Here's a list of status variables returned by SHOW STATUS 
that are useful in validating the our SQL statements and the 
MySQL server settings.  Generally we want to watch out for
operations that hit the disk unnecessarily or search/sort
data without using an index.

----------------------

INDEX USAGE:

Select_scan
The number of joins that did a full scan of the first table.

Handler_read_key
The number of requests to read a row based on a key. If this is high,
it is a good indication that your queries and tables are properly
indexed.

Handler_read_next
The number of requests to read the next row in key order. This will be
incremented if you are querying an index column with a range
constraint or if you are doing an index scan.

Handler_read_prev
The number of requests to read the previous row in key order. This
read method is mainly used to optimize ORDER BY ... DESC.

Handler_read_rnd
The number of requests to read a row based on a fixed position. This
will be high if you are doing a lot of queries that require sorting of
the result. You probably have a lot of queries that require MySQL to
scan whole tables or you have joins that don't use keys properly.

Handler_read_rnd_next
The number of requests to read the next row in the data file. This
will be high if you are doing a lot of table scans. Generally this
suggests that your tables are not properly indexed or that your
queries are not written to take advantage of the indexes you have.

Key_blocks_unused

Key_blocks_used
The number of used blocks in the key cache. You can use this value to
determine how much of the key cache is in use; see the discussion of
key_buffer_size in section 5.2.3 Server System Variables.

Key_read_requests
The number of requests to read a key block from the cache.

Key_reads
The number of physical reads of a key block from disk. If Key_reads is
big, then your key_buffer_size value is probably too small. The cache
miss rate can be calculated as Key_reads/Key_read_requests.

-----------------

QUERY PERFORMANCE:

Slow_queries
The number of queries that have taken more than long_query_time
seconds. See section 5.9.5 The Slow Query Log.

----------------

SORTING:

Sort_merge_passes
The number of merge passes the sort algorithm has had to do. If this
value is large, you should consider increasing the value of the
sort_buffer_size system variable.

Sort_range
The number of sorts that were done with ranges.

Sort_rows
The number of sorted rows.

Sort_scan
The number of sorts that were done by scanning the table.

--------------

QUERY CACHE:

Qcache_free_blocks
The number of free memory blocks in query cache.

Qcache_free_memory
The amount of free memory for query cache.

Qcache_hits
The number of cache hits.

Qcache_inserts
The number of queries added to the cache.

Qcache_lowmem_prunes
The number of queries that were deleted from the cache because of low
memory.

Qcache_not_cached
The number of non-cached queries (not cachable, or due to
query_cache_type).

Qcache_queries_in_cache
The number of queries registered in the cache.

Qcache_total_blocks
The total number of blocks in the query cache.

----------------

MISC:

Uptime
The number of seconds the server has been up.

Binlog_cache_disk_use
The number of transactions that used the temporary binary log cache
but that exceeded the value of binlog_cache_size and used a temporary
file to store statements from the transaction.

Created_tmp_disk_tables
The number of temporary tables on disk created automatically by the
server while executing statements.

Opened_tables
The number of tables that have been opened. If Opened_tables is big,
your table_cache value is probably too small.