Execution planning process (best understood with the diagram)

The browser (client) sends a request. Before executing the database query, it finds the connection pool (recycling), connects with the database driver, finds the mysql service after obtaining the connection, then the connection pool finds the query cache, and then parses it, find the parse tree parsing, generate a new parse tree after preprocessing, query the optimizer after the new parse tree, execute sql, find the data file through the engine, read the data, return it to the cache, and finally return it to the user.

Some optimizers: (understand)

CBO: Cost-based optimizer, look at which index, and count the index according to the cost value.

RBO: Rule-based optimizer, with indexes using indexes.Then all tables with indexes will go through the index in any case

Second, the values ​​and parameters of the execution plan

Code

explain(select * from t_users id=1unionselect * from t_users id=2);

Running Results

• select_type: category, mainly used to distinguish complex queries (union) such as ordinary queries, union queries, sub-queries, etc.

1. simple: a simple select query without subqueries or unions

2. primary: The query contains any complex subparts, the outermost query is marked

3. derived: The subquery is used in the from list, the table is used as the query condition, and the subquery is also calculated

4. union: Included in the subquery of the from clause, a temporary table will be generated when the subquery occurs

• paritiions: If the table data volume is large, set the partition conditions.
• possible_keys: possible keys (indexes)
• key: the index actually used
• key_len: length of key used (bytes)
• ref: Shows which field or constant is used with the key
• rows: how much data to traverse to find
• type: is a more important indicator, the result value is (performance) in order from best to worst

system > const > eq_ref > ref > fulltext > ref_or_null > index_merge > index_subquery > range > index > all

Note: (marked in red means that the system has gone to the index)

• system: There is only one row in the table (equal to the system table);

• const means that it is found once by index;

• eq_ref: one-dimensional record;

• ref: non-unique index scan;

• range: retrieve only rows in a given range

• all: full table scan

• index: index coverage (scan all)

• null: execute without even accessing the table or index;

• Extra: Indicates important extra information that doesn't fit in other columns

1. Using index: Covering the index to avoid accessing the table.

2. Using temporary: sorting temporary table

3. Using where: filter after retrieval

How to optimize the performance of myql?

The server checks the query cache first, and if it hits the cache, it immediately returns the result stored in the cache.Otherwise go to the next stage.

If a column is indexed, try to avoid using the null value, because its value is more complex and it is not easy to create an index.