How does a PostgreSQL update get a lock?
An UPDATE acquires a row level lock on each target row before updating. If a concurrent transaction tries to UPDATE the same row, it will see the lock on the row and wait till the blocking transaction is finished ( ROLLBACK or COMMIT ), then being the first in the lock queue: If committed, recheck the condition.
How to add a check constraint in PostgreSQL?
At some later date, you can attempt to VALIDATE the constraint (when a lock on the table is ok) This form adds a new constraint to a table using the same syntax as CREATE TABLE, plus the option NOT VALID, which is currently only allowed for foreign key and CHECK constraints.
How to update and insert values in PostgreSQL?
I suggest a single statement with a data-modifying CTE (something that MySQL also doesn’t have) because it’s convenient to pass values from one table to the other directly (if you should need that). If you don’t need anything from the coupon table you can use a transaction with separate UPDATE and INSERT statements just as well.
What happens if you roll back PostgreSQL update?
Else the UPDATE now finds no qualifying row and does nothing, returning no row, so the INSERT also does nothing. If rolled back, lock the row and proceed. There is no potential for a race condition. There is no potential for a deadlock unless you put more writes into the same transaction or otherwise lock more rows than just the one.
What’s the difference between lock modes in PostgreSQL?
The only real difference between one lock mode and another is the set of lock modes with which each conflicts (see Table 13-2 ). Two transactions cannot hold locks of conflicting modes on the same table at the same time. (However, a transaction never conflicts with itself.
Is the access exclusive lock self-conflicting in PostgreSQL?
Notice in particular that some lock modes are self-conflicting (for example, an ACCESS EXCLUSIVE lock cannot be held by more than one transaction at a time) while others are not self-conflicting (for example, an ACCESS SHARE lock can be held by multiple transactions). Conflicts with the ACCESS EXCLUSIVE lock mode only.