How to resolve long running queries in Oracle 19c

Introduction

Oracle database is powerful and used by almost all the companies in their environment to handle their data. Oracle is known for its strong performance, scalability, and dependability along with various features like Backup and recovery, Security, High availability, and many more.

In this article, we will explore a detailed, step-by-step guide on how to effectively troubleshoot and resolve issues related to long running Queries in Oracle.

As we all know, whenever the application team experiences slowness or an SQL running slow in Oracle, they often go straight to the DBA team without checking anything else. Their usual request is, “Please check the database. Our query was running perfectly fine yesterday, but today it’s taking much longer to finish”.

So today we will see a real-time scenario of the long running queries in Oracle along with the steps and guide to resolve them.

How to find the long running queries in Oracle

We need to follow the below steps to find the long running queries in Oracle.

1. Login and check the status of the Database.

set lines 250 pages 250
col HOST_NAME for a15
col DB_Start_Time for a20
SELECT NAME as DB_NAME,OPEN_MODE,instance_name,status,HOST_NAME,database_role,logins,to_char(startup_time,’DD-MON-YYYY HH24:MI’) DB_Start_Time FROM
gV$INSTANCE,v$database;

2. Check the Blocking Session

set lines 200 pages 200
SELECT DISTINCT S1.USERNAME || ‘@’ || S1.MACHINE
|| ‘ ( INST=’ || S1.INST_ID || ‘ SID=’ || S1.SID || ‘ ) IS BLOCKING ‘
|| S2.USERNAME || ‘@’ || S2.MACHINE || ‘ ( INST=’ || S1.INST_ID || ‘ SID=’ || S2.SID || ‘ ) ‘ AS BLOCKING_STATUS
FROM GV$LOCK L1, GV$SESSION S1, GV$LOCK L2, GV$SESSION S2
WHERE S1.SID=L1.SID AND S2.SID=L2.SID
AND S1.INST_ID=L1.INST_ID AND S2.INST_ID=L2.INST_ID
AND L1.BLOCK > 0 AND L2.REQUEST > 0
AND L1.ID1 = L2.ID1 AND L1.ID2 = L2.ID2;

3. Find the long-running session

ACTIVE Sessions Details

SET LINES 999 PAGES 999
col username for a15
col sql_id for a15
alter session set nls_date_format=’DD-MON-YYYY HH24:MI:SS’;
select sysdate from dual;
col status for a10
col machine for a20
COL PROGRAM FOR A25
COL OSUSER FOR A15
COL USERNAME FOR a30
col EVENT for a20
SELECT sid,sql_id,serial#,STATUS, osuser,username,last_call_et, logon_time,state
FROM gv$session where STATUS=’ACTIVE’ and username not in (‘SYS’,’SYSTEM’,’PUBLIC’) order by logon_time;

INACTIVE Sessions Details

SET LINES 999 PAGES 999
col username for a15
col sql_id for a15
alter session set nls_date_format=’DD-MON-YYYY HH24:MI:SS’;
select sysdate from dual;
col status for a10
col machine for a20
COL PROGRAM FOR A25
COL OSUSER FOR A15
COL USERNAME FOR a30
col EVENT for a20
SELECT sid,sql_id,serial#,STATUS, osuser,username,last_call_et, logon_time,state
FROM gv$session where STATUS=’INACTIVE’ and username not in (‘SYS’,’SYSTEM’,’PUBLIC’) order by logon_time;

Long Running Session Details

set lines 999 pages 999
SELECT sid, serial#, context, sofar, totalwork,round(sofar/totalwork*100,2) “% Complete” FROM v$session_longops;

====================

set pagesize 999
set linesize 999
column opname format a18
col %CMP format 99999
select sid, opname, sofar, totalwork,
TIME_REMAINING REMAINING , ELAPSED_SECONDS ELPSecs, round(sofar/totalwork*100,2) “%CMP”
from v$session_longops
where time_remaining > 0
/

=====================

col TIME_REMAINING for 999999
col opname for a30
col target for a30
SELECT inst_id,sID, SERIAL#, CONTEXT, SOFAR, TOTALWORK,opname,target,
ROUND(SOFAR/TOTALWORK*100,2) “%_COMPLETE”,TIME_REMAINING
FROM gV$SESSION_LONGOPS where OPNAME NOT LIKE ‘%aggregate%’ AND TOTALWORK != 0 AND SOFAR <> TOTALWORK;

Output

4. Find the sql_text for a sql_id

We have got the SQL_ID from the above output and now we need to find out the text from SQL_ID.

Command to find out the SQL_TEXT from SQL_ID

set long 99999
select sql_id,sql_fulltext from gv$sql where sql_id IN (‘&SQL_ID‘);

Command to check the wait event

select sid,serial#,username,machine, event,sql_id from gv$session where sid =’&SID‘;

5. Check the Stats of the Tables

SET LINES 200 PAGES 200
COL OWNER FOR A30
COL TABLE_NAME FOR A30
COL PARTITION_NAME for a20
COL SUBPARTITION_NAME for a20
select table_name,OWNER,PARTITION_NAME,SUBPARTITION_NAME, stale_stats, last_analyzed from dba_tab_statistics where table_name IN (‘&TABLE_NAME‘);

Output

Note: – We can check from the above output table is in the STALE state which is not good.

