Does Serializable isolation level block read queries in Postgres

Question

I have table "Transactions", where customers accounts transactions will be hold. So I want to set transaction level Serializable to prevent all types of anomalies.

Lets assume transactions goes this way:

BEGIN TRANSACTION ISOLATION LEVEL SERIALIZABLE;
INSERT INTO transaction_test (sum, currency) VALUES (333, '820');

I doubt whether this table will be accessible for users to read, because our Application needs to run multiple SELECTs from this table for statistics?

After some googling I'm confused a little bit:

• From the one hand Postgres PostgreSQL actually treats every SQL statement as being executed within a transaction., so regular SELECT query for the data could be blocked
• From the other hand that “readers never block writers” and “writers never block readers.” due Postgres MVCC.

So the question is: can the Transaction table be safely use for SELECT queries in parallel of being changed with transactions of SERIAL level isolation?

Answer 1

PostgreSQL does not guarantee SERIALIZABLE by locking out every potentially conflicting transactions. Rather, it takes special predicate locks, which don't block anything, but track what data a transaction has read. If a situation arises that could violate serializability, PostgreSQL would abort one of the transactions with a serialization error.

Concurrent read-only transactions won't be blocked. As recommended by the documentation, you should wrap read-only queries in a SERIALIZABLE READ ONLY transaction for best performance.