How to force query plans in MariaDB knowledge base?
However in some cases it does not have enough information to choose a perfect plan and in these cases you may have to provide hints to force the optimizer to use another plan. You can examine the query plan for a SELECT by writing EXPLAIN before the statement. SHOW EXPLAIN shows the output of a running query.
When to show the explain output in MariaDB?
The output is always accompanied with a warning which shows the query the target thread is running (this shows what the EXPLAIN is for): The output can be only produced if the target thread is currently running a query, which has a ready query plan. If this is not the case, the output will be:
How to get the thread ID in MariaDB?
The thread id can be obtained with SHOW PROCESSLIST. The output is always accompanied with a warning which shows the query the target thread is running (this shows what the EXPLAIN is for):
What does the select column do in MariaDB?
The SELECT is a UNION of the PRIMARY. The result of the UNION. This column contains information on how the table is accessed. A full table scan is done for the table (all rows are read). This is bad if the table is large and the table is joined against a previous table!
Is there an optimizer switch in MariaDB 5.3?
In MariaDB 5.3 we added an optimizer switch which allows you to specify which algorithms will be considered when optimizing a query. See the optimizer section for more information about the different algorithms which are used. Copyright © 2021 MariaDB.
What happens if you don’t force index in MariaDB?
Without the FORCE INDEX option we would have ‘ Using where; Using temporary; Using filesort ‘ in the ‘Extra’ column, which means that the optimizer would created a temporary table and sort it. The optimizer uses several strategies to optimize GROUP BY and ORDER BY: Scan the table in index order and output data as we go.
How to create a temporary table in MariaDB?
Create a temporary table to hold the GROUP BY result with an index that matches the GROUP BY fields. If a row with the GROUP BY key exists in the temporary table, add the new result row to it. If not, create a new row. Before sending the results to the user, sort the rows with filesort to get the results in the GROUP BY order.