I am looking to compare indexes and further optimize my code. What I would like to do is force the query to run without an index so I can see what difference it has made. Is it possible to do this?
Yes, you can ask the query to not use indexes by adding IGNORE INDEX (index1, index2, ...) to your query. More details can be found here.
Note: I know this is an old question and you would already have found the answer, but I hope this will help others looking for the same answer, and they wouldn't have to spend their valuable time digging through the MySQL docs.
Related
I found this picture in a document. It shows the queries running on a given SQL database. It looks similar to DBeaver software. However, I couldn't find a way to get to this screen from DBeaver. Can someone please help me to figure out what software this is? It would be really helpful to troubleshoot performance-related issues.
That's MySQL's SHOW PROCESSLIST.
IN ( SELECT ... ) often optimizes poorly; try to rewrite using a JOIN.
This may help:
INDEX(status, Calendar_Date, Date_Received)
For more help we need to see the queries and SHOW CREATE TABLE. You can obfuscate the names, but don't make it too hard to read.
{The TIMESTAMP qualifier is not required.)
Frameworks (eg, DBeaver) are handy for getting started. But ultimately you need to understand the underlying database.
Is there any way to know how many rows where actually scanned in a sql?
I don't mean using explain.
MySQL introduces a new command EXPLAIN ANALYZE in version 8.0.18. Run
EXPLAIN ANALYZE your_query
to see a detailed execution plan that was executed to run the query.
Is it possible to find out how often an index of a MySQL table was used?
I have several indices for a table and I'd like to find out, if there are indice which are not used by MySQL.
Yes, it is. You should query Performance Schema:
select * from performance_schema.table_io_waits_summary_by_index_usage
where object_schema = 'your_schema'
The count_star column show how many times each index was used since MySQL was started. If you add the following, you got the never used indexes:
and count_star = 0
Addition to #user1970667's answer, you may also use:
select * from sys.schema_unused_indexes;
to get a list of unused indexes.
NOTE: This answer is no longer valid as of 5.5.3!
See https://stackoverflow.com/a/43205887/1251127
Original answer below.
Currently, MySQL does not offer statistics on index usage.
One way to generate those stats on your own would be to log all queries (Be careful of the extra i/o here) and parse it out. Percona has a nice tool for this, pt-index-usage.
Ive been using PHP and MySQL for a while now and I still don't understand when I should add Indexes to colums in my MySQL tables to increase performance. Are there any simple resources to help explain this to me?
In short, anything that will appear in a WHERE or ON clause should probably have an index on it. As well, you can use MySQL's EXPLAIN command to see how it actually executes the query which can point out where indexes are needed.
is there a way to preview a mysql queries results before processing?
If you're asking whether it's possible to execute MySQL commands, look at the results and decide whether you actually want these changes, look at transactions.
http://dev.mysql.com/doc/refman/5.0/en/commit.html
I've never done it but my guess would be you could create a temporary table from the table in question and run the query on that table. Hopefully this gets you going in the right direction.
It depends on what you want to achieve, if you want to know what mysql will do with your query you might be interessted in
http://dev.mysql.com/doc/refman/5.0/en/explain.html
Turn autocommit off, run your query and then rollback. It's universal and I know of no other way that's as simple as this.