Identifying SQLs Causing Wait Events Between Snapshots in Oracle

Introduction
What are Wait Events?
SQL Script to Identify Wait Events Between Snapshots
Example: Analyzing Wait Events Between Snapshots
Conclusion

Introduction

When managing an Oracle Database, identifying SQL statements that cause wait events between snapshots can be crucial for diagnosing performance issues and optimizing SQL execution. By analyzing wait events over specific snapshot intervals, database administrators (DBAs) can pinpoint problematic SQLs and take corrective action. This blog post will guide you through identifying SQL statements causing wait events between snapshots using a detailed SQL script.

What are Wait Events?

Wait events in Oracle Database represent the time spent waiting for various resources during SQL execution. These events are essential for understanding performance bottlenecks and identifying inefficiencies in SQL statements. By examining wait events between snapshots, DBAs can track the impact of specific SQLs over time and improve database performance.

SQL Script to Identify Wait Events Between Snapshots

The following SQL script retrieves information about SQL statements causing wait events between specified snapshot IDs. It provides details such as the wait event, SQL ID, username, program, and a snippet of the SQL text: 

select distinct
    ash.event,
    ash.sql_id,
    users.username,
    ash.program,
    dbms_lob.substr(sql_text.sql_text, 4000, 1) sql_text
from
    dba_hist_active_sess_history ash
    left outer join
    dba_users users
        on ash.user_id = users.user_id
    left outer join
    dba_hist_sqltext sql_text
        on ash.sql_id = sql_text.sql_id
where
    ash.snap_id between BEGIN_SNAP_ID and END_SNAP_ID   --REPLACE THE BEGIN AND END SNAPSHOT ID HERE
and
    ash.event = 'wait event'

Explanation of the Script

This script performs the following tasks:
  • Joins dba_hist_active_sess_history with dba_users to retrieve username information.
  • Joins dba_hist_active_sess_history with dba_hist_sqltext to fetch the SQL text.
  • Filters the results to include only rows where the snapshot ID is between BEGIN_SNAP_ID and END_SNAP_ID and the event is 'wait event'.
  • Displays distinct wait events, SQL IDs, usernames, programs, and SQL text snippets.

Example: Analyzing Wait Events Between Snapshots

Consider an example where you want to analyze SQL statements causing wait events between two snapshots. Assume the following details: 

--SQL TO RETRIVE SNAPSHOT IDS
    
select DBID, INSTANCE_NUMBER, SNAP_ID, BEGIN_INTERVAL_TIME from DBA_HIST_SNAPSHOT order by 4 asc;

Begin Snapshot ID: 1000
End Snapshot ID: 1010
Wait Event: 'db file sequential read'


Step-by-Step Example:
1. **Specify the Snapshot IDs**: Replace BEGIN_SNAP_ID and END_SNAP_ID with 1000 and 1010, respectively.
2. **Specify the Wait Event**: Replace 'wait event' with 'db file sequential read'.
3. **Run the Script**: Execute the modified script to retrieve the SQL statements causing the specified wait event between the snapshots.
4. **Analyze the Results**: Review the SQL statements, users, and programs causing the wait events to identify performance bottlenecks.

Example Query: 

select distinct
    ash.event,
    ash.sql_id,
    users.username,
    ash.program,
    dbms_lob.substr(sql_text.sql_text, 4000, 1) sql_text
from
    dba_hist_active_sess_history ash
    left outer join
    dba_users users
        on ash.user_id = users.user_id
    left outer join
    dba_hist_sqltext sql_text
        on ash.sql_id = sql_text.sql_id
where
    ash.snap_id between 1000 and 1010
and
    ash.event = 'db file sequential read'


Interpreting the Results:
- Wait Event: The specific wait event encountered.
- SQL ID: The ID of the SQL statement causing the wait event.
- Username: The user executing the SQL statement.
- Program: The program associated with the SQL execution.
- SQL Text: A snippet of the SQL text causing the wait event.

By analyzing these details, you can identify problematic SQL statements and take corrective actions, such as tuning queries, optimizing indexes, or modifying application logic.

Conclusion

Identifying SQL statements causing wait events between snapshots is a powerful technique for diagnosing and resolving performance issues in Oracle Database. By leveraging the provided SQL script, DBAs can gain insights into the impact of specific SQLs over time and optimize database performance. Regular analysis of wait events between snapshots helps maintain a healthy and efficient Oracle Database environment. Understanding and addressing wait events will lead to faster query performance and a more efficient Oracle Database, ultimately benefiting your overall system performance.

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Last updated in July, 2024

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