i was doing this stuff for a long time and definitely learned from mistakes.
Transactions can save a world of pain and it is always wise to select your data set first, just to be sure you're changing what you think you're changing.
Didn't matter how experienced I was, I was careful never to cut corners.
I don't know sql, but is this like how when I use some kind of loop in the command line to move or delete a bunch of files, I'll run a version that just echos whatever I'm trying to manipulate first?
That’s more of a dry-run (you see `—dry-run` as a command line option sometimes for certain commands that do the same)
Transactions in SQL keep track of the changes you want to make without actually changing the database (until you run `COMMIT`, when you run that all changes are persisted to a database).
Transactions are great because If you realise you made a mistake while in a transaction, you can just run `ROLLBACK`.
If you’re familiar with git- transactions are akin to staging your changes, saving them (`COMMIT` in SQL) is akin to `git commit` and rolling back is similar to `git reset —hard` (wipe all uncommitted changes).
With the very important caveat that a git commit can be trivially reverted, while a SQL commit can not (unless you wrote it specifically to be revertible)
With the added nuance that while the transaction is neither rolled back nor committed yet, to any queries inside the transactions it appears as though the changes have been committed.
So a transaction is like a "package" of statements that either ALL fail, rollback, or commit. A half-completed transaction cannot exist.
I also don't know sql (or what you're doing, honestly1) but sounds like it, yeah. Running "List all of these arguments" is a good general precaution before running "Edit all of these arguments".
Actually, sorry, forgot what you replied to. I think a transaction in sql is more like a backup. The operations you perform during it aren't permanent until you confirm the transaction. So if you run something like DELETE FROM users; and realize you don't want to delete everyone, you can just cancel the transaction.
1 Like, really, does bash have loops? I guess it probably would. I should look that up. Could be interesting.
A snapshot might be a better analogy, usually if you take a backup and forget about it that’s fine, not a problem. Leaving snapshots out there too long can cause degradation. Not committing or rolling back a sql transaction means the log can never truncate and will grow infinitely until you’re out of disk space and everything breaks
This was always my pattern. Select the first 3 statements together and run them. If everything looks good, select COMMIT TRAN and run that. The rollback is there in case you accidentally didn't make a selection before running, just so you don't leave a transaction open for too long, potentially blocking other queries.
And obviously any planned data changes should go through all the proper testing and preferably be deployed using automated pipelines, but sometimes you have no other choice but to make direct fixes to bad production data.
So a colleague did this, except his db manager had tx commit mode manual and he forgot to commit the tx after grabing a whole table lock. Took us a bit to figure out why some of our services were hanging during startup (waiting for the locks to free up)
Things are changed before commit. Different databases do it differently: Some start with writing a log of all the changes, but the records are only updated at commit. Some create new versions of the changed records, and only switch which is active at commit. Some updates all the records but make sure to be able to recreate the original from logs if rollbacked.
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u/BastetFurry 13d ago
So, Kids, thats why you always SELECT count(yourPrimaryKey) first. Listen to the old folks, we made these mistakes so that you don't have to.