6. Find out the PLAN_HASH_VALUE for the SQL_ID

Command to find our PLAN_HASH_VALUE history for the SQL_ID

set lines 999 pages 999
col END_TIME for a30
alter session set nls_date_format=’mm/dd/yyyy hh24:mi’;
SELECT
a.sql_id,
TO_CHAR (begin_interval_time, ‘mm/dd/yyyy hh24:mi’) as START_TIME,
TO_CHAR (end_interval_time, ‘mm/dd/yyyy hh24:mi’) as END_TIME,
plan_hash_value,
executions_delta,
rows_processed_delta,
ROUND (elapsed_time_delta / 1000000) / executions_delta
“Elapsed_per_exec_sec”
FROM dba_hist_sqlstat a, dba_hist_snapshot b
WHERE sql_id IN (‘&SQL_ID’)
AND a.snap_id = b.snap_id
AND executions_delta > 0
ORDER BY 2;

Output

Note: – We can check from the above output PLAN_HASH_VALUE has been changed which picks up the terrible plan because it has not processed a single row, but it has taken lots of time due to the app team facing a slowness issue.

PLAN_HASH_VALUE can be changed due to some factors like :

a) Stats have not been gathered properly due to which Optimizer will not get updated statistics and as a result it may pick a bad plan.

b) If any changes are made in the SQL query by the application team then optimizes different plan hash values.

c) Performace issues also occur if there is a full table scan happening for large tables so we need to suggest the application team to do proper indexing, if we want to see some recommendations from Oracle we can use SQL TUNING ADVISOR for the same.

7. Gather the stats of the Table

We make sure to update the application team that they should inform the DBA team in advance if there are lots of DML statements running and tables are modifying regularly. So that the DBA team makes scripts and automates them with the help of CRON jobs or other tools to gather the stats of those tables at some intervals of time to avoid any STALE issues.

TABLE STAT Gather Script

execute dbms_stats.gather_table_stats(‘&SCHEMA_NAME‘,’&TABLE_NAME‘,cascade=>true,estimate_percent=>dbms_stats.auto_sample_size,degree=>dbms_stats.auto_degree);

Command to check the STATS of a TABLE

SET LINES 200 PAGES 200
COL OWNER FOR A30
COL TABLE_NAME FOR A30
COL PARTITION_NAME for a20
COL SUBPARTITION_NAME for a20
select table_name,OWNER,PARTITION_NAME,SUBPARTITION_NAME, stale_stats, last_analyzed from dba_tab_statistics where table_name IN (‘&TABLE_NAME‘);

Output

8. Query to Find Execution Plan Details

Command to find the Execution Plan so that we will get to know what all operations like Full Table Scan, Index Scan, Rows, CPU cost, and much more have performed in a Tree shape structure.

Execution Plan Details from Cursor for a SQL_ID

SET LINESIZE 150
SET PAGESIZE 2000
SELECT * FROM table (DBMS_XPLAN.DISPLAY_CURSOR(‘&SQL_ID‘));

Execution Plan Details from AWR Repository for a SQL_ID

SET LINESIZE 150
SET PAGESIZE 2000
SELECT * FROM table (DBMS_XPLAN.DISPLAY_AWR(sql_id => ‘&SQL_ID‘, plan_hash_value => &PLAN_HASH_VALUE));

9. Purge the Shared Pool for the SQL_ID

set lines 999 pages 999
select inst_id ,address||’,’||hash_value from gv$sqlarea where sql_id like ‘&SQL_ID’;

exec sys.dbms_shared_pool.purge(‘&ADDRESS , &HASH_VALUE ‘,’C’,1);

Output

10. Pin the Good Execution Plan

We have to execute the coe_xfr_sql_profile.sql script to pin the good execution plan.

Output

sys@ORAXXXX> @coe_xfr_sql_profile.sql

Parameter 1:
SQL_ID (required)

Enter a value for 1: 7q70u4a3817k2 —————–> Enter the SQL_ID here

PLAN_HASH_VALUE AVG_ET_SECS
—————– ———————————
3583374745 2.858————————-> Good PLAN_HASH_VALUE (3583374745)
1194960972 9921.22————————> Bad PLAN_HASH_VALUE (1194960972)

Parameter 2:
PLAN_HASH_VALUE (required)

Enter a value for 2: 3583374745———————-> Enter the Good PLAN_HASH_VALUE

Values passed:
————–
SQL_ID : “7q70u4a3817k2”
PLAN_HASH_VALUE: “3583374745”

Execute coe_xfr_sql_profile_7q70u4a3817k2_3583374745.sql on the TARGET system to create a custom SQL Profile with plan 3583374745 linked to adjusted sql_text.

COE_XFR_SQL_PROFILE completed.

Note: – Now we need to execute coe_xfr_sql_profile_7q70u4a3817k2_3583374745.sql script to make the changes in the PLAN_HASH_VALUE for the SQL_ID.

Output

sys@ORAXXXX> @coe_xfr_sql_profile_7q70u4a3817k2_3583374745.sql

11. Check the status of the SQL Profile

set lines 999 pages 999
col CREATED for a30
col NAME for a40
col FORCE_MATCHING for a20
select NAME,TYPE,STATUS,FORCE_MATCHING,CREATED from dba_sql_profiles where NAME IN (‘&SQL_PROFILE‘);

Output

Note: – After performing all the steps we asked the application team to re-run the query again and this time their query was completed in just a few minutes.

Conclusion

We have seen all the above steps which helped me to resolve the long running queries in Oracle. You can also take the reference if you face this type of issue in your environment.

FAQ’s

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