MySQL SHOW STATUS Explained

        The server maintains many status variables that provide operational information. You can view these variables and their values using the SHOW STATUS statement. These values can help uncover key issues currently present in the database, such as

If Opened_tables is too large, your table_cache variable might be too small. 
If key_reads is too large, your key_cache might be too small. The cache hit rate can be calculated as key_reads/key_read_requests. 
If Handler_read_rnd is too large, you likely have many queries that require MySQL to scan entire tables or you have joins that do not use keys correctly.

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The following is an excerpt from the MySQL 5 Chinese manual, detailing the meaning of each variable in show status.

mysql> SHOW STATUS;Reference Manual.

·         Aborted_clients

The number of connections that were aborted because the client died without closing the connection properly. See Section A.2.10, “Communication Errors and Aborted Connections”.

·         Aborted_connects

The number of failed attempts to connect to the MySQL server. See Section A.2.10, “Communication Errors and Aborted Connections”.

·         Binlog_cache_disk_use

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

·         Binlog_cache_use

The number of transactions that used the temporary binary log cache.

·         Bytes_received

The number of bytes received from all clients.

·         Bytes_sent

The number of bytes sent to all clients.

·         Com_xxx

The Com_xxx statement counter variables indicate the number of times each xxx statement has been executed. There is one status variable for each type of statement. For example, Com_delete and Com_insert count the number of DELETE and INSERT statements executed, respectively.

The Com_stmt_xxx status variables are:

o        Com_stmt_prepare

o        Com_stmt_execute

o        Com_stmt_fetch

o        Com_stmt_send_long_data

o        Com_stmt_reset

o        Com_stmt_close

These variables stand for prepared statement commands. Their names correspond to the COM_xxx command series used at the network layer; in other words: their values increase whenever prepared statement API calls such as mysql_stmt_prepare(), mysql_stmt_execute(), and so forth are executed. However, Com_stmt_prepare, Com_stmt_execute and Com_stmt_close also increase when the following SQL statements are executed: PREPARE, EXECUTE, or DEALLOCATE PREPARE. Furthermore, the values of the older (available since MySQL 4.1.3) statement counter variables Com_prepare_sql, Com_execute_sql, and Com_dealloc_sql also increase with PREPARE, EXECUTE, and DEALLOCATE PREPARE statements. Com_stmt_fetch stands for the total number of network round-trips fetched through cursors.

All Com_stmt_xxx variables will increase, even if a statement argument is unknown or an error occurs during execution. In other words, their values correspond to the number of requests issued, not the number of requests successfully completed.

·         Connections

The number of connection attempts (successful or not) to the MySQL server.

·         Created_tmp_disk_tables

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

·         Created_tmp_files

The number of temporary files mysqld has created.

·         Created_tmp_tables

The number of in-memory temporary tables created automatically by the server while executing statements. If Created_tmp_disk_tables is large, you may want to increase the tmp_table_size value to cause temporary tables to be memory-based instead of disk-based.

·         Delayed_errors

The number of rows written with INSERT DELAYED for which some error occurred (probably duplicate key).

·         Delayed_insert_threads

The number of INSERT DELAYED handler threads in use.

·         Delayed_writes

The number of INSERT DELAYED rows written.

·         Flush_commands

The number of executed FLUSH statements.

·         Handler_commit

The number of internal COMMIT statements.

·         Handler_discover

The MySQL server can ask the NDB CLUSTER storage engine if it knows about a table with a given name. This is called discovery. Handler_discover indicates the number of times that tables have been discovered via this method.

·         Handler_delete

The number of times that rows have been deleted from tables.

·         Handler_read_first

The number of times the first entry in an index was read. If this is high, it suggests that the server is doing a lot of full index scans; for example, SELECT col1 FROM foo, assuming col1 is indexed.

·         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 value is increased if you are querying an index column with a range constraint or if you are performing 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 value is 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 the entire table 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 value is high if you are doing a lot of table scans. Generally this suggests that your tables are not properly indexed or that your written queries do not take advantage of indexes.

