Row-level vs statement-level triggers in PostgreSQL - Visual Side-by-Side Comparison

Concept Flow - Row-level vs statement-level triggers

Trigger Event Occurs

↓

Is Trigger Row-level?

No→Execute Statement-level Trigger Once

Yes↓

For Each Affected Row

↓

Execute Row-level Trigger

↓

End of Trigger Execution

When a trigger event happens, PostgreSQL checks if the trigger is row-level or statement-level. Row-level triggers run once per affected row, statement-level triggers run once per statement.

Execution Sample

PostgreSQL

CREATE TRIGGER trg_row AFTER INSERT ON employees
FOR EACH ROW EXECUTE FUNCTION log_row_insert();

CREATE TRIGGER trg_stmt AFTER INSERT ON employees
FOR EACH STATEMENT EXECUTE FUNCTION log_stmt_insert();

Defines two triggers on the employees table: one runs after each inserted row, the other runs once after the entire insert statement.

Execution Table

Step	Trigger Type	Rows Affected	Trigger Execution Count	Action
1	Row-level	3	1	Trigger fires for first inserted row
2	Row-level	3	2	Trigger fires for second inserted row
3	Row-level	3	3	Trigger fires for third inserted row
4	Statement-level	3	1	Trigger fires once after all rows inserted
5	End	3	N/A	No more triggers to execute

💡 All affected rows processed for row-level trigger; statement-level trigger executed once after statement completes

Variable Tracker

Variable	Start	After 1	After 2	After 3	Final
Row Insert Count	0	1	2	3	3
Row-level Trigger Calls	0	1	2	3	3
Statement-level Trigger Calls	0	0	0	0	1

Key Moments - 2 Insights

Why does the row-level trigger execute multiple times but the statement-level trigger only once?

Because row-level triggers run once for each affected row (see execution_table steps 1-3), while statement-level triggers run once per statement regardless of rows affected (step 4).

If no rows are affected, will the row-level trigger run?

No, row-level triggers only run for affected rows. If zero rows are affected, they do not execute, but statement-level triggers still run once per statement.

Visual Quiz - 3 Questions

Looking at the execution_table, how many times does the row-level trigger execute when 3 rows are inserted?

A 0 times

B 1 time

C 3 times

D 4 times

💡 Hint

Check the 'Trigger Execution Count' column for row-level trigger in steps 1-3.

At which step does the statement-level trigger execute?

A Step 4

B Step 1

C Step 3

D Step 5

💡 Hint

Look for 'Statement-level' in the 'Trigger Type' column in the execution_table.

If the insert statement affects zero rows, what happens to the statement-level trigger?

A It does not execute

B It executes once

C It executes multiple times

D It executes only if rows are affected

💡 Hint

Remember statement-level triggers run once per statement regardless of rows affected (see key_moments).

Concept Snapshot

Row-level triggers run once for each affected row.
Statement-level triggers run once per statement.
Row-level triggers can access individual row data.
Statement-level triggers cannot access row data.
Use row-level for detailed per-row actions.
Use statement-level for summary or batch actions.

Full Transcript

When a trigger event happens in PostgreSQL, the system checks if the trigger is row-level or statement-level. Row-level triggers execute once for each row affected by the event, allowing access to that row's data. Statement-level triggers execute once after the entire statement completes, regardless of how many rows were affected. For example, if an insert statement adds three rows, a row-level trigger fires three times, once per row, while a statement-level trigger fires once after all rows are inserted. If no rows are affected, row-level triggers do not run, but statement-level triggers still execute once per statement. This distinction helps decide which trigger type to use depending on whether you need per-row processing or a single action per statement.

Practice

1. What is the main difference between a row-level trigger and a statement-level trigger in PostgreSQL?

easy

A. Row-level triggers only work on INSERT; statement-level triggers only work on UPDATE.

B. Row-level triggers execute once per SQL statement; statement-level triggers execute once for each affected row.

C. Row-level triggers execute once for each affected row; statement-level triggers execute once per SQL statement.

D. Row-level triggers cannot modify data; statement-level triggers can modify data.

Solution

1. Step 1: Understand trigger execution scope

   Row-level triggers run once for every row affected by the SQL command, meaning if 10 rows are updated, the trigger runs 10 times.

2. Step 2: Understand statement-level trigger behavior

   Statement-level triggers run only once per SQL command, regardless of how many rows are affected.

3. Final Answer:

   Row-level triggers execute once for each affected row; statement-level triggers execute once per SQL statement. -> Option C

4. Quick Check:

   Row-level = per row, Statement-level = per statement [OK]

Hint: Row-level = per row; statement-level = per statement [OK]

Common Mistakes:

• Confusing which trigger runs per row vs per statement
• Thinking row-level triggers run only once per statement
• Assuming statement-level triggers run per row
• Believing trigger types depend on operation type (INSERT/UPDATE)

2. Which of the following is the correct syntax to create a row-level trigger in PostgreSQL?

easy

A. CREATE TRIGGER trg AFTER INSERT ON table FOR EACH ROW EXECUTE FUNCTION func();

B. CREATE TRIGGER trg AFTER INSERT ON table FOR EACH STATEMENT EXECUTE FUNCTION func();

C. CREATE TRIGGER trg AFTER INSERT ON table EXECUTE FUNCTION func();

D. CREATE TRIGGER trg FOR EACH ROW EXECUTE FUNCTION func();

Solution

1. Step 1: Identify correct trigger syntax

   The syntax for creating a row-level trigger requires the clause FOR EACH ROW to specify it runs per affected row.

