Statement Summary Tables
To better handle SQL performance issues, MySQL has provided statement summary tables in performance_schema
to monitor SQL with statistics. Among these tables, events_statements_summary_by_digest
is very useful in locating SQL problems with its abundant fields such as latency, execution times, rows scanned, and full table scans.
Therefore, starting from v4.0.0-rc.1, TiDB provides system tables in information_schema
(not performance_schema
) that are similar to events_statements_summary_by_digest
in terms of features.
statements_summary
statements_summary_history
cluster_statements_summary
cluster_statements_summary_history
statements_summary_evicted
This document details these tables and introduces how to use them to troubleshoot SQL performance issues.
statements_summary
statements_summary
is a system table in information_schema
. statements_summary
groups the SQL statements by the SQL digest and the plan digest, and provides statistics for each SQL category.
The "SQL digest" here means the same as used in slow logs, which is a unique identifier calculated through normalized SQL statements. The normalization process ignores constant, blank characters, and is case insensitive. Therefore, statements with consistent syntaxes have the same digest. For example:
SELECT * FROM employee WHERE id IN (1, 2, 3) AND salary BETWEEN 1000 AND 2000;
select * from EMPLOYEE where ID in (4, 5) and SALARY between 3000 and 4000;
After normalization, they are both of the following category:
select * from employee where id in (...) and salary between ? and ?;
The "plan digest" here refers to the unique identifier calculated through normalized execution plan. The normalization process ignores constants. The same SQL statements might be grouped into different categories because the same statements might have different execution plans. SQL statements of the same category have the same execution plan.
statements_summary
stores the aggregated results of SQL monitoring metrics. In general, each of the monitoring metrics includes the maximum value and average value. For example, the execution latency metric corresponds to two fields: AVG_LATENCY
(average latency) and MAX_LATENCY
(maximum latency).
To make sure that the monitoring metrics are up to date, data in the statements_summary
table is periodically cleared, and only recent aggregated results are retained and displayed. The periodical data clearing is controlled by the tidb_stmt_summary_refresh_interval
system variable. If you happen to make a query right after the clearing, the data displayed might be very little.
The following is a sample output of querying statements_summary
:
SUMMARY_BEGIN_TIME: 2020-01-02 11:00:00
SUMMARY_END_TIME: 2020-01-02 11:30:00
STMT_TYPE: Select
SCHEMA_NAME: test
DIGEST: 0611cc2fe792f8c146cc97d39b31d9562014cf15f8d41f23a4938ca341f54182
DIGEST_TEXT: select * from employee where id = ?
TABLE_NAMES: test.employee
INDEX_NAMES: NULL
SAMPLE_USER: root
EXEC_COUNT: 3
SUM_LATENCY: 1035161
MAX_LATENCY: 399594
MIN_LATENCY: 301353
AVG_LATENCY: 345053
AVG_PARSE_LATENCY: 57000
MAX_PARSE_LATENCY: 57000
AVG_COMPILE_LATENCY: 175458
MAX_COMPILE_LATENCY: 175458
...........
AVG_MEM: 103
MAX_MEM: 103
AVG_DISK: 65535
MAX_DISK: 65535
AVG_AFFECTED_ROWS: 0
FIRST_SEEN: 2020-01-02 11:12:54
LAST_SEEN: 2020-01-02 11:25:24
QUERY_SAMPLE_TEXT: select * from employee where id=3100
PREV_SAMPLE_TEXT:
PLAN_DIGEST: f415b8d52640b535b9b12a9c148a8630d2c6d59e419aad29397842e32e8e5de3
PLAN: Point_Get_1 root 1 table:employee, handle:3100
statements_summary_history
The table schema of statements_summary_history
is identical to that of statements_summary
. statements_summary_history
saves the historical data of a time range. By checking historical data, you can troubleshoot anomalies and compare monitoring metrics of different time ranges.
The fields SUMMARY_BEGIN_TIME
and SUMMARY_END_TIME
represent the start time and the end time of the historical time range.
statements_summary_evicted
The tidb_stmt_summary_max_stmt_count
variable controls the maximum number of statements that the statement_summary
table stores in memory. The statement_summary
table uses the LRU algorithm. Once the number of SQL statements exceeds the tidb_stmt_summary_max_stmt_count
value, the longest unused record is evicted from the table. The number of evicted SQL statements during each period is recorded in the statements_summary_evicted
table.
