How to test run an UPDATE statement in PostgreSQL?

sql

transactions

database-safety

best-practices

byAlex Kataev·Nov 25, 2024

The secret sauce to testing your UPDATE statement without causing a data apocalypse is to tuck it inside a transaction block. Use SELECT to peek at the impending changes, and finally execute ROLLBACK to undo them:

BEGIN;
UPDATE my_table SET column = 'new_value' WHERE criteria; -- Change is coming!
SELECT * FROM my_table WHERE criteria; -- Peekaboo! Did I get it right?
ROLLBACK; -- Nevermind. CTRL+Z

This approach allows you to mimic an UPDATE without really committing the changes. You get to see the outcome without the commitment.

If you want an eagle-eye view on how many bird...err..rows would be affected by your UPDATE, use the EXPLAIN command. Remember, it does not give you the real data changes, but emphasizes on the range and efficiency:

EXPLAIN UPDATE my_table SET column = 'new_value' WHERE criteria; -- What's the damage?

And if you're the thrill-seeking kind, wanting a total simulation of your queries in the wild, SQL Fiddle lets you relish in an external environment where you can test your SQL logic and witness the events before implementing in your actual database.

Guarding your digital kingdom

When you're tweaking your database with an UPDATE, your foremost thought should be safety. Unforeseen changes are like a bull in a china shop. That’s where transactions come in, your knights in shining armor protecting your data kingdom. After your SELECT, if you spot anomalies, you can give the command to ROLLBACK. This magic lets you test rigorously and sleep peacefully knowing your data is unchanged.

Let's hope for the best, but prepare for the worst when testing:

Use SELECT to foresee constraint violations (e.g., Foreign Key, CHECK)
Be a trigger whisperer. Remember the active database triggers that might set off as a domino effect from your UPDATE.

Just to double-check if your safety protocol is in order:

BEGIN;
-- Update operation here
-- Validate data integrity and ensure no dragons (constraints) are angered
SELECT * FROM my_table WHERE ...;
-- Confirm if all's well
ROLLBACK; -- or COMMIT if the coast is clear

Understand your UPDATE's mission with EXPLAIN

Before your UPDATE embarks on its mission inside a transaction, use the EXPLAIN command for a briefing session. This debrief lets you peek into how the database plans to execute your UPDATE and whether it'll take the highway (indexes) or go local (full table scans).

Taming the beast: Complex UPDATES

When your UPDATE morphs from a cat to a beast, with JOINs or subqueries, you need to be extra careful. Accidentally updating unintended rows is like stepping on a LEGO piece in the middle of the night, painful and totally avoidable.

Here's how you can handle your beast:

Break down the beast into kittens. Test simplified UPDATEs individually.
Use WITH to describe the execution steps. Think of your complex UPDATE as an epic saga.
For multi-table updates, validate joining conditions before letting the UPDATE loose.

Practice makes perfect: Safe UPDATE simulations

Before you UPDATE data on your live database, rehearse. Here’s how:

Set up a stage (testing environment), a duplicate of your production schema.
Take regular snapshots (backups) of your data kingdom to revert in case of disaster.
Version control your database schema and data-related changes. They are as crucial as your coffee breaks.

These practices act as a safety net for your data, making sure it stays put during testing and you stay sane during UPDATEs. They are not just best practices, but sanity practices.

explain-codes / Sql / How to test run an UPDATE statement in PostgreSQL?

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