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Identify Slow Queries

To help users identify slow queries, analyze and improve the performance of SQL execution, TiDB outputs the statements whose execution time exceeds slow-threshold (The default value is 300 milliseconds) to slow-query-file (The default value is "tidb-slow.log").

Usage example

# Time: 2019-08-14T09:26:59.487776265+08:00 # Txn_start_ts: 410450924122144769 # User: root@ # Conn_ID: 3086 # Query_time: 1.527627037 # Process_time: 0.07 Request_count: 1 Total_keys: 131073 Process_keys: 131072 Prewrite_time: 0.335415029 Commit_time: 0.032175429 Get_commit_ts_time: 0.000177098 Local_latch_wait_time: 0.106869448 Write_keys: 131072 Write_size: 3538944 Prewrite_region: 1 # DB: test # Is_internal: false # Digest: 50a2e32d2abbd6c1764b1b7f2058d428ef2712b029282b776beb9506a365c0f1 # Stats: t:pseudo # Num_cop_tasks: 1 # Cop_proc_avg: 0.07 Cop_proc_p90: 0.07 Cop_proc_max: 0.07 Cop_proc_addr: # Cop_wait_avg: 0 Cop_wait_p90: 0 Cop_wait_max: 0 Cop_wait_addr: # Mem_max: 525211 # Succ: false insert into t select * from t;

Fields description

Slow query basics:

  • Time: The print time of log.
  • Query_time: The execution time of a statement.
  • Query: A SQL statement. Query is not printed in the slow log, but the corresponding field is called Query after the slow log is mapped to the memory table.
  • Digest: The fingerprint of the SQL statement.
  • Txn_start_ts: The start timestamp and the unique ID of a transaction. You can use this value to search for the transaction-related logs.
  • Is_internal: Whether a SQL statement is TiDB internal. true indicates that a SQL statement is executed internally in TiDB and false indicates that a SQL statement is executed by the user.
  • Index_ids: The IDs of the indexes involved in a statement.
  • Succ: Whether a statement is executed successfully.
  • Backoff_time: The waiting time before retry when a statement encounters errors that require a retry. The common errors as such include: lock occurs, Region split, and tikv server is busy.

Memory usage fields:

  • Memory_max: The maximum memory space used during the execution period of a SQL statement (the unit is byte).

User fields:

  • User: The name of the user who executes this statement.
  • Conn_ID: The Connection ID (session ID). For example, you can use the keyword con:3 to search for the log whose session ID is 3.
  • DB: The current database.

TiKV Coprocessor Task fields:

  • Request_count: The number of Coprocessor requests that a statement sends.
  • Total_keys: The number of keys that Coprocessor has scanned.
  • Process_time: The total processing time of a SQL statement in TiKV. Because data is sent to TiKV concurrently, this value might exceed Query_time.
  • Wait_time: The total waiting time of a statement in TiKV. Because the Coprocessor of TiKV runs a limited number of threads, requests might queue up when all threads of Coprocessor are working. When a request in the queue takes a long time to process, the waiting time of the subsequent requests increases.
  • Process_keys: The number of keys that Coprocessor has processed. Compared with total_keys, processed_keys does not include the old versions of MVCC. A great difference between processed_keys and total_keys indicates that many old versions exist.
  • Cop_proc_avg: The average execution time of cop-tasks.
  • Cop_proc_p90: The P90 execution time of cop-tasks.
  • Cop_proc_max: The maximum execution time of cop-tasks.
  • Cop_proc_addr: The address of the cop-task with the longest execution time.
  • Cop_wait_avg: The average waiting time of cop-tasks.
  • Cop_wait_p90: The P90 waiting time of cop-tasks.
  • Cop_wait_max: The maximum waiting time of cop-tasks.
  • Cop_wait_addr: The address of the cop-task whose waiting time is the longest.

Memory mapping in slow log

You can query the contents of the slow query log by querying the INFORMATION_SCHEMA.SLOW_QUERY table. Each column name in the table corresponds to one field name in the slow log. For table structure, see the introduction to the SLOW_QUERY table in Information Schema.

Query example of SLOW_QUERY

Top-N slow queries

Query the Top 2 slow queries of users. Is_internal=false means excluding slow queries inside TiDB and only querying slow queries of users.

select query_time, query from information_schema.slow_query where is_internal = false order by query_time desc limit 2;

Usage example:

+--------------+------------------------------------------------------------------+ | query_time | query | +--------------+------------------------------------------------------------------+ | 12.77583857 | select * from t_slim, t_wide where t_slim.c0=t_wide.c0; | | 0.734982725 | select t0.c0, t1.c1 from t_slim t0, t_wide t1 where t0.c0=t1.c0; | +--------------+------------------------------------------------------------------+

Query the Top-N slow queries of the test user

In the following example, the slow queries executed by the test user are queried, and the first two results are displayed in reverse order of execution time.

select query_time, query, user from information_schema.slow_query where is_internal = false and user = "test" order by query_time desc limit 2;

Usage example:

+-------------+------------------------------------------------------------------+----------------+ | Query_time | query | user | +-------------+------------------------------------------------------------------+----------------+ | 0.676408014 | select t0.c0, t1.c1 from t_slim t0, t_wide t1 where t0.c0=t1.c1; | test | +-------------+------------------------------------------------------------------+----------------+

Query similar slow queries with the same SQL fingerprints

After querying the Top-N SQL statements, continue to query similar slow queries using the same fingerprints.

