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MySQL’s slow query log is disabled by default. To capture slow statements, enable slow_query_log, make sure log_output includes FILE (or deliberately choose TABLE), and set a useful long_query_time. If the log is enabled but empty, the usual causes are log_output=NONE, a threshold that your test query does not exceed, an unmet min_examined_row_limit, or filters that exclude administrative and no-index statements.
This guide shows the runtime and startup configuration, explains where the file is written, provides a systematic fix for an empty or rapidly growing log, and shows how to turn entries into safer index and SQL changes.
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The slow query log is a diagnostic stream of SQL statements that exceed the configured execution-time threshold and, under the documented rule, examine at least the configured minimum number of rows. MySQL’s official wording is: “The slow query log consists of SQL statements that take more than long_query_time seconds to execute and require at least min_examined_row_limit rows to be examined.”
It is separate from the general query log. The general log records accepted statements broadly, while the slow log is intended to identify work that deserves performance investigation. A slow-log entry includes timing and row-count fields such as:
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Query_time: elapsed execution time.Lock_time: time spent waiting for locks.Rows_sent: rows returned to the client.Rows_examined: rows MySQL inspected while executing the statement.
A large difference between rows examined and rows sent can indicate inefficient filtering, a missing or poorly ordered index, or a plan that is unsuitable for the statement’s parameter values. It is a prioritization clue, not proof that an index is required; confirm the cause with an execution plan.
Check the current configuration first
Run this query before changing anything. It records the effective global settings on the server you are troubleshooting:
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'slow_query_log','slow_query_log_file','log_output','long_query_time',
'min_examined_row_limit','log_queries_not_using_indexes',
'log_slow_admin_statements','log_throttle_queries_not_using_indexes'
);Interpret the result as a set rather than checking only one variable:
slow_query_logmust beONfor capture to occur.log_outputdetermines the destination.FILEwrites a file,TABLEwrites tomysql.slow_log, andNONEsuppresses query output.slow_query_log_fileidentifies the file name or path used for file output.long_query_timeis the execution-time threshold. Its documented default is 10 seconds.min_examined_row_limitadds a rows-examined requirement. A query can exceed the time threshold and still be omitted if it examines fewer rows than this value.log_queries_not_using_indexescontrols whether statements that do not use indexes are included.log_slow_admin_statementscontrols logging of qualifying administrative statements.log_throttle_queries_not_using_indexeslimits bursts created by no-index queries.
Use SHOW GLOBAL, not a session-only value, when diagnosing what the server is applying to new connections. A setting changed only for your session will not enable the server-wide log.
Enable the log immediately at runtime
Runtime changes are useful for a bounded diagnostic window because they take effect without restarting the server. The account issuing SET GLOBAL must have the privileges required by your MySQL version and deployment.
Write entries to a file
For ordinary file-based troubleshooting, select FILE, enable the logger, and choose a threshold:
SET GLOBAL log_output = 'FILE';
SET GLOBAL slow_query_log = ON;
SET GLOBAL long_query_time = 1;This example captures statements taking more than one second. One second is a diagnostic choice, not a universal production setting. On a busy system it can generate a substantial volume, so use the lowest threshold that answers the question and return it to a sensible value afterward.
Choose a specific file
Where the server permits it, set the global slow_query_log_file variable:
SET GLOBAL slow_query_log_file = '/var/log/mysql/app-slow.log';When the file name changes, MySQL closes the already open slow-log file and opens the new one. Confirm the resulting value with:
SELECT @@global.slow_query_log_file;The path must be valid on the database server’s host, not on the workstation from which you run the SQL client. The MySQL service account also needs permission to create or write the file and to traverse each directory in the path.
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Use the internal slow-log table instead
If you prefer SQL access to the records, include TABLE in log_output:
SET GLOBAL log_output = 'TABLE';
SET GLOBAL slow_query_log = ON;Entries are written to mysql.slow_log. A deployment can include more than one destination, for example FILE,TABLE, when both a file and SQL queries are needed. Be deliberate: every enabled destination receives the captured entries and therefore adds storage or maintenance work.
Make the configuration persistent across restarts
SET GLOBAL changes are generally runtime changes. A restart, package upgrade, container replacement, or managed-service operation can restore the startup configuration. To keep the setting, add the equivalent server options to the MySQL option file or launch configuration used by your installation:
slow_query_log=ON
slow_query_log_file=/var/log/mysql/app-slow.log
log_output=FILE
long_query_time=1The exact option-file location and persistence mechanism differ by operating system, package, container image, and managed MySQL service. Edit the configuration source that actually launches your server, then restart it according to that environment’s procedure. Afterward, repeat the SHOW GLOBAL VARIABLES query and confirm that the effective values match the file. A setting added to an unused option file will appear to have no effect even though its syntax is correct.
For a short investigation, runtime configuration avoids a restart and limits the chance of accidentally leaving a diagnostic threshold in production. For a permanent policy, startup configuration prevents the logger from silently disappearing after the next restart.
