MySQL isn’t too concerned about table handler memory usage – it will allocate row size buffer thrice per each table invocation. There’s a few year old bug discussing UNION memory usage – for each mention in an union one can allocate nearly 200k of unaccounted memory – so a megabyte sized query can consume 7GB of RAM already.
Partitioning though adds even more pain here – it will allocate those three buffers per each partition, so opening a table with 1000 partitions looks like this on memory profile:
Click to enlarge, and you will see 191MB sent to execute a simple single-row fetching query from a table (I filed a bug on this).
There’re multiple real life situations when this is painful (e.g. any kind of server stall may lead to multiple concurrent threads reading from same table, consuming additional gigabytes or tens of gigabytes of memory). It gets even more painful when combined with UNION bug – a megabyte query on an empty table can now consume 7TB of memory and I doubt anyone has that much on their MySQL servers :-)