View mysql query cache contents

Question

I have a large database application that uses mysqls query cache ability. I'd like to inspect the contents of the query cache to identify entries that should not get cached (e.g. by using SELECT SQL_NO_CACHE...) because I want to optimize my application.

Is there a way to get a list of cached entries from the query cache?

-- Edit: additional info as suggested by Rick ---

1) RAM size is 16GB

2) show variables;

auto_increment_increment    1
auto_increment_offset   1
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automatic_sp_privileges ON
back_log    50
basedir /usr/
big_tables  OFF
binlog_cache_size   32768
binlog_direct_non_transactional_updates OFF
binlog_format   STATEMENT
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character_set_connection    utf8
character_set_database  utf8
character_set_filesystem    binary
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character_set_system    utf8
character_sets_dir  /usr/share/mysql/charsets/
collation_connection    utf8_general_ci
collation_database  utf8_general_ci
collation_server    utf8_general_ci
completion_type 0
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datadir /var/lib/mysql/
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datetime_format %Y-%m-%d %H:%i:%s
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flush   OFF
flush_time  0
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ft_boolean_syntax   + -><()~*:""&|
ft_max_word_len 84
ft_min_word_len 4
ft_query_expansion_limit    20
ft_stopword_file    (built-in)
general_log OFF
general_log_file    /var/run/mysqld/mysqld.log
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have_community_features YES
have_compress   YES
have_crypt  YES
have_csv    YES
have_dynamic_loading    YES
have_geometry   YES
have_innodb YES
have_ndbcluster NO
have_openssl    DISABLED
have_partitioning   NO
have_query_cache    YES
have_rtree_keys YES
have_ssl    DISABLED
have_symlink    YES
hostname    www2
identity    0
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init_slave   
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innodb_autoextend_increment 8
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innodb_change_buffering inserts
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innodb_commit_concurrency   0
innodb_concurrency_tickets  500
innodb_data_file_path   ibdata1:10M:autoextend
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innodb_doublewrite  ON
innodb_fast_shutdown    1
innodb_file_format  Antelope
innodb_file_format_check    Barracuda
innodb_file_per_table   ON
innodb_flush_log_at_trx_commit  1
innodb_flush_method  
innodb_force_recovery   0
innodb_io_capacity  200
innodb_lock_wait_timeout    50
innodb_locks_unsafe_for_binlog  OFF
innodb_log_buffer_size  67108864
innodb_log_file_size    268435456
innodb_log_files_in_group   2
innodb_log_group_home_dir   ./
innodb_max_dirty_pages_pct  75
innodb_max_purge_lag    0
innodb_mirrored_log_groups  1
innodb_old_blocks_pct   37
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innodb_open_files   300
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innodb_read_io_threads  4
innodb_replication_delay    0
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innodb_spin_wait_delay  6
innodb_stats_method nulls_equal
innodb_stats_on_metadata    ON
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innodb_strict_mode  OFF
innodb_support_xa   ON
innodb_sync_spin_loops  30
innodb_table_locks  ON
innodb_thread_concurrency   0
innodb_thread_sleep_delay   10000
innodb_use_sys_malloc   ON
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key_cache_age_threshold 300
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optimizer_prune_level   1
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optimizer_switch    index_merge=on,index_merge_union=on,index_merge_so...
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version 5.1.56-log
version_comment Gentoo Linux mysql-5.1.56
version_compile_machine x86_64
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3) show global status;

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Answer 1

You can know the amount of queries cached with

Show status like 'Qcache_queries_in_cache'

It will give you the amount of cached query in value column. Only the hash of queries are stored in Qcache. It is not possible to find which queries are actually cached. One possible way to know it is to execute one of your query and to check changes of value column.

Answer 2

Observations:

Version: 5.1.56-log
16 GB of RAM
Uptime = 53d 21:20:56
You are not running on Windows.
Running 64-bit version
It appears that you are running both MyISAM and InnoDB.

