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POMySQL query randomly takes "forever"
 

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  1. POMySQL query randomly takes "forever"
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    <p>We have an XMPP application that uses MySQL to store information. We are not experiencing any specific load problem so far, but I am trying to be prepared for the worst (or the best, in terms of users;)).</p> <p>The host on which this MySQL server is installed is a Slicehost slice with 2GB of RAM.</p> <p>Yesterday, I activated slow query logging to be sure we actually had nothing slow. Unfortunately, it seems that a lot of slow queries were actually found:</p> <pre> Reading mysql slow query log from /var/log/mysql/mysql-slow.log Count: 109 Time=25.57s (2787s) Lock=0.00s (0s) Rows=1.0 (109), xxxxx[xxxxx]@[172.21.xxx.xxx] SELECT * FROM `feeds` WHERE (`id` = 'S') LIMIT N </pre> <p>This is really strange for me, since id is actually a primary key... The Table is InnoDB</p> <p>I did an EXPLAIN:</p> <pre> mysql&gt; EXPLAIN SELECT * FROM `feeds` WHERE (`id` = '2650') LIMIT 1; +----+-------------+-------+-------+---------------+---------+---------+-------+------+-------+ | id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra | +----+-------------+-------+-------+---------------+---------+---------+-------+------+-------+ | 1 | SIMPLE | feeds | const | PRIMARY | PRIMARY | 4 | const | 1 | | +----+-------------+-------+-------+---------------+---------+---------+-------+------+-------+ 1 row in set (0.00 sec) </pre> <p>There must be another reason why this happens. And there are actaully a lot of similar slow queries (query that use primary keys) in our log.</p> <p>I think it would make sense to post the MySQL settings here :</p> <pre> mysql&gt; SHOW VARIABLES; +---------------------------------+-----------------------------+ | Variable_name | Value | +---------------------------------+-----------------------------+ | auto_increment_increment | 1 | | auto_increment_offset | 1 | | automatic_sp_privileges | ON | | back_log | 50 | | basedir | /usr/ | | binlog_cache_size | 32768 | | bulk_insert_buffer_size | 8388608 | | character_set_client | latin1 | | character_set_connection | latin1 | | character_set_database | latin1 | | character_set_filesystem | binary | | character_set_results | latin1 | | character_set_server | latin1 | | character_set_system | utf8 | | character_sets_dir | /usr/share/mysql/charsets/ | | collation_connection | latin1_swedish_ci | | collation_database | latin1_swedish_ci | | collation_server | latin1_swedish_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 | | keep_files_on_create | OFF | | engine_condition_pushdown | OFF | | expire_logs_days | 10 | | flush | OFF | | flush_time | 0 | | ft_boolean_syntax | + -&gt;&lt;()~*:""&| | | ft_max_word_len | 84 | | ft_min_word_len | 4 | | ft_query_expansion_limit | 20 | | ft_stopword_file | (built-in) | | group_concat_max_len | 1024 | | have_archive | YES | | have_bdb | NO | | have_blackhole_engine | YES | | have_compress | YES | | have_crypt | YES | | have_csv | YES | | have_dynamic_loading | YES | | have_example_engine | NO | | have_federated_engine | DISABLED | | have_geometry | YES | | have_innodb | YES | | have_isam | NO | | have_merge_engine | YES | | have_ndbcluster | DISABLED | | have_openssl | DISABLED | | have_ssl | DISABLED | | have_query_cache | YES | | have_raid | NO | | have_rtree_keys | YES | | have_symlink | YES | | hostname | SuperfeedrDatabase | | init_connect | | | init_file | | | init_slave | | | innodb_additional_mem_pool_size | 1048576 | | innodb_autoextend_increment | 8 | | innodb_buffer_pool_awe_mem_mb | 0 | | innodb_buffer_pool_size | 1073741824 | | innodb_checksums | ON | | innodb_commit_concurrency | 0 | | innodb_concurrency_tickets | 500 | | innodb_data_file_path | ibdata1:10M:autoextend | | innodb_data_home_dir | | | innodb_adaptive_hash_index | ON | | innodb_doublewrite | ON | | innodb_fast_shutdown | 1 | | innodb_file_io_threads | 4 | | innodb_file_per_table | ON | | innodb_flush_log_at_trx_commit | 2 | | innodb_flush_method | O_DIRECT | | innodb_force_recovery | 0 | | innodb_lock_wait_timeout | 50 | | innodb_locks_unsafe_for_binlog | OFF | | innodb_log_arch_dir | | | innodb_log_archive | OFF | | innodb_log_buffer_size | 4194304 | | innodb_log_file_size | 5242880 | | innodb_log_files_in_group | 2 | | innodb_log_group_home_dir | ./ | | innodb_max_dirty_pages_pct | 90 | | innodb_max_purge_lag | 0 | | innodb_mirrored_log_groups | 1 | | innodb_open_files | 300 | | innodb_rollback_on_timeout | OFF | | innodb_support_xa | ON | | innodb_sync_spin_loops | 20 | | innodb_table_locks | ON | | innodb_thread_concurrency | 8 | | innodb_thread_sleep_delay | 10000 | | interactive_timeout | 28800 | | join_buffer_size | 131072 | | key_buffer_size | 16777216 | | key_cache_age_threshold | 300 | | key_cache_block_size | 1024 | | key_cache_division_limit | 100 | | language | /usr/share/mysql/english/ | | large_files_support | ON | | large_page_size | 0 | | large_pages | OFF | | lc_time_names | en_US | | license | GPL | | local_infile | ON | | locked_in_memory | OFF | | log | OFF | | log_bin | OFF | | log_bin_trust_function_creators | OFF | | log_error | | | log_queries_not_using_indexes | ON | | log_slave_updates | OFF | | log_slow_queries | ON | | log_warnings | 1 | | long_query_time | 3 | | low_priority_updates | OFF | | lower_case_file_system | OFF | | lower_case_table_names | 0 | | max_allowed_packet | 16777216 | | max_binlog_cache_size | 18446744073709547520 | | max_binlog_size | 104857600 | | max_connect_errors | 10 | | max_connections | 2000 | | max_delayed_threads | 20 | | max_error_count | 64 | | max_heap_table_size | 16777216 | | max_insert_delayed_threads | 20 | | max_join_size | 18446744073709551615 | | max_length_for_sort_data | 1024 | | max_prepared_stmt_count | 16382 | | max_relay_log_size | 0 | | max_seeks_for_key | 18446744073709551615 | | max_sort_length | 1024 | | max_sp_recursion_depth | 0 | | max_tmp_tables | 32 | | max_user_connections | 0 | | max_write_lock_count | 18446744073709551615 | | multi_range_count | 256 | | myisam_data_pointer_size | 6 | | myisam_max_sort_file_size | 9223372036853727232 | | myisam_recover_options | BACKUP | | myisam_repair_threads | 1 | | myisam_sort_buffer_size | 8388608 | | myisam_stats_method | nulls_unequal | | ndb_autoincrement_prefetch_sz | 1 | | ndb_force_send | ON | | ndb_use_exact_count | ON | | ndb_use_transactions | ON | | ndb_cache_check_time | 0 | | ndb_connectstring | | | net_buffer_length | 16384 | | net_read_timeout | 30 | | net_retry_count | 10 | | net_write_timeout | 60 | | new | OFF | | old_passwords | OFF | | open_files_limit | 10000 | | optimizer_prune_level | 1 | | optimizer_search_depth | 62 | | pid_file | /var/run/mysqld/mysqld.pid | | plugin_dir | | | port | 3306 | | preload_buffer_size | 32768 | | profiling | OFF | | profiling_history_size | 15 | | protocol_version | 10 | | query_alloc_block_size | 8192 | | query_cache_limit | 1048576 | | query_cache_min_res_unit | 4096 | | query_cache_size | 16777216 | | query_cache_type | ON | | query_cache_wlock_invalidate | OFF | | query_prealloc_size | 8192 | | range_alloc_block_size | 4096 | | read_buffer_size | 131072 | | read_only | OFF | | read_rnd_buffer_size | 262144 | | relay_log | | | relay_log_index | | | relay_log_info_file | relay-log.info | | relay_log_purge | ON | | relay_log_space_limit | 0 | | rpl_recovery_rank | 0 | | secure_auth | OFF | | secure_file_priv | | | server_id | 0 | | skip_external_locking | ON | | skip_networking | OFF | | skip_show_database | OFF | | slave_compressed_protocol | OFF | | slave_load_tmpdir | /tmp/ | | slave_net_timeout | 3600 | | slave_skip_errors | OFF | | slave_transaction_retries | 10 | | slow_launch_time | 2 | | socket | /var/run/mysqld/mysqld.sock | | sort_buffer_size | 2097144 | | sql_big_selects | ON | | sql_mode | | | sql_notes | ON | | sql_warnings | OFF | | ssl_ca | | | ssl_capath | | | ssl_cert | | | ssl_cipher | | | ssl_key | | | storage_engine | MyISAM | | sync_binlog | 0 | | sync_frm | ON | | system_time_zone | UTC | | table_cache | 64 | | table_lock_wait_timeout | 50 | | table_type | MyISAM | | thread_cache_size | 8 | | thread_stack | 131072 | | time_format | %H:%i:%s | | time_zone | SYSTEM | | timed_mutexes | OFF | | tmp_table_size | 33554432 | | tmpdir | /tmp | | transaction_alloc_block_size | 8192 | | transaction_prealloc_size | 4096 | | tx_isolation | READ-COMMITTED | | updatable_views_with_limit | YES | | version | 5.0.67-0ubuntu6-log | | version_comment | (Ubuntu) | | version_compile_machine | x86_64 | | version_compile_os | debian-linux-gnu | | wait_timeout | 28800 | +---------------------------------+-----------------------------+ 237 rows in set (0.00 sec) </pre> <p>Most of our requests are "basic", however, we <em>need</em> a tremendous speed!</p> <p>Any thoughts on what could actually make MySQL so slow?</p> <p>[<strong>SUMMARY</strong>] To sum up the various answers :</p> <ul> <li>Remove "LIMIT", change WHERE id = "X" into WHERE id = X</li> <li>Make sure I don't have any script(backup or else) that runs from time to time that would consume a lot of resources</li> <li>Make sure the "host" is not actually the culprit.</li> </ul>
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    1. USJulien Genestoux
    1. USJulien Genestoux
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      1. USmacbirdie
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      1. USTim Haines
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    1. COHow many records are in the table? I've had this happen with tables that weren't exceptionaly big and fixed it by adding an index..
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      1. USTim
    2. COIndeed, the table is rather small now : 20k rows.
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      1. USJulien Genestoux
    3. COisn't the primary key automatically indexed, by virtue of being a primary key?
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      1. USKip
 

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