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Updated on 2024-04-25 GMT+08:00
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Analyzing SQL Statements that Are Being Executed

During development, developers often encounter problems such as excessive SQL connections, long SQL query time, and SQL query blocking. You can use the PG_STAT_ACTIVITY and PGXC_THREAD_WAIT_STATUS views to analyze and locate SQL problems. This section describes some common locating methods.

Table 1 Some PG_STAT_ACTIVITY fields

Name

Type

Description

usename

name

Name of the user logging in to the backend

client_addr

inet

IP address of the client connected to the backend null indicates either that the client is connected via a Unix socket on the server machine or that this is an internal process such as autovacuum.

application_name

text

Name of the application connected to the backend

state

text

Overall state of the backend. The value can be:

  • active: The backend is executing queries.
  • idle: The backend is waiting for new client commands.
  • idle in transaction: The backend is in a transaction, but there is no statement being executed in the transaction.
  • idle in transaction (aborted): The backend is in a transaction, but there are statements failed in the transaction.
  • fastpath function call: The backend is executing a fast-path function.
  • disabled: This state is reported if track_activities is disabled in this backend.
NOTE:

Common users can view only the session status of their own accounts. That is, the state information of other accounts is empty.

waiting

boolean

If the back end is currently waiting for a lock, the value is t. Otherwise, the value is f.

  • t stands for true.
  • f stands for false.

enqueue

text

Queuing status of a statement. Its value can be:

  • waiting in global queue: The statement is queuing in the global concurrency queue. The number of concurrent statements exceeds the value of max_active_statements configured for a single CN.
  • waiting in respool queue: The statement is queuing in the resource pool and the concurrency of simple jobs is limited. The main reason is that the concurrency of simple jobs exceeds the upper limit max_dop of the fast track.
  • waiting in ccn queue: The job is in the CCN queue, which may be global memory queuing, slow lane memory queuing, or concurrent queuing. The scenarios are:
    1. The available global memory exceeds the upper limit, the job is queuing in the global memory queue.
    2. Concurrent requests on the slow lane in the resource pool exceed the upper limit, which is specified by active_statements.
    3. The slow lane memory of the resource pool exceeds the upper limit, that is, the estimated memory of concurrent jobs in the resource pool exceeds the upper limit specified by mem_percent.
  • Empty or no waiting queue: The statement is running.

pid

bigint

ID of the backend thread.

Viewing Connection Information

  • Set track_activities to on.
    SET track_activities = on;

    The database collects the running information about active queries only if this parameter is set to on.

  • You can run the following SQL statements to check the current connection user, connection address, connection application, status, whether to wait for a lock, queuing status, and thread ID. 
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    SELECT usename,client_addr,application_name,state,waiting,enqueue,pid FROM PG_STAT_ACTIVITY WHERE DATNAME='database name';

    The following command output is displayed:

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     usename |  client_addr  | application_name | state  | waiting | enqueue |       pid       
---------+---------------+------------------+--------+---------+---------+-----------------
 leo     | 192.168.0.133 | gsql             | idle   | f       |         | 139666091022080
 dbadmin | 192.168.0.133 | gsql             | active | f       |         | 139666212681472
 joe     | 192.168.0.133 |                  | idle   | f       |         | 139665671489280
(3 rows)
  • End a session (only the system administrator has the permission). 
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    SELECT PG_TERMINATE_BACKEND(pid);

Viewing SQL Running Information

  • Run the following command to obtain all SQL information that the current user has permission to view (if the current user has administrator or preset role permission, all user query information can be displayed): 
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    SELECT usename,state,query FROM PG_STAT_ACTIVITY WHERE DATNAME='database name';
    If the value of state is active, the query column indicates the SQL statement that is being executed. In other cases, the query column indicates the previous query statement. If the value of state is idle, the connection is idle and waits for the user to enter a command. The following command output is displayed: 
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     usename | state  |                                   query                                   
---------+--------+---------------------------------------------------------------------------
 leo     | idle   | select * from joe.mytable;
 dbadmin | active | SELECT usename,state,query FROM PG_STAT_ACTIVITY WHERE DATNAME='gaussdb';
 joe     | idle   | GRANT SELECT ON TABLE mytable to leo;
(3 rows)
  • Run the following command to view the information about the SQL statements that are not in the idle state: 
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    SELECT datname,usename,query FROM PG_STAT_ACTIVITY WHERE state != 'idle' ;

Viewing Time-Consuming Statements

  • Check the SQL statements that take a long time to execute. 
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    SELECT current_timestamp - query_start as runtime, datname, usename, query FROM PG_STAT_ACTIVITY WHERE state != 'idle' order by 1 desc;

    Query statements are returned and sorted by execution time length in descending order. The first record is the query statement that takes the longest time to execute.

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         runtime     | datname  | usename |                                                                  query                                                                  
-----------------+----------+---------+-----------------------------------------------------------------------------------------------------------------------------------------
 00:04:47.054958 | gaussdb  | leo     | insert into mytable1 select generate_series(1, 10000000);
 00:00:01.72789  | gaussdb  | dbadmin | SELECT current_timestamp - query_start as runtime, datname, usename, query FROM PG_STAT_ACTIVITY WHERE state != 'idle' order by 1 desc;
(2 rows)
  • Alternatively, you can set current_timestamp - query_start to be greater than a threshold to identify query statements that are executed for a duration longer than this threshold. 
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    SELECT query from PG_STAT_ACTIVITY WHERE current_timestamp - query_start > interval '2 days';

Querying Blocked Statements

  • Run the following command to view blocked query statements: 
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    SELECT pid, datname, usename, state, query FROM PG_STAT_ACTIVITY WHERE state <> 'idle' and waiting=true;
    Run the following statement to end the blocked SQL session: 
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    SELECT PG_TERMINATE_BACKEND(pid);
    • In most cases, blocking is caused by internal locks and waiting=true is displayed. You can view the blocking in the pg_stat_activity view.
    • The blocked statements about file write and event schedulers cannot be viewed in the pg_stat_activity view.
  • View information about the blocked query statements, tables, and schemas. 
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    SELECT w.query as waiting_query,
w.pid as w_pid,
w.usename as w_user,
l.query as locking_query,
l.pid as l_pid,
l.usename as l_user,
t.schemaname || '.' || t.relname as tablename
from pg_stat_activity w join pg_locks l1 on w.pid = l1.pid
and not l1.granted join pg_locks l2 on l1.relation = l2.relation
and l2.granted join pg_stat_activity l on l2.pid = l.pid join pg_stat_user_tables t on l1.relation = t.relid
where w.waiting;

    The command output includes a session ID, user information, query status, and table or schema that caused the block.

    After finding the blocked table or schema information, end the faulty session.

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    SELECT PG_TERMINATE_BACKEND(pid);

    If information similar to the following is displayed, the session is successfully terminated:

    PG_TERMINATE_BACKEND 
----------------------  
t 
(1 row)

    If information similar to the following is displayed, the user is attempting to terminate the session, but the session will be reconnected rather than terminated.

    FATAL:  terminating connection due to administrator command 
FATAL:  terminating connection due to administrator command 
The connection to the server was lost. Attempting reset: Succeeded.

    If the PG_TERMINATE_BACKEND function is used by the gsql client to terminate the background threads of the session, the client will be reconnected automatically rather than be terminated.

Parent topic: Database Management