  1. Acquire Top-N slow queries and the corresponding SQL fingerprints.

    select query_time, query, digest from information_schema.slow_query where is_internal = false order by query_time desc limit 1;

    Usage example:

    +-------------+-----------------------------+------------------------------------------------------------------+ | query_time | query | digest | +-------------+-----------------------------+------------------------------------------------------------------+ | 0.302558006 | select * from t1 where a=1; | 4751cb6008fda383e22dacb601fde85425dc8f8cf669338d55d944bafb46a6fa | +-------------+-----------------------------+------------------------------------------------------------------+
  2. Query similar slow queries with the fingerprints.

    select query, query_time from information_schema.slow_query where digest = "4751cb6008fda383e22dacb601fde85425dc8f8cf669338d55d944bafb46a6fa";

    Usage example:

    +-----------------------------+-------------+ | query | query_time | +-----------------------------+-------------+ | select * from t1 where a=1; | 0.302558006 | | select * from t1 where a=2; | 0.401313532 | +-----------------------------+-------------+

Query slow queries with pseudo stats

select query, query_time, stats from information_schema.slow_query where is_internal = false and stats like '%pseudo%';

Usage example:

+-----------------------------+-------------+---------------------------------+ | query | query_time | stats | +-----------------------------+-------------+---------------------------------+ | select * from t1 where a=1; | 0.302558006 | t1:pseudo | | select * from t1 where a=2; | 0.401313532 | t1:pseudo | | select * from t1 where a>2; | 0.602011247 | t1:pseudo | | select * from t1 where a>3; | 0.50077719 | t1:pseudo | | select * from t1 join t2; | 0.931260518 | t1:407872303825682445,t2:pseudo | +-----------------------------+-------------+---------------------------------+

Parse other TiDB slow log files

TiDB uses the session variable tidb_slow_query_file to control the files to be read and parsed when querying INFORMATION_SCHEMA.SLOW_QUERY. You can query the contents of other slow query log files by modifying the value of the session variable.

set tidb_slow_query_file = "/path-to-log/tidb-slow.log"

Parse TiDB slow logs with pt-query-digest

Use pt-query-digest to parse TiDB slow logs.

For example:

pt-query-digest --report tidb-slow.log

Usage example:

# 320ms user time, 20ms system time, 27.00M rss, 221.32M vsz # Current date: Mon Mar 18 13:18:51 2019 # Hostname: localhost.localdomain # Files: tidb-slow.log # Overall: 1.02k total, 21 unique, 0 QPS, 0x concurrency _________________ # Time range: 2019-03-18-12:22:16 to 2019-03-18-13:08:52 # Attribute total min max avg 95% stddev median # ============ ======= ======= ======= ======= ======= ======= ======= # Exec time 218s 10ms 13s 213ms 30ms 1s 19ms # Query size 175.37k 9 2.01k 175.89 158.58 122.36 158.58 # Commit time 46ms 2ms 7ms 3ms 7ms 1ms 3ms # Conn ID 71 1 16 8.88 15.25 4.06 9.83 # Process keys 581.87k 2 103.15k 596.43 400.73 3.91k 400.73 # Process time 31s 1ms 10s 32ms 19ms 334ms 16ms # Request coun 1.97k 1 10 2.02 1.96 0.33 1.96 # Total keys 636.43k 2 103.16k 652.35 793.42 3.97k 400.73 # Txn start ts 374.38E 0 16.00E 375.48P 1.25P 89.05T 1.25P # Wait time 943ms 1ms 19ms 1ms 2ms 1ms 972us . . .

Identify problematic SQL statements

Not all of the SLOW_QUERY statements are problematic. Only those whose process_time is very large increase the pressure on the entire cluster.

The statements whose wait_time is very large and process_time is very small are usually not problematic. This is because the statement is blocked by real problematic statements and it has to wait in the execution queue, which leads to a much longer response time.

admin show slow command

In addition to the TiDB log file, you can identify slow queries by running the admin show slow command:

admin show slow recent N admin show slow top [internal | all] N

recent N shows the recent N slow query records, for example:

admin show slow recent 10

top N shows the slowest N query records recently (within a few days). If the internal option is provided, the returned results would be the inner SQL executed by the system; If the all option is provided, the returned results would be the user's SQL combinated with inner SQL; Otherwise, this command would only return the slow query records from the user's SQL.

admin show slow top 3 admin show slow top internal 3 admin show slow top all 5

TiDB stores only a limited number of slow query records because of the limited memory. If the value of N in the query command is greater than the records count, the number of returned records is smaller than N.

The following table shows output details:

Column nameDescription
startThe starting time of the SQL execution
durationThe duration of the SQL execution
detailsThe details of the SQL execution
succWhether the SQL statement is executed successfully. 1 means success and 0 means failure.
conn_idThe connection ID for the session
transcation_tsThe commit ts for a transaction commit
userThe user name for the execution of the statement
dbThe database involved when the statement is executed
table_idsThe ID of the table involved when the SQL statement is executed
index_idsThe ID of the index involved when the SQL statement is executed
internalThis is a TiDB internal SQL statement
digestThe fingerprint of the SQL statement
sqlThe SQL statement that is being executed or has been executed
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