Why is MySQL is using so many temporary tables?

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http://serverfault.com – 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%.. engine myisam group_concat_max_len = 32768 key_buffer_size = 3.7 GB, th (HowTos)