Why does PostgreSQL serializable transaction think this as conflict?

Question

In my understanding PostgreSQL use some kind of monitors to guess if there\'s a conflict in serializable isolation level. Many examples are about modifying same resource in

Answer 1

You can fix this problem with the following index:

CREATE INDEX accounts_user_idx ON accounts(user_id);

Since there are so few data in your example table, you will have to tell PostgreSQL to use an index scan:

SET enable_seqscan=off;

Now your example will work!

If that seems like black magic, take a look at the query execution plans of your SELECT and UPDATE statements.

Without the index both will use a sequential scan on the table, thereby reading all rows in the table. So both transactions will end up with a SIReadLock on the whole table.

This triggers the serialization failure.

Answer 2

To my knowledge serializable has the highest level of isolation, therefore lowest level of concurrency. The transactions occur one after the other with zero concurrency.