Why is MySQL is creating so many temporary tables on disk? Can any configuration mistake lead to creating too many temp tables by mysql..mysql tuner shows

Current max_heap_table_size = 200 M
Current tmp_table_size = 200 M
Of 17158 temp tables, 30% were created on disk

table_open_cache = 125 tables
table_definition_cache = 256 tables
You have a total of 97 tables
You have 125 open tables.
Current table_cache hit rate is 3%


Earlier temp table was "of the 23725 temp tables 38% were created on disk" but I changed max_heap and tmp_table to 200m from 16m and it lowered to 30%..

Configuration:

engine myisam 
group_concat_max_len = 32768
key_buffer_size = 3.7 GB,
thread_stack = 256k,
table_cache = 125
query_cache_limit = 1M
query_cache_size = 16M
join_buffer_size = 2.00 M
max_connections = 800


Another system with default configuration is showing "of 23725 temp tables, 1% were created on disk" with the same database.

I tried changing to default on the machine with this issue and it still shows "Of 580 temp tables, 16% were created on disk".

I am using Ubuntu 11.4 64 bit with 48 gb ram. Can any one suggest a solution?

Will changing the db engine from "myisam" to "memory" on tables using "group by" fix this? As explained here: http://www.mysqlperformanceblog.com/2007/08/16/how-much-overhead-is-caused-by-on-disk-temporary-tables/
mysqltuner rarely provides any useful information.  It uses mostly irrelevant statistics about "hit rates" and puts arbitrary limits on what is an acceptable number of widgets are acceptable.  If you are not facing a performance problem, then you don't actually need to solve any of the problems that it presents to you.  That being said, here's a little background information about temporary tables...

MySQL internally uses the MEMORY storage engine for creating implicit temporary tables.  On disk temporary tables use the MyISAM storage engine.

Temporary tables are created on disk when:


TEXT or BLOB fields are present (because MEMORY doesn't support these types)
the size of the resulting implicit temporary table exceeds the lesser of tmp_table_size or max_heap_table_size
If a column w/ more than 512 bytes is used with either a GROUP BY or UNION or ORDER BY


Read the MySQL Documentation on Internal Temporary Tables for more details.

What can you do about this?  Presuming that it actually represents a performance problem (rather than just bothering you intelluctually):


Avoid TEXT/BLOB fields and instead use appropriately sized VARCHAR or CHAR fields where possible.
If TEXT/BLOB are unavoidable, sequester them to separate tables with a foreign key relationship and JOIN only when you need them.
Treat large columns, more than 512 bytes as you would the above mentioned TEXT/BLOB fields.
Make sure your queries are returning only the result set you need (appropriately selective WHERE clauses, avoid SELECT *)
Avoid subqueries and replace them with joins, especially if they return a large result set
Last resort - raise both tmp_table_size and max_heap_table_size.  Don't do this unless you find that your queries cannot be optimized.


If you are concerned about your MySQL configuration and are not comfortable with the available settings yourself, you might want to check out the Percona Configuration Wizard as a starting point.


  Will changing the db engine from "myisam" to "memory" on tables using
  "group by" fix this? as explained here


No, it won't and it will make it such that your tables are never persisted to disk.  Don't do this.