The statements_summary_evicted
table is updated only when a SQL record is evicted from the statement_summary
table. The statements_summary_evicted
only records the period during which the eviction occurs and the number of evicted SQL statements.
The cluster
tables for statement summary
The statements_summary
, statements_summary_history
, and statements_summary_evicted
tables only show the statement summary of a single TiDB server. To query the data of the entire cluster, you need to query the cluster_statements_summary
, cluster_statements_summary_history
, or cluster_statements_summary_evicted
tables.
cluster_statements_summary
displays the statements_summary
data of each TiDB server. cluster_statements_summary_history
displays the statements_summary_history
data of each TiDB server. cluster_statements_summary_evicted
displays the statements_summary_evicted
data of each TiDB server. These tables use the INSTANCE
field to represent the address of the TiDB server. The other fields are the same as those in statements_summary
, statements_summary_history
, and statements_summary_evicted
.
Parameter configuration
The following system variables are used to control the statement summary:
tidb_enable_stmt_summary
: Determines whether to enable the statement summary feature.1
representsenable
, and0
meansdisable
. The feature is enabled by default. The statistics in the system table are cleared if this feature is disabled. The statistics are re-calculated next time this feature is enabled. Tests have shown that enabling this feature has little impact on performance.tidb_stmt_summary_refresh_interval
: The interval at which thestatements_summary
table is refreshed. The time unit is second (s). The default value is1800
.tidb_stmt_summary_history_size
: The size of each SQL statement category stored in thestatements_summary_history
table, which is also the maximum number of records in thestatements_summary_evicted
table. The default value is24
.
tidb_stmt_summary_max_stmt_count
: Limits the number of SQL statements that can be stored in statement summary tables. The default value is3000
. If the limit is exceeded, TiDB clears the SQL statements that recently remain unused. These cleared SQL statements are represented as rows withDIGEST
set toNULL
and recorded in thestatements_summary_evicted
table. On the SQL statements page of TiDB Dashboard, the information of these rows is displayed asOthers
.
tidb_stmt_summary_max_sql_length
: Specifies the longest display length ofDIGEST_TEXT
andQUERY_SAMPLE_TEXT
. The default value is4096
.tidb_stmt_summary_internal_query
: Determines whether to count the TiDB SQL statements.1
means to count, and0
means not to count. The default value is0
.
An example of the statement summary configuration is shown as follows:
set global tidb_stmt_summary_max_stmt_count = 3000;
set global tidb_enable_stmt_summary = true;
set global tidb_stmt_summary_refresh_interval = 1800;
set global tidb_stmt_summary_history_size = 24;
After the preceding configuration takes effect, the statements_summary
table is cleared every 30 minutes and the statements_summary_history
table stores up to 3000 types of SQL statements. For each type, the statements_summary_history
table stores data for the recent 24 periods. The statements_summary_evicted
table records the recent 24 periods during which SQL statements are evicted from the statement summary. The statements_summary_evicted
table is updated every 30 minutes.
Set a proper size for statement summary
After the system has run for a period of time (depending on the system load), you can check the statement_summary
table to see whether SQL eviction has occurred. For example:
select @@global.tidb_stmt_summary_max_stmt_count;
select count(*) from information_schema.statements_summary;
+-------------------------------------------+
| @@global.tidb_stmt_summary_max_stmt_count |
+-------------------------------------------+
| 3000 |
+-------------------------------------------+
1 row in set (0.001 sec)
+----------+
| count(*) |
+----------+
| 3001 |
+----------+
1 row in set (0.001 sec)
You can see that the statements_summary
table is full of records. Then check the evicted data from the statements_summary_evicted
table:
select * from information_schema.statements_summary_evicted;
+---------------------+---------------------+---------------+
| BEGIN_TIME | END_TIME | EVICTED_COUNT |
+---------------------+---------------------+---------------+
| 2020-01-02 16:30:00 | 2020-01-02 17:00:00 | 59 |
+---------------------+---------------------+---------------+
| 2020-01-02 16:00:00 | 2020-01-02 16:30:00 | 45 |
+---------------------+---------------------+---------------+
2 row in set (0.001 sec)
From the preceding result, you can see that a maximum of 59 SQL categories are evicted. In this case, it is recommended that you increase the size of the statement_summary
table by at least 59 records, which means increasing the size to at least 3059 records.
Limitation
The statement summary tables have the following limitation:
All data of the statement summary tables above will be lost when the TiDB server is restarted. This is because statement summary tables are all memory tables, and the data is cached in memory instead of being persisted on storage.
