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
autocommit  ON
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
bulk_insert_buffer_size 4294967296
character_set_client    utf8
character_set_connection    utf8
character_set_database  utf8
character_set_filesystem    binary
character_set_results   utf8
character_set_server    utf8
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
concurrent_insert   1
connect_timeout 10
datadir /var/lib/mysql/
date_format %Y-%m-%d
datetime_format %Y-%m-%d %H:%i:%s
default_week_format 0
delay_key_write ON
delayed_insert_limit    100
delayed_insert_timeout  300
delayed_queue_size  1000
div_precision_increment 4
engine_condition_pushdown   ON
error_count 0
event_scheduler ON
expire_logs_days    0
flush   OFF
flush_time  0
foreign_key_checks  ON
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
group_concat_max_len    1024
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
ignore_builtin_innodb   ON
init_connect     
init_file    
init_slave   
innodb_adaptive_flushing    ON
innodb_adaptive_hash_index  ON
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innodb_autoextend_increment 8
innodb_autoinc_lock_mode    1
innodb_buffer_pool_size 1073741824
innodb_change_buffering inserts
innodb_checksums    ON
innodb_commit_concurrency   0
innodb_concurrency_tickets  500
innodb_data_file_path   ibdata1:10M:autoextend
innodb_data_home_dir     
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
innodb_old_blocks_time  0
innodb_open_files   300
innodb_read_ahead_threshold 56
innodb_read_io_threads  4
innodb_replication_delay    0
innodb_rollback_on_timeout  OFF
innodb_spin_wait_delay  6
innodb_stats_method nulls_equal
innodb_stats_on_metadata    ON
innodb_stats_sample_pages   8
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
innodb_version  1.0.15
innodb_write_io_threads 4
insert_id   0
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key_buffer_size 4294967296
key_cache_age_threshold 300
key_cache_block_size    1024
key_cache_division_limit    100
language    /usr/share/mysql/english/
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lc_time_names   en_US
license GPL
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log OFF
log_bin OFF
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log_output  FILE
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myisam_stats_method nulls_unequal
myisam_use_mmap OFF
net_buffer_length   1048576
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new OFF
old OFF
old_alter_table OFF
old_passwords   OFF
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optimizer_prune_level   1
optimizer_search_depth  62
optimizer_switch    index_merge=on,index_merge_union=on,index_merge_so...
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slow_query_log_file /var/log/mysql/slowqueries.log
socket  /var/run/mysqld/mysqld.sock
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version 5.1.56-log
version_comment Gentoo Linux mysql-5.1.56
version_compile_machine x86_64
version_compile_os  pc-linux-gnu
wait_timeout    30
warning_count   0

3) show global status;

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Halter

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.

Suchta

Do SHOW STATUS LIKE 'Qcache%'; and you will get even more variables to check ;) – Thoron

OK, that sounds quite convincing. If all queries would be stored this would require serious amounts of memory, storing only query hashes is probably the only way to go. At leadst Qcache_queries_in_cache will give me a hint if the last issued query is added to the cache. – Halter

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.

Dav

Wow, that's a whole lot of data to work on. Did you use some kind of tool for analysis? I'm gonna discuss this with my hosting provider as some of those settings don't mean much to me ;-) Thank you! – Halter

I forgot to add: 23 issues flagged, out of 144 computed Variables/Status/Expressions. Settings not mentioned are mostly OK. Yeah it is an automated tool, but I am struggling with making it really automated. The "More Important Issues" is a hand-done interpretation of the gibberish put out by the tool. It can't automatically resolve contradictions ("increase X; no decrease X") and interactions. For now, I provide it as a service. – Dav

I logged in just to give this answer a thumbsup. This is a lot of information. Thanks @RickJames – Overmatch

@rickydj - If you would like a similar review of your system, start a new Question and provide the two SHOWs, plus ram size. – Dav

Answer 3

If you have a "large" db app, this implies that there are a lot of writes? Keep in mind that all QC entries for a table are purged when any write to that table occurs. That implies that you may not benefit from the QC.

Furthermore, that purging takes effort, thereby slowing down the system. For query_cache_size under, say, 50M, the cost is not big. But if you have a 1G QC, you will spend more time purging the QC than using it!

The actual queries in the QC cannot be discovered. Here is what might help: Put SQL_NO_CACHE on any queries involving table(s) that change frequently. Put SQL_CACHE on the rest. Then use DEMAND. Monitor STATUS LIKE Qc%.

If you would like an analysis of your system (QC and otherwise), provide (1) RAM size, (2) SHOW VARIABLES, (3) SHOW GLOBAL STATUS after the server has been up for at least a day.

Dav

Thanks for your help, additional server info attached to my original question. Would be glad if you could analyse the data. – Halter