Trigger Performance FAQS

1. What is an Oracle trigger?

A trigger is a stored PL/SQL block that automatically executes in response to certain events on a table, view, schema, or database.

• Common types: BEFORE INSERT, AFTER UPDATE, INSTEAD OF DELETE (for views)
• Use case: Auditing, enforcing complex business rules, or cascading actions

 

2. How do triggers affect performance?

Triggers can impact performance because they execute automatically on DML operations (INSERT, UPDATE, DELETE)

Factors affecting performance:

1.    Frequency of firing: Triggers on heavily updated tables can add overhead

2.    Complexity of trigger code: Loops, queries, or heavy PL/SQL logic inside triggers slow down DML

3.    Row vs. statement triggers:

o   Row-level triggers (FOR EACH ROW) execute for every affected row → heavier load on large updates

o   Statement-level triggers execute once per DML statement → lighter impact

 

3. Row-level vs. statement-level triggers – which is better?

• Row-level: Necessary if you need to examine or modify each row. Slower for bulk operations
• Statement-level: Preferred when you only need to act once per DML statement

Tip: For bulk inserts/updates, minimize row-level triggers to improve performance

 

4. Can triggers cause blocking or deadlocks?

Yes. Triggers that modify other tables can cause locks

• Avoid triggers that update the same table that fired the trigger (mutating table errors)
• Minimize DML inside triggers, or use compound triggers (Oracle 11g+) to handle row-level changes efficiently

 

5. What is a mutating table, and how to avoid it?

• A mutating table is the table currently being modified by a DML operation. Accessing it inside a row-level trigger can cause the ORA-04091 error

Avoidance strategies:

1.    Use AFTER statement-level triggers to perform queries on the table

2.    Use compound triggers to store row changes in a collection and process after all rows are done

 

6. How can I monitor trigger performance?

1.    DBA_TRIGGERS – check triggers on tables

2.    DBMS_TRACE / DBMS_SESSION – trace trigger execution

3.    SQL Monitoring – check if triggers contribute to slow DML

4.    Use AUTOTRACE in SQL*Plus to measure additional cost of trigger execution

 

7. Best practices for performance optimization

• Only use triggers when necessary – don’t replace application logic blindly
• Keep trigger code minimal; avoid complex queries or loops
• Prefer statement-level triggers when possible
• Use compound triggers to avoid mutating table problems efficiently
• Avoid recursive triggers (a trigger that fires another trigger)
• Consider instead-of triggers on views for selective logic, but monitor performance

 

8. Common performance issues

• Bulk DML on tables with row-level triggers → slow
• Triggers performing SELECT or INSERT/UPDATE on large tables → heavy I/O
• Recursive triggers causing excessive CPU usage
• Complex logic or PL/SQL functions inside triggers → hidden latency

Triggers Best Practices FAQS

1. When should I use a trigger?

• Enforce business rules that cannot be handled by constraints.
• Automatically audit or log changes to tables.
• Implement derived column values or cascading changes.

Avoid triggers for: simple validations or logic that can be done in application code or constraints—they add hidden overhead.

2. Should I use row-level or statement-level triggers?

• Row-level (FOR EACH ROW): Use when you need access to :OLD and :NEW values of each row.
• Statement-level: Use when you only need to react once per statement, e.g., logging the operation.

Best practice: Prefer statement-level triggers unless row-level is absolutely necessary.

3. How do I avoid mutating table errors?

Mutating table errors occur when a row-level trigger queries/modifies the same table it is triggered on.

Best practices:

• Use compound triggers (Oracle 11g+) to handle row-level changes efficiently.
• Use AFTER statement-level triggers to process data after DML finishes.
• Store intermediate changes in a PL/SQL collection during row-level execution.

4. Should triggers contain complex logic?

No. Keep triggers lightweight:

• Avoid large queries, loops, or external procedure calls inside triggers.
• Complex logic should move to stored procedures or application code.

Reason: Triggers execute synchronously and can slow down DML operations significantly.

5. Is it okay to have triggers calling triggers?

• Recursive triggers (triggers firing other triggers) can cause performance issues or infinite loops.
• If cascading triggers are needed, use controlled logic with flags or compound triggers to avoid recursion.

6. How can I optimize triggers for bulk operations?

• Use statement-level triggers instead of row-level triggers for bulk DML.
• Minimize DML inside row-level triggers; instead, collect changes in a PL/SQL collection and process them at the statement level.
• Avoid triggers on high-transaction tables unless necessary.

7. How do I monitor trigger performance?

• Check DBA_TRIGGERS to review trigger definitions.
• Use AUTOTRACE or SQL Monitoring to measure the impact of DML.
• Use DBMS_TRACE or DBMS_SESSION to trace PL/SQL execution inside triggers.

8. Should triggers replace application logic?

• No. Triggers are meant for data integrity, auditing, or automatic system-level operations, not business logic.
• Moving complex logic to the application layer can improve maintainability and performance.

9. How do I handle exceptions in triggers?

• Always include exception handling in triggers to prevent them from stopping DML.
• Log errors instead of raising them unless absolutely required.

Example:

EXCEPTION

  WHEN OTHERS THEN

    INSERT INTO error_log(table_name, err_msg) VALUES ('MY_TABLE', SQLERRM);

END;

10. Key overall best practices

1.    Keep triggers simple and minimal.

2.    Prefer statement-level triggers.

3.    Use compound triggers to avoid mutating table errors.

4.    Avoid recursive triggers.

5.    Move complex logic to procedures, packages, or application code.

6.    Monitor triggers to ensure they do not slow down high-volume DML.

7.    Document triggers well – hidden logic can confuse DBAs and developers.