Set thresholds and capture scope deliberately
long_query_time
long_query_time is the main execution-time filter. The documented default is 10 seconds, which is often too high to reveal user-facing latency in a responsive application. Lowering it to one second, or to another value based on your service-level objective, makes the log more useful for an investigation.
Do not set it to zero casually. A near-zero threshold can record almost every statement, creating I/O, storage, and review costs and making important entries harder to find. Use a defined observation period, collect enough representative traffic, then restore the threshold.
min_examined_row_limit
This variable is a second eligibility test. If it is set above zero, a statement must examine at least that many rows in addition to exceeding long_query_time. For a controlled test, inspect its value and temporarily lower it if you need to observe low-row-count statements. Keep the change tied to the diagnostic interval so that the production policy remains intentional.
No-index queries
By default, queries that do not use indexes are excluded from the slow log unless they meet the normal slow-query criteria and the relevant option is enabled. To broaden capture, set:
SET GLOBAL log_queries_not_using_indexes = ON;This option can be noisy on workloads that legitimately scan small tables. If you enable it, use log_throttle_queries_not_using_indexes to limit bursts:
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SET GLOBAL log_throttle_queries_not_using_indexes = 10;The number is the throttle limit you choose for the deployment; select it according to traffic and the volume you can safely review. Throttling limits repeated no-index entries but does not make an inefficient query efficient.
Administrative statements
Administrative statements are excluded by default. To include qualifying administrative work, enable:
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Find the file and verify that MySQL can write it
Never guess the file location. Read the server’s effective value:
SELECT @@global.slow_query_log_file;If the value is an absolute path, inspect that exact directory on the database host. If it is not absolute, MySQL uses its data directory together with a default host-based file name. The file may therefore be in the data directory rather than in a distribution’s commonly assumed log directory.
Check all of the following on the server host:
- The parent directory exists.
- The MySQL service account can traverse the directory and create or append to the file.
- Mandatory access-control policies, such as a host security profile, allow the service to write there.
- The filesystem has free space and the file has not hit an operational quota.
- You are inspecting the active database host, not a replica, container shell, or client machine.
After generating a deliberately slow test in a safe environment, refresh the file and check for a new timestamp or entry. Avoid running an expensive query merely to test logging on a production system; use an existing representative statement or a controlled test that will not damage availability.
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Why an enabled slow log is empty
log_output is NONE
slow_query_log=ON does not guarantee file output. If log_output=NONE, MySQL writes no query entries even though the slow-log switch is enabled. Set log_output='FILE' for a file or log_output='TABLE' for mysql.slow_log, then verify the value again.
The test does not cross the filters
Check long_query_time and min_examined_row_limit. A statement that completes below the time threshold, or examines fewer rows than the minimum, is correctly absent. Remember that the threshold concerns execution time on the server; time spent waiting in an application, connection pool, network, or client is not automatically represented as query execution time.
The statement is excluded by scope
Administrative statements and no-index statements are not included by default. Enable log_slow_admin_statements or log_queries_not_using_indexes only when that is the behavior you need. For no-index capture, configure throttling as well if the workload can produce a burst.
You are reading the wrong destination
With TABLE, the records are in mysql.slow_log, not in the file named by slow_query_log_file. With multiple destinations, check each one. Query the table directly when that is the selected output:
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SELECT start_time, query_time, lock_time, rows_sent, rows_examined, sql_text
FROM mysql.slow_log
ORDER BY start_time DESC
LIMIT 20;The exact columns available can vary with MySQL version, so inspect the table definition if a column in the example is unavailable.
Why the file cannot be found or opened
Start with @@global.slow_query_log_file, then distinguish a wrong path from a permissions failure. A relative file name resolves under MySQL’s data directory; an absolute name points elsewhere. If the directory exists but the file never appears, inspect ownership, directory permissions, security policy, disk space, and service logs. A path that is writable by your interactive shell may still be unwritable by the account running mysqld.
Changing the global file variable causes MySQL to close the current file and open the new one. If the new path is invalid or inaccessible, correct the directory and permissions, set the variable again, and confirm the effective value. Do not delete the active file while the server is writing to it; use your platform’s supported log rotation process or change the file name first.
Control a log that grows too quickly
A fast-growing log usually reflects an intentionally broad capture setting rather than a broken logger. The most common triggers are lowering long_query_time toward zero and enabling log_queries_not_using_indexes on a high-volume workload.
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- Record the current values of
long_query_time,log_queries_not_using_indexes, andlog_throttle_queries_not_using_indexes. - Limit the diagnostic interval and monitor disk usage while collecting data.
- Restore a useful threshold when you have enough samples.
- Leave no-index logging disabled unless it answers a specific question, or retain it with a deliberate throttle.
- Archive or rotate the completed file according to your operating system and retention policy.
The slow log is itself workload. Writing, storing, rotating, and later parsing every entry consumes resources. There is no universal overhead percentage that applies to all MySQL versions and workloads, so measure the effect in your environment rather than relying on a generic number.