·         Handler_rollback

The number of internal ROLLBACK statements.

·         Handler_update

The number of requests to update a row in a table.

·         Handler_write

The number of requests to insert a row into a table.

·         Innodb_buffer_pool_pages_data

The number of pages containing data (dirty or clean).

·         Innodb_buffer_pool_pages_dirty

The current number of dirty pages.

·         Innodb_buffer_pool_pages_flushed

The number of buffer pool pages that have been requested to be flushed.

·         Innodb_buffer_pool_pages_free

The number of free pages.

·         Innodb_buffer_pool_pages_latched

The number of latched pages in the InnoDB buffer pool. These are pages currently being read or written or that cannot be flushed or removed for some other reason.

·           Innodb_buffer_pool_pages_misc

The number of pages that are busy because they have been allocated for administrative overhead such as row locks or the adaptive hash index. This value can also be calculated as Innodb_buffer_pool_pages_total – Innodb_buffer_pool_pages_free – Innodb_buffer_pool_pages_data.

·         Innodb_buffer_pool_pages_total

The total size of the buffer pool, in pages.

·         Innodb_buffer_pool_read_ahead_rnd

The number of “random” read-aheads initiated by InnoDB. This happens when a query scans a large portion of a table but in random order.

·         Innodb_buffer_pool_read_ahead_seq

The number of sequential read-aheads initiated by InnoDB. This happens when InnoDB performs a sequential full table scan.

·         Innodb_buffer_pool_read_requests

The number of logical read requests InnoDB has done.

·         Innodb_buffer_pool_reads

The number of logical reads that InnoDB could not satisfy from the buffer pool and had to read a single page directly.

·         Innodb_buffer_pool_wait_free

Normally, writes to the InnoDB buffer pool happen in the background. However, if it is necessary to read or create a page and no clean pages are available, it is also necessary to wait for pages to be flushed first. This counter counts instances of these waits. If the buffer pool size has been set properly, this value should be small.

·         Innodb_buffer_pool_write_requests

The number of writes to the InnoDB buffer pool.

·         Innodb_data_fsyncs

The number of fsync() operations.

·         Innodb_data_pending_fsyncs

The current number of pending fsync() operations.

·         Innodb_data_pending_reads

The current number of pending reads.

·         Innodb_data_pending_writes

The current number of pending writes.

·         Innodb_data_read

The amount of data read so far, in bytes.

·         Innodb_data_reads

The total number of data reads.

·         Innodb_data_writes

The total number of data writes.

·         Innodb_data_written

The amount of data written so far (in bytes).

·         Innodb_dblwr_writes, Innodb_dblwr_pages_written

The number of doublewrite operations that have been performed and the number of pages that have been written for this purpose. See Section 15.2.14.1, “Disk I/O”.

·         Innodb_log_waits

The number of times we had to wait because the log buffer was too small, and we had to wait for it to be flushed before continuing.

·         Innodb_log_write_requests

The number of log write requests.

·         Innodb_log_writes

The number of physical writes to the log file.

·         Innodb_os_log_fsyncs

The number of fsync() writes completed to the log file.

·         Innodb_os_log_pending_fsyncs

The number of pending log file fsync() operations.

·         Innodb_os_log_pending_writes

Pending log file writes.

·         Innodb_os_log_written

The number of bytes written to the log file.

·         Innodb_page_size

The compiled-in InnoDB page size (default 16KB). Many values are counted in pages; the page size enables easy conversion to bytes.

·         Innodb_pages_created

The number of pages created.

·         Innodb_pages_read

The number of pages read.

·         Innodb_pages_written

The number of pages written.

·         Innodb_row_lock_current_waits

The number of row locks currently being waited for.

·         Innodb_row_lock_time

The total time spent in acquiring row locks, in milliseconds.

·         Innodb_row_lock_time_avg

The average time to acquire a row lock, in milliseconds.

·         Innodb_row_lock_time_max

The maximum time to acquire a row lock, in milliseconds.