Troubleshooting examples
This section provides two examples to show how to use the statement summary feature to troubleshoot SQL performance issues.
Could high SQL latency be caused by the server end?
In this example, the client shows slow performance with point queries on the employee
table. You can perform a fuzzy search on SQL texts:
SELECT avg_latency, exec_count, query_sample_text
FROM information_schema.statements_summary
WHERE digest_text LIKE 'select * from employee%';
1ms
and 0.3ms
are considered within the normal range of avg_latency
. Therefore, it can be concluded that the server end is not the cause. You can troubleshoot with the client or the network.
+-------------+------------+------------------------------------------+
| avg_latency | exec_count | query_sample_text |
+-------------+------------+------------------------------------------+
| 1042040 | 2 | select * from employee where name='eric' |
| 345053 | 3 | select * from employee where id=3100 |
+-------------+------------+------------------------------------------+
2 rows in set (0.00 sec)
Which categories of SQL statements consume the longest total time?
If the QPS decrease significantly from 10:00 to 10:30, you can find out the three categories of SQL statements with the longest time consumption from the history table:
SELECT sum_latency, avg_latency, exec_count, query_sample_text
FROM information_schema.statements_summary_history
WHERE summary_begin_time='2020-01-02 10:00:00'
ORDER BY sum_latency DESC LIMIT 3;
The result shows that the following three categories of SQL statements consume the longest time in total, which need to be optimized with high priority.
+-------------+-------------+------------+-----------------------------------------------------------------------+
| sum_latency | avg_latency | exec_count | query_sample_text |
+-------------+-------------+------------+-----------------------------------------------------------------------+
| 7855660 | 1122237 | 7 | select avg(salary) from employee where company_id=2013 |
| 7241960 | 1448392 | 5 | select * from employee join company on employee.company_id=company.id |
| 2084081 | 1042040 | 2 | select * from employee where name='eric' |
+-------------+-------------+------------+-----------------------------------------------------------------------+
3 rows in set (0.00 sec)
Fields description
statements_summary
fields description
The following are descriptions of fields in the statements_summary
table.
Basic fields:
STMT_TYPE
: SQL statement type.SCHEMA_NAME
: The current schema in which SQL statements of this category are executed.DIGEST
: The digest of SQL statements of this category.DIGEST_TEXT
: The normalized SQL statement.QUERY_SAMPLE_TEXT
: The original SQL statements of the SQL category. Only one original statement is taken.TABLE_NAMES
: All tables involved in SQL statements. If there is more than one table, each is separated by a comma.INDEX_NAMES
: All SQL indexes used in SQL statements. If there is more than one index, each is separated by a comma.SAMPLE_USER
: The users who execute SQL statements of this category. Only one user is taken.PLAN_DIGEST
: The digest of the execution plan.PLAN
: The original execution plan. If there are multiple statements, the plan of only one statement is taken.BINARY_PLAN
: The original execution plan encoded in binary format. If there are multiple statements, the plan of only one statement is taken. Execute theSELECT tidb_decode_binary_plan('xxx...')
statement to parse the specific execution plan.PLAN_CACHE_HITS
: The total number of times that SQL statements of this category hit the plan cache.PLAN_IN_CACHE
: Indicates whether the previous execution of SQL statements of this category hit the plan cache.
Fields related to execution time:
SUMMARY_BEGIN_TIME
: The beginning time of the current summary period.SUMMARY_END_TIME
: The ending time of the current summary period.FIRST_SEEN
: The time when SQL statements of this category are seen for the first time.LAST_SEEN
: The time when SQL statements of this category are seen for the last time.
Fields related to TiDB server:
EXEC_COUNT
: Total execution times of SQL statements of this category.SUM_ERRORS
: The sum of errors occurred during execution.SUM_WARNINGS
: The sum of warnings occurred during execution.SUM_LATENCY
: The total execution latency of SQL statements of this category.MAX_LATENCY
: The maximum execution latency of SQL statements of this category.MIN_LATENCY
: The minimum execution latency of SQL statements of this category.AVG_LATENCY
: The average execution latency of SQL statements of this category.AVG_PARSE_LATENCY
: The average latency of the parser.MAX_PARSE_LATENCY
: The maximum latency of the parser.AVG_COMPILE_LATENCY
: The average latency of the compiler.MAX_COMPILE_LATENCY
: The maximum latency of the compiler.AVG_MEM
: The average memory (byte) used.MAX_MEM
: The maximum memory (byte) used.AVG_DISK
: The average disk space (byte) used.MAX_DISK
: The maximum disk space (byte) used.