2. Step 2: Check full syntax correctness

   CREATE TRIGGER trg AFTER INSERT ON table FOR EACH ROW EXECUTE FUNCTION func(); correctly includes AFTER INSERT, ON table, FOR EACH ROW, and EXECUTE FUNCTION func(); which is the proper syntax.

3. Final Answer:

   CREATE TRIGGER trg AFTER INSERT ON table FOR EACH ROW EXECUTE FUNCTION func(); -> Option A

4. Quick Check:

   Row-level triggers use FOR EACH ROW [OK]

Hint: Row-level triggers always use FOR EACH ROW clause [OK]

Common Mistakes:

• Omitting FOR EACH ROW for row-level triggers
• Using FOR EACH STATEMENT for row-level triggers
• Missing EXECUTE FUNCTION keyword
• Incorrect order of clauses

3. Consider this trigger function and trigger:

CREATE FUNCTION trg_func() RETURNS trigger AS $$ BEGIN RAISE NOTICE 'Triggered'; RETURN NEW; END; $$ LANGUAGE plpgsql;
CREATE TRIGGER trg AFTER UPDATE ON employees FOR EACH ROW EXECUTE FUNCTION trg_func();
UPDATE employees SET salary = salary * 1.1 WHERE department = 'Sales';

What will be the output when the UPDATE affects 3 rows?

medium

A. The notice 'Triggered' will appear 3 times.

B. The notice 'Triggered' will appear once.

C. No notice will appear because AFTER UPDATE triggers do not raise notices.

D. The notice 'Triggered' will appear once per statement plus once per row.

Solution

1. Step 1: Identify trigger type and execution count

   The trigger is defined FOR EACH ROW, so it runs once for every row updated.

2. Step 2: Calculate total trigger executions

   Since 3 rows are updated, the trigger function runs 3 times, each raising the notice 'Triggered'.

3. Final Answer:

   The notice 'Triggered' will appear 3 times. -> Option A

4. Quick Check:

   Row-level trigger runs per row = 3 notices [OK]

Hint: FOR EACH ROW triggers run once per affected row [OK]

Common Mistakes:

• Assuming notice appears only once per statement
• Confusing FOR EACH ROW with FOR EACH STATEMENT
• Thinking AFTER UPDATE triggers don't raise notices
• Believing trigger runs multiple times per row

4. You created a statement-level trigger but it seems to run multiple times when you update multiple rows. What is the most likely cause?

medium

A. PostgreSQL does not support statement-level triggers.

B. Statement-level triggers always run once per row by design.

C. The trigger function contains a loop causing multiple executions.

D. You accidentally defined the trigger as FOR EACH ROW instead of FOR EACH STATEMENT.

Solution

1. Step 1: Understand trigger definition impact

   If a trigger runs multiple times per row update, it is likely defined as FOR EACH ROW, not FOR EACH STATEMENT.

2. Step 2: Verify PostgreSQL trigger capabilities

   PostgreSQL supports both row-level and statement-level triggers; statement-level triggers run once per statement.

3. Final Answer:

   You accidentally defined the trigger as FOR EACH ROW instead of FOR EACH STATEMENT. -> Option D

4. Quick Check:

   FOR EACH ROW triggers run per row, causing multiple executions [OK]

Hint: Check FOR EACH ROW vs FOR EACH STATEMENT clause [OK]

Common Mistakes:

• Believing statement-level triggers run per row
• Ignoring trigger definition syntax
• Assuming PostgreSQL lacks statement-level triggers
• Blaming trigger function code without checking trigger type

5. You want to log a summary message once after any UPDATE statement on a table, regardless of how many rows are changed. Which trigger type and timing should you use?

hard

A. A BEFORE UPDATE row-level trigger

B. An AFTER UPDATE statement-level trigger

C. An AFTER UPDATE row-level trigger

D. A BEFORE UPDATE statement-level trigger

Solution

1. Step 1: Determine trigger timing for logging after update

   Logging after the update completes requires an AFTER trigger.

2. Step 2: Choose trigger level for single summary message

   To log once per statement regardless of rows, use a statement-level trigger (FOR EACH STATEMENT).

3. Final Answer:

   An AFTER UPDATE statement-level trigger -> Option B

4. Quick Check:

   Summary logging = AFTER + statement-level trigger [OK]

Hint: Use AFTER statement-level trigger for single summary action [OK]

Common Mistakes:

• Using row-level triggers causing multiple logs
• Using BEFORE triggers missing final state
• Confusing timing and level for logging
• Assuming row-level triggers can log once per statement