Tired of MySQL Making You Wait? Webinar: Questions and Answers

MySQLWe’d like to thank everybody for joining us on January 7th for our “Tired of MySQL Making You Wait?” webinar with Percona’s Alexander Rubin, Principal Consultant and SolarWinds’ Janis Griffin, Database Evangelist.

Too often developers and DBAs struggle to pinpoint the root cause of performance issues and then spend too much time in trying to fix them. In the webinar, we discussed how you can significantly increase the performance of your applications while also reducing database response time.

You can find an archived version of the webinar here.

Below are the questions that were asked during the webinar, with responses from Alex and Janis. If you need further clarification, please respond in the comments.

Thanks again, and we look forward to you joining us at our next webinar (with Percona CEO Peter Zaitsev), Compression In Open Source Databases!


Q: Are there special tuning tips for Galera Cluster?

A: Since Galera Cluster (Percona XtraDB Cluster) is based on MySQL, all query tuning tips will apply as well. There are a number of Galera Cluster configuration tips available: for example the blog post at this link talks about tuning the PXC for maximum write throughput: https://www.percona.com/blog/2015/06/03/optimizing-percona-xtradb-cluster-write-hotspots/


Q: Does DPA support Galera Cluster ?

A: Yes, DPA has the ability to group the cluster together to see load balancing, top 15 SQLs across the cluster, plus the top wait states.


Q: Can I create a covered index when I have “group by” and “order by” instructions together?

A: Yes, you can create a covered index and MySQL will use it to satisfy the query (you will see “using index”). If you have “group by” and “order by” on a different columns, however, MySQL will still have to perform a filesort and create a temp table. To create this index, specify all the following fields in your query in the index:

  1. All fields in the “where” condition
  2. The “group by” fields
  3. The “order by” fields
  4. The fields that the query is selecting.

Please note the limitations of such approach described here: http://dev.mysql.com/doc/refman/5.7/en/order-by-optimization.html


Q: Can we use DPA with Azure MySQL?

A: Yes, DPA will monitor, tune and analyze the SQL server performance running on Microsoft Azure.


Q: Do you know if MariaDB has or is planning to follow with these virtual fields and/or SYS schema enhancements from MySQL 5.7?

A: MariaDB has had virtual or computed columns since version 5.2. I don’t believe MariaDB comes with the sys schema already installed, but you can download and install it.


Q: Does DPA support PostgreSQL? If not, is it in the roadmap?

A: Currently, DPA does not support PostgresSQL. However, we continually re-evaluate it with each new release.


Q: Does DPA support RDS instances?

A: Yes, DPA supports the monitoring of RDS instances.


Q: Does the performance schema show any information about how the load data is performing?

A: MySQL 5.5 performance_schema became available in 5.5.3 and has only 11 tables. Most of the tables deal with wait events and file information. In addition, you would need turn on the consumers and enable the instrumentation of the wait events. Once you’ve done that, you will be able to see the threads and what they are waiting on.


Q: I didn’t understand the reasoning that leads to the index on ORDER BY. I can’t link it to the previous slide query.

A: I assume this question is about the ORDER BY + LIMIT optimization. When you create an index on the ORDER BY field only, MySQL can start reading the whole table in the order of the index. As the index is sorted, it can start fetching the rows and filter out the rows that don’t match the ORDER BY condition. As there is a LIMIT N on the query, MySQL will stop after fetching N rows.


Q: How can I analyze parts of a stored procedure that runs nightly to see where by bottlenecks are? It has 100+ update queries that it performs every night to build a table with one million plus rows.

A: You can do it using the slow query log in Percona Server (5.5/5.6) and/or Performance Schema in MySQL 5.7. If you are running Percona Server, you can enable extended stored procedures logging as described here: https://www.percona.com/doc/percona-server/5.6/diagnostics/slow_extended.html. Another way is using a deprecated “show profile” method as described here: https://www.percona.com/blog/2009/01/19/profiling-mysql-stored-routines/


Q: How will DPA use the index when there are more than five columns in the “where” conditions? How would you create indexes?

A: I would suggest checking the “cardinality” of the fields (= number of unique values). Usually (unless you create a covered index or are optimizing the group by) it makes much more sense to limit the number of fields in an index, and only include the fields with the high cardinality. For example, PRIMARY KEY or UNIQUE INDEX works best, whereas the “gender” field (with only two unique values, “male” and “female”) would not be very useful.


Q: How would the analytics tool work in an open stack VM environment, where we have 100 database servers?

A: One installation of DPA can monitor hundreds of database servers. In fact, we have several very large companies that monitor 1000s of servers worldwide.


Q: If you have a small table with only 100 records, is it worth creating indexes on specific fields or just do a table scan?

A: If the table is only 100 records and you are not joining it with other tables, it usually does not make sense to add indexes. But because the table is so small it doesn’t really matter either way.


Q: Is the SolarWinds tool better than MONyog, and how expensive is the license cost for this?

A: MONyog is also a monitoring tool, but it doesn’t have the advisors, alarms, granularity, history, or customizations that DPA gives you. The retail cost per server is currently $1,995 per monitored server, but is heavily discounted the more you purchase.


Q: In many cases, due to the randomness and complexity of queries thrown at various tables, I end up creating a lot of indexes. At what point would there be too many indexes? Should I then create MySQL views instead of indexes? Should one use MySQL views at all to optimize searches?

A: First of all there are no “materialized views” in MySQL, so it is not a useful replacement for indexes. You can create “summary” tables manually, which will usually help a lot. Although it is hard to say when you have too many indexes, lots of indexes can decrease the performance of your insert/update/delete operations, as well as confuse MySQL. So a great many indexes might cause MySQL to start choosing a wrong index when doing selects.


Q: Sometime, we need to add indices for different queries for the same table. Eventually, the table has too many indices. Any suggestion for such cases?

A: See the response to the previous question.


Q: Is there a way in DPA to see what queries are currently running? In other words, to know about slow queries as they run rather than only knowing about them historically?

A: Yes. In the “Current” dashboard, click the “Currently Active Sessions” box. With this option, you can sort by longest running, etc.


Q: Why is delay indexed in the composite key? It only covers the query, but the temp table can be avoided by the first two fields?

A: You are referring to this example:

mysql> alter table ontime_2012
add key covered(dayofweek, Carrier, DepDelayMinutes);
explain select max(DepDelayMinutes), Carrier, dayofweek from ontime_2012 where dayofweek =7 group by CarrierG
possible_keys: DayOfWeek,covered
          key: covered
          key_len: 2
          ref: const
          rows: 905138
          Extra: Using where; Using index

The reason we add DepDelayMinutes is to make the index covered, so MySQL will be able to satisfy the query with an index only.


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