Thread: Deadlocks and transactions
I have a process that inserts a record in one table and, after that, a record in another table is updated. Because there are 2 DB operations, I decided to perform both of them in a transaction.
Can a deadlock take place even if transactions are used?
Best regards,
Jorge Maldonado
I have a process that inserts a record in one table and, after that, a record in another table is updated. Because there are 2 DB operations, I decided to perform both of them in a transaction.Can a deadlock take place even if transactions are used?
Its impossible to deadlock without transactions.
Simplistically, a deadlock happens when there are two processes - one holds lock A and wants lock B while the other wants lock A while holding lock B.
Your choice to use a transaction here is good but you will have at least some risk of deadlock with others parts of the system. Other processes running this same exact code, however, should not pose a risk since the locking order would be consistent.
David J.
Hi Jorge,
Here are the two advices I give developpers to reduce the risk a deadlock occures :2018-03-19 23:18 GMT+01:00 David G. Johnston <david.g.johnston@gmail.com>:
I have a process that inserts a record in one table and, after that, a record in another table is updated. Because there are 2 DB operations, I decided to perform both of them in a transaction.Can a deadlock take place even if transactions are used?Its impossible to deadlock without transactions.Simplistically, a deadlock happens when there are two processes - one holds lock A and wants lock B while the other wants lock A while holding lock B.Your choice to use a transaction here is good but you will have at least some risk of deadlock with others parts of the system. Other processes running this same exact code, however, should not pose a risk since the locking order would be consistent.David J.
My application is a website.
Let´s suppose the following scenario.
* User 1 has already loaded a web page and clicks a button that triggers a transaction that includes 2 tables.
* User 2 loads the same page and SELECTs data from the 2 tables currently in the transaction generated by User 1. User 2 does not trigger a transaction because he/she only gets data from the DB.
Is there any issue/problem for User 2?
(Is it correct to reply-to-all when posting back to a question?)
Best regards,
Jorge Maldonado
On Tue, Mar 20, 2018 at 4:01 AM, Lætitia Avrot <laetitia.avrot@gmail.com> wrote:
LætitiaCheers,- Keep your transactions short- Always change(update, delete...) data in the same orderHi Jorge,Here are the two advices I give developpers to reduce the risk a deadlock occures :2018-03-19 23:18 GMT+01:00 David G. Johnston <david.g.johnston@gmail.com>:I have a process that inserts a record in one table and, after that, a record in another table is updated. Because there are 2 DB operations, I decided to perform both of them in a transaction.Can a deadlock take place even if transactions are used?Its impossible to deadlock without transactions.Simplistically, a deadlock happens when there are two processes - one holds lock A and wants lock B while the other wants lock A while holding lock B.Your choice to use a transaction here is good but you will have at least some risk of deadlock with others parts of the system. Other processes running this same exact code, however, should not pose a risk since the locking order would be consistent.David J.
My application is a website.Let´s suppose the following scenario.* User 1 has already loaded a web page and clicks a button that triggers a transaction that includes 2 tables.* User 2 loads the same page and SELECTs data from the 2 tables currently in the transaction generated by User 1. User 2 does not trigger a transaction because he/she only gets data from the DB.
Well, if you want a consistent point-in-time picture of the data in those two tables you should probably perform the selects in a transaction too.
Is there any issue/problem for User 2?
MVCC mechanics eliminate the possibility of deadlocking between update queries and select queries (not select-for-update though). Because of it deadlocking is only possible between two updating transactions.
(Is it correct to reply-to-all when posting back to a question?)
Yes, reply-to-all is preferred; top-posting replies is not. Please inline or bottom-post (and trim quoting) as appropriate.
David J.
On 21 March 2018 at 13:10, JORGE MALDONADO <jorgemal1960@gmail.com> wrote: > Let´s suppose the following scenario. > * User 1 has already loaded a web page and clicks a button that triggers a > transaction that includes 2 tables. > * User 2 loads the same page and SELECTs data from the 2 tables currently in > the transaction generated by User 1. User 2 does not trigger a transaction > because he/she only gets data from the DB. > > Is there any issue/problem for User 2? There's no deadlock risk between a read and a write transaction. This is mentioned in the 2nd paragraph in https://www.postgresql.org/docs/10/static/mvcc-intro.html That might be interesting for you to read as it explains in a bit of detail how concurrency is handled in PostgreSQL. > (Is it correct to reply-to-all when posting back to a question?) Yes. It's also our preference that you don't top post in replies. We generally use the interleaved style described in https://en.wikipedia.org/wiki/Posting_style#Interleaved_style -- David Rowley http://www.2ndQuadrant.com/ PostgreSQL Development, 24x7 Support, Training & Services