What is REF IN explain in MySQL?
What is REF IN explain in MySQL?
ref – Shows the columns or constants that are compared to the index named in the key column. MySQL will either pick a constant value to be compared or a column itself based on the query execution plan. For a complete list of possible values and their meaning, check out the MySQL documentation.
What is filtered column in MySQL explain?
The filtered column indicates an estimated percentage of table rows that will be filtered by the table condition. That is, rows shows the estimated number of rows examined and rows × filtered / 100 shows the number of rows that will be joined with previous tables.
How do you optimize MySQL query using explain?
Optimizing Database Schema
- Limiting the number of columns: MySQL has a limit of 4096 columns per table.
- Normalize Tables: Normalizing keeps all data non-redundant.
- Use the Most Appropriate Data Types: There are more than 20 different data types in MySQL designed for different uses.
- Avoid Null Values.
How MySQL works explain?
The EXPLAIN statement provides information about how MySQL executes statements:
- EXPLAIN works with SELECT , DELETE , INSERT , REPLACE , and UPDATE statements.
- When EXPLAIN is used with an explainable statement, MySQL displays information from the optimizer about the statement execution plan.
What is query explain?
1 : to ask questions of especially with a desire for authoritative information. 2 : to ask questions about especially in order to resolve a doubt. 3 : to put as a question.
What is explain extended?
The EXPLAIN statement produces extra (“extended”) information that is not part of EXPLAIN output but can be viewed by issuing a SHOW WARNINGS statement following EXPLAIN . As of MySQL 8.0. 12, extended information is available for SELECT , DELETE , INSERT , REPLACE , and UPDATE statements. Prior to 8.0.
What is filtered in explain?
What is MySQL explain with example?
The EXPLAIN keyword is used to obtain information about how our SQL databases execute the queries in MySQL. It is synonyms to the DESCRIBE statement. In practice, the DESCRIBE keyword provides table structure information, whereas the EXPLAIN keyword gives the query execution plan.
What is a column in MySQL?
Columns in the table are a series of cells that can stores text, numbers, and images. Every column stores one value for each row in a table. When we work with the MySQL server, it is common to display the column information from a particular table.
What is explain keyword in SQL?
The EXPLAIN keyword is used throughout various SQL databases and provides information about how your SQL database executes a query. Instead of the usual result output, MySQL would then show its statement execution plan by explaining which processes take place in which order when executing the statement.
What is the difference between Refref and EQ_ref In SQL?
Ref: represents the join matching criteria of the above table, that is, which columns or constants are used to find values on index columns Eq_ Ref: similar to ref, the difference is that the index used is a unique index. For each index key value, there is only one record matching in the table.
What does the filtered column mean in InnoDB tables?
For InnoDB tables, this number is an estimate, and may not always be exact. The filtered column indicates an estimated percentage of table rows that are filtered by the table condition. The maximum value is 100, which means no filtering of rows occurred.
What is the filtered column in the table condition?
The filtered column indicates an estimated percentage of table rows that are filtered by the table condition. The maximum value is 100, which means no filtering of rows occurred.
What are the possible values of the columns returned by query?
The columns returned by the query are: id – a sequential identifier for each SELECT within the query (for when you have nested subqueries) select_type – the type of SELECT query. Possible values are: SIMPLE – the query is a simple SELECT query without any subqueries or UNION s PRIMARY – the SELECT is in the outermost query in a JOIN
