DO blocks for anonymous code in PostgreSQL - Step-by-Step Execution

Concept Flow - DO blocks for anonymous code

Start DO block

↓

Declare variables (optional)

↓

Execute procedural statements

↓

Handle exceptions (optional)

↓

End DO block

The DO block runs anonymous procedural code inside the database without creating a stored function. It executes statements step-by-step and ends.

Execution Sample

PostgreSQL

DO $$
BEGIN
  RAISE NOTICE 'Hello, world!';
END $$;

This DO block prints a message 'Hello, world!' as a notice when run.

Execution Table

Step	Action	Evaluation	Result
1	Enter DO block	Start execution	Ready to run statements
2	Execute RAISE NOTICE	Print message	NOTICE: Hello, world!
3	End DO block	No more statements	DO block finishes successfully

💡 All statements executed, DO block ends

Variable Tracker

Variable	Start	After 1	Final

Key Moments - 2 Insights

Why doesn't the DO block return a result set like a SELECT query?

The DO block runs procedural code for side effects like printing or updating data. It does not return rows, as shown in execution_table step 2 where it prints a notice instead of returning data.

Can I declare variables inside a DO block?

Yes, variables can be declared inside the DO block before procedural statements. This is optional and not shown in the simple example, but the concept_flow shows the declaration step before execution.

Visual Quiz - 3 Questions

Look at the execution_table, what happens at step 2?

A Variables are declared

B The DO block ends

C A message is printed as a notice

D An error occurs

💡 Hint

Check the 'Action' and 'Result' columns in execution_table row 2

At which step does the DO block finish execution?

A Step 1

B Step 3

C Step 2

D There is no end step

💡 Hint

Look at the 'Step' and 'Result' columns in execution_table row 3

If you add variable declarations, where would they appear in the flow?

A Between start and executing statements

B After executing statements

C Before entering the DO block

D After the DO block ends

💡 Hint

Refer to the concept_flow diagram showing declaration before execution

Concept Snapshot

DO blocks run anonymous procedural code in PostgreSQL.
Syntax: DO $$ BEGIN statements; END $$;
Used for running code without creating functions.
Can declare variables and handle exceptions.
Does not return query results, only side effects.

Full Transcript

A DO block in PostgreSQL lets you run anonymous procedural code without creating a stored function. It starts by optionally declaring variables, then executes procedural statements like printing messages or updating data. The example shows a DO block printing 'Hello, world!' as a notice. The execution table traces entering the block, executing the print statement, and finishing the block. Variables can be declared inside the block but are optional. The DO block does not return rows like a SELECT query; it is for side effects. This visual helps beginners see each step the database takes when running a DO block.