mysql slow on first query, then fast for related queries

Question

I have been struggling with a problem that only happens when the database has been idle for a period of time for the data queried. The first query will be extremely slow, on the order of 30 seconds and then related queries will be fast like 0.1 seconds. I am assuming this is related to caching, but I have been unable to find the cause of it.

Changing the mysql variables tmp_table_size, max_heap_table_size to a larger size had no effect except to create the temp tables in memory.

I do not think this is related to the query itself as it is well indexed and after the first slow query, variants of the same query do not show up in the slow query log. I am most interested in trying to determine the cause of this or a way to reset the offending cache so I can troubleshoot the issue.

Answer 1

Pages of the innodb data files get cached in the innodb buffer pool. This is what you'd expect. Reading files is slow, even on good hard drives, especially random reads which is mostly what databases see.

It may be that your first query is doing some kind of table scan which pulls a lot of pages into the buffer pool, then accessing them is fast. Or something similar.

This is what I'd expect.

Ideally, use the same engine for all tables (exceptions: system tables, temporary tables (perhaps) and very small tables or short-lived ones). If you don't do this then they have to fight for ram.

Assuming all your tables are innodb, make the buffer pool use up to 75% of the server's physical ram (assuming you don't run too many other tasks on the machine).

Then you will be able to fit around 12G of your database into ram, so once it's "warmed up", the "most used" 12G of your database will be in ram, where accessing it is nice and fast.

Some users of mysql tend to "warm up" production servers following a restart by sending them queries copied from another machine for a while (these will be replication slaves) until they add them into their production pool. This avoids the extreme slowness seen while the cache is cold. For example, Youtube does this (or at least it used to; Google bought them and they may now use Google-fu)

Answer 2

MySQL Workbench:

The below isn't 100% related to this SO question, but the symptoms are very related and this is the first SO result when searching for "mysql workbench slow" or related terms, so hopefully it's useful for others.

Clear the query history! - following the process at MySql workbench query history ( last executed query / queries ) i.e. create / alter table, select, insert update queries to clear MySQL Workbench's query history really sped up the program for me.

In summary: change the Output pane to History Output, right click on a Date and select Delete All Logs.

The issue I was experiencing was "slow first query" in that it would take a few seconds to load the results even though the duration/fetch were well under 1 second. After clearing my query history, the duration/fetch times stayed the same (well under 1 second, so no DB behavior actually changed), but now the results loaded instantly rather than after a few second delay.