Replication: why a replica may not show statements
Replicas do not log replicated queries by default. In MySQL 8.0.26 and later, the relevant option is log_slow_replica_statements. Earlier releases use log_slow_slave_statements. Configure the option on the replica where you need visibility, and verify the release-specific variable name before changing it.
Row-based replication limits the effect of these settings because the replica applies row events rather than replaying the original SQL statement in the same way as statement-based replication. Consequently, a primary’s slow log and a replica’s apply workload answer different questions. Check the server and replication format before concluding that missing replica entries indicate a logging failure.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.Summarize entries and choose what to fix first
Reading a large file line by line is inefficient. MySQL ships mysqldumpslow to summarize recurring statements. For the 20 statements with the greatest total time, use:
mysqldumpslow -s t -t 20 /path/to/host-slow.logThe path must be the active file on the server or a copy you have exported for analysis. Grouping normalizes similar statements so that repeated work is visible even when literal values differ.
Prioritize candidates using four signals:
- Total time: repeated moderate delays can cost more than one rare, very slow statement.
- Frequency: a statement executed thousands of times may deserve attention before a one-off operation.
- Rows examined: high values suggest excess scanning or filtering work.
- Rows sent: compare this with rows examined to see whether the server does much more work than the client receives.
A high rows-examined-to-rows-sent gap combined with high repetition is a strong candidate for investigation. It is not a license to add an index blindly: writes must maintain every additional index, indexes consume storage, and a new index can destabilize plans for other parameter values.
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Investigate a representative statement
Take a real statement pattern from the summary and run an execution plan with representative parameters:
EXPLAIN SELECT ...;Use the deployed MySQL version’s execution-analysis facility where it is supported and where the additional execution work is safe. Compare the plan with the observed slow-log fields. Look for full scans on large tables, joins that examine many rows before filtering, predicates that cannot use the available index, unnecessary sorting or temporary tables, and estimates that differ greatly from actual work.
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Remediation options and their trade-offs
| Option | When it helps | Costs and risks to check | How to validate |
|---|---|---|---|
| Add or adjust an index | Filtering, joining, or ordering repeatedly scans many rows that could be narrowed by a selective key. | Extra storage and write maintenance; possible duplicate or redundant indexes; plan changes for other queries. | Compare execution plans, rows examined, write latency, and slow-log recurrence before and after. |
| Rewrite SQL | Predicates, joins, subqueries, sorting, or selected columns force unnecessary work. | Different semantics, changed locking behavior, and regressions for other parameter values. | Run correctness tests and compare plans and timings under representative parameters. |
| Change schema or data access pattern | Repeated aggregation, archival, or access patterns cannot be served efficiently by the current design. | Migration complexity, backfill time, application changes, and operational risk. | Measure the complete request path, not only the isolated query. |
| Adjust server logging thresholds | The immediate need is better diagnosis or less diagnostic volume. | It does not make a query faster; broad capture adds I/O, storage, and review work. | Confirm the intended entries are captured and return to a sustainable threshold. |
Use the slow log to identify and verify a problem, not as a substitute for an execution plan. The best fix reduces latency and rows examined without creating unacceptable write cost, storage use, plan instability, or deployment risk.
A repeatable troubleshooting checklist
- Run the global-variable query and save the current values.
- Confirm
slow_query_log=ON. - Confirm
log_outputincludes the destination you are checking. - Read
@@global.slow_query_log_fileif using file output. - Check
long_query_timeandmin_examined_row_limitagainst the statement you expect to capture. - Decide whether administrative or no-index statements need explicit inclusion.
- Verify the server host, service-account permissions, security policy, and free disk space.
- Generate or observe a safe, representative test during a bounded interval.
- Summarize recurring statements with
mysqldumpslow. - Use
EXPLAINand version-supported execution analysis before changing SQL or indexes. - Test the change under realistic data and concurrency.
- Restore thresholds, disable temporary broad-capture options, and retain only the startup settings you intend to operate.
Conclusion
Enabling the MySQL slow query log is a three-part configuration task: turn on slow_query_log, select an output destination, and choose thresholds that match the diagnostic question. An empty log is usually explained by the destination, threshold, row-count filter, statement category, or an incorrect file path—not by the logger switch alone. Once entries appear, summarize recurring work, validate representative statements with execution plans, and weigh latency gains against index write cost, storage, plan stability, and operational risk.
Frequently Asked Questions
Does setting slow_query_log=ON automatically create a file?
No. File output requires log_output to include FILE. If output is TABLE, records go to mysql.slow_log; if it is NONE, no query entries are written.
What is the default MySQL slow-query threshold?
The documented default for long_query_time is 10 seconds. A statement must also satisfy min_examined_row_limit when that value is greater than zero.
Why is a query that does not use an index missing from the log?
No-index queries are excluded by default. Enable log_queries_not_using_indexes for that diagnostic purpose, and consider log_throttle_queries_not_using_indexes to limit bursts.
Why can a replica lack slow-log entries for replicated work?
Replicas do not log replicated queries by default. MySQL 8.0.26 and later use log_slow_replica_statements; earlier releases use log_slow_slave_statements. Row-based replication also limits what these settings reveal.
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