The More Important Issues

key_buffer_size = 1500M
innodb_buffer_pool_size = 2000M
join_buffer_size = 1M   -- This may be the most important
tmp_table_size = 256M   -- Also important
query_cache_size = 50M  -- as already mentioned
Use slowlog to find tmp tables spilling to disk

Three settings were especially high and could lead to paging/swapping. MySQL performance suffers terribly when paged/swapped. It is better to lower settings than to have that happen.

Details and Minor Issues

( innodb_buffer_pool_size / _ram ) = 1024M / 16384M = 6.2% -- % of RAM used for InnoDB buffer_pool

( Innodb_pages_written / Innodb_buffer_pool_write_requests ) = 29,673,396 / 196667000 = 15.1% -- Write requests that had to hit disk -- Check innodb_buffer_pool_size

( Innodb_buffer_pool_read_ahead_evicted / Innodb_buffer_pool_read_ahead ) = 4,691,916 / 30144135 = 15.6% -- Utility of read_ahead. -- Turn off read-ahead

( Innodb_os_log_written / (Uptime / 3600) / innodb_log_files_in_group / innodb_log_file_size ) = 34,746,359,296 / (4656056 / 3600) / 2 / 256M = 0.05 -- Ratio

( Uptime / 60 * innodb_log_file_size / Innodb_os_log_written ) = 4,656,056 / 60 * 256M / 34746359296 = 599 -- Minutes between InnoDB log rotations Beginning with 5.6.8, this can be changed dynamically; be sure to also change my.cnf. -- (The recommendation of 60 minutes between rotations is somewhat arbitrary.)

( table_open_cache ) = 4,096 -- Number of table descriptors to cache -- Several hundred is usually good.

( innodb_stats_on_metadata ) = ON -- Re-analyze table when touching stats. -- ON is likely to slow down certain SHOWs and information_schema accesses.

( join_buffer_size ) = 2,147,483,648 = 2048MB -- 0-N per thread. May speed up JOINs (better to fix queries/indexes) (all engines) Used for index scan, range index scan, full table scan, each full JOIN, etc. -- Use the default.

( join_buffer_size / _ram ) = 2,147,483,648 / 16384M = 12.5% -- 0-N per thread. May speed up JOINs (better to fix queries/indexes) (all engines) Used for index scan, range index scan, full table scan, each full JOIN, etc. -- Decrease join_buffer_size to avoid memory pressure. Suggest less than 1% of RAM.

( min( tmp_table_size, max_heap_table_size ) / _ram ) = min( 2147483648, 256M ) / 16384M = 1.6% -- Percent of RAM to allocate when needing MEMORY table (per table), or temp table inside a SELECT (per temp table per some SELECTs). Too high may lead to swapping. -- Decrease tmp_table_size and max_heap_table_size to, say, 1% of ram.

( (key_buffer_size - 1.2 * Key_blocks_used * 1024) / _ram ) = (4294967296 - 1.2 * 864035 * 1024) / 16384M = 18.8% -- Percent of RAM wasted in key_buffer. -- Decrease key_buffer_size

( Key_blocks_used * 1024 / key_buffer_size ) = 864,035 * 1024 / 4294967296 = 20.6% -- Percent of key_buffer used . High-water-mark. -- Lower key_buffer_size to avoid unnecessary memory usage.

( Key_writes / Key_write_requests ) = 7,764,232 / 105012022 = 7.4% -- key_buffer effectiveness for writes

( query_cache_size ) = 1024M -- Size of QC -- Too small = not of much use. Too large = too much overhead. Recommend either 0 or no more than 50M.

( Created_tmp_disk_tables / (Created_tmp_disk_tables + Created_tmp_tables) ) = 2,151,917 / (2151917 + 3928177) = 35.4% -- Percent of temp tables that spilled to disk

( tmp_table_size ) = 2,147,483,648 = 2048MB -- Limit on size of MEMORY temp tables used to support a SELECT -- Decrease tmp_table_size to avoid running out of RAM. Perhaps no more than 64M.

( back_log / max_connections ) = 50 / 120 = 41.7%

( max_connect_errors ) = 100,000 -- A small protection against hackers. -- Perhaps no more than 200.