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