Fields related to TiKV Coprocessor task:
SUM_COP_TASK_NUM
: The total number of Coprocessor requests sent.MAX_COP_PROCESS_TIME
: The maximum execution time of Coprocessor tasks.MAX_COP_PROCESS_ADDRESS
: The address of the Coprocessor task with the maximum execution time.MAX_COP_WAIT_TIME
: The maximum waiting time of Coprocessor tasks.MAX_COP_WAIT_ADDRESS
: The address of the Coprocessor task with the maximum waiting time.AVG_PROCESS_TIME
: The average processing time of SQL statements in TiKV.MAX_PROCESS_TIME
: The maximum processing time of SQL statements in TiKV.AVG_WAIT_TIME
: The average waiting time of SQL statements in TiKV.MAX_WAIT_TIME
: The maximum waiting time of SQL statements in TiKV.AVG_BACKOFF_TIME
: The average waiting time before retry when a SQL statement encounters an error that requires a retry.MAX_BACKOFF_TIME
: The maximum waiting time before retry when a SQL statement encounters an error that requires a retry.AVG_TOTAL_KEYS
: The average number of keys that Coprocessor has scanned.MAX_TOTAL_KEYS
: The maximum number of keys that Coprocessor has scanned.AVG_PROCESSED_KEYS
: The average number of keys that Coprocessor has processed. Compared withavg_total_keys
,avg_processed_keys
does not include the old versions of MVCC. A great difference betweenavg_total_keys
andavg_processed_keys
indicates that many old versions exist.MAX_PROCESSED_KEYS
: The maximum number of keys that Coprocessor has processed.
Transaction-related fields:
AVG_PREWRITE_TIME
: The average time of the prewrite phase.MAX_PREWRITE_TIME
: The longest time of the prewrite phase.AVG_COMMIT_TIME
: The average time of the commit phase.MAX_COMMIT_TIME
: The longest time of the commit phase.AVG_GET_COMMIT_TS_TIME
: The average time of gettingcommit_ts
.MAX_GET_COMMIT_TS_TIME
: The longest time of gettingcommit_ts
.AVG_COMMIT_BACKOFF_TIME
: The average waiting time before retry when a SQL statement encounters an error that requires a retry during the commit phase.MAX_COMMIT_BACKOFF_TIME
: The maximum waiting time before retry when a SQL statement encounters an error that requires a retry during the commit phase.AVG_RESOLVE_LOCK_TIME
: The average time for resolving lock conflicts occurred between transactions.MAX_RESOLVE_LOCK_TIME
: The longest time for resolving lock conflicts occurred between transactions.AVG_LOCAL_LATCH_WAIT_TIME
: The average waiting time of the local transaction.MAX_LOCAL_LATCH_WAIT_TIME
: The maximum waiting time of the local transaction.AVG_WRITE_KEYS
: The average count of written keys.MAX_WRITE_KEYS
: The maximum count of written keys.AVG_WRITE_SIZE
: The average amount of written data (in byte).MAX_WRITE_SIZE
: The maximum amount of written data (in byte).AVG_PREWRITE_REGIONS
: The average number of Regions involved in the prewrite phase.MAX_PREWRITE_REGIONS
: The maximum number of Regions during the prewrite phase.AVG_TXN_RETRY
: The average number of transaction retries.MAX_TXN_RETRY
: The maximum number of transaction retries.SUM_BACKOFF_TIMES
: The sum of retries when SQL statements of this category encounter errors that require a retry.BACKOFF_TYPES
: All types of errors that require retries and the number of retries for each type. The format of the field istype:number
. If there is more than one error type, each is separated by a comma, liketxnLock:2,pdRPC:1
.AVG_AFFECTED_ROWS
: The average number of rows affected.PREV_SAMPLE_TEXT
: When the current SQL statement isCOMMIT
,PREV_SAMPLE_TEXT
is the previous statement toCOMMIT
. In this case, SQL statements are grouped by the digest andprev_sample_text
. This means thatCOMMIT
statements with differentprev_sample_text
are grouped to different rows. When the current SQL statement is notCOMMIT
, thePREV_SAMPLE_TEXT
field is an empty string.
statements_summary_evicted
fields description
BEGIN_TIME
: Records the starting time.END_TIME
: Records the ending time.EVICTED_COUNT
: The number of SQL categories that are evicted during the record period.