·         Innodb_row_lock_waits

The number of times a row lock had to be waited for.

·         Innodb_rows_deleted

The number of rows deleted from InnoDB tables.

·         Innodb_rows_inserted

The number of rows inserted into InnoDB tables.

·         Innodb_rows_read

The number of rows read from InnoDB tables.

·         Innodb_rows_updated

The number of rows updated in InnoDB tables.

·         Key_blocks_not_flushed

The number of key blocks in the key cache that have been changed but have not yet been flushed to disk.

·         Key_blocks_unused

The number of unused 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 5.3.3, “Server System Variables”.

·         Key_blocks_used

The number of used blocks in the key cache. This value is a high-water mark, indicating the maximum number of blocks that have ever been in use at one time.

·         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 large, then your Key_buffer_size value may be too small. The cache miss rate can be calculated as Key_reads/Key_read_requests.

·         Key_write_requests

The number of requests to write a key block to the cache.

·         Key_writes

The number of physical writes of a key block to disk.

·         Last_query_cost

The total cost of the last compiled query as computed by the query optimizer. This is useful for comparing the cost of different query plans for the same query. The default value of 0 means that no query has been compiled yet. The default value is 0. Last_query_cost has session scope.

·         Max_used_connections

The maximum number of connections that have been in use simultaneously since the server started.

·         Not_flushed_delayed_rows

The number of rows waiting to be written in INSERT DELAYED queues.

·         Open_files

The number of files that are open.

·         Open_streams

The number of streams that are open (used mainly for logging).

·         Open_tables

The number of tables that are currently open.

·         Opened_tables

The number of tables that have been opened. If Opened_tables is large, your table_cache value may be too small.

·         QCACHE_free_blocks

The number of free memory blocks in the query cache.

·         QCACHE_free_memory

The amount of free memory for the query cache.

·         QCACHE_hits

The number of query 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 cacheable, or not cached due to the query_cache_type setting).

·         Qcache_queries_in_cache

The number of queries registered in the cache.

·         Qcache_total_blocks

The total number of blocks in the query cache.

·         Questions

The number of queries that have been sent to the server.

·         Rpl_status

The fail-safe replication status (not yet in use).

·         Select_full_join

The number of joins that do not use indexes. If this value is not 0, you should carefully check the indexes of your tables.

·         Select_full_range_join

The number of joins that used a range search on a reference table.

·         Select_range

The number of joins that used ranges on the first table. This is normally not a critical issue even if the value is quite large.

·         Select_range_check

The number of joins without keys that check for key usage after each row. If this is not 0, you should carefully check the indexes of your tables.

·         Select_scan

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

·         Slave_open_temp_tables

The number of temporary tables that are currently open by the slave SQL thread.

·         Slave_running

This is ON if this server is a slave that is connected to a master.

·         Slave_retried_transactions

The total number of times since startup that the replication slave SQL thread has retried transactions.

·         Slow_launch_threads

The number of threads that have taken more than slow_launch_time seconds to create.

·         Slow_queries

The number of queries that have taken more than long_query_time seconds. See Section 5.11.4, “The Slow Query Log”.

·         Sort_merge_passes

The number of merge passes that 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 using ranges.

·         Sort_rows

The number of sorted rows.

·         Sort_scan

The number of sorts completed by scanning the table.

·         Ssl_xxx

Variables used for SSL connections.

·         Table_locks_immediate

The number of times a table lock was acquired immediately.

·         Table_locks_waited

The number of times a table lock could not be acquired immediately. If this value is high and you have performance issues, you should first optimize your queries, then split tables or use replication.

·         Threads_cached

The number of threads in the thread cache.

·         Threads_connected

The number of currently open connections.

·         Threads_created

The number of threads created to handle connections. If Threads_created is large, you may want to increase the thread_cache_size value. The cache hit rate can be calculated as Threads_created/Connections.

·           Threads_running

The number of active (non-sleeping) threads.

·         Uptime

The number of seconds the server has been running.