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Comment by Fire-Dragon-DoL | Hacker News Reader

Oh that's super nasty, is it mentioned somewhere in the doc?

Is it the same for repeatable read?

I have read the docs plenty of times, but it never stuck for me until I read the (free!) PostgreSQL 14 Internals ebook: https://postgrespro.com/community/books/internals

Quoted from Page 70:

If you use the Serializable level, it must be observed by all transactions of the application. When combined with other levels, Serializable behaves as Repeatable Read without any notice. So if you decide to use the Serializable level, it makes read sense to modify the default_transaction_isolation parameter value accordingly -- even though someone can still overwrite it by explicitly setting a different level.

I had a real "WTF?" moment when I read this the first time.

Interesting, this book looks really cool!

It is mentioned in the doc, but can be easy to mis-understand:

"If a pattern of reads and writes among concurrent serializable transactions would create a situation which could not have occurred for any serial (one-at-a-time) execution of those transactions, one of them will be rolled back with a serialization_failure error."

Note that it says nothing about the non-serializable transactions.

https://www.postgresql.org/docs/current/sql-set-transaction....

It is in the Wiki: https://wiki.postgresql.org/wiki/Serializable Any transaction which is run at a transaction isolation level other than SERIALIZABLE will not be affected by SSI. If you want to enforce business rules through SSI, all transactions should be run at the SERIALIZABLE transaction isolation level, and that should probably be set as the default.

I guess it is the same for all MVCC databases. They don't want to acquire a read lock just in case another transaction is in Serializable

The OP mentioned that SQLServer does lock (but then, it doesn't use SSI - just SI).

The recommendation in PG docs to use a combination of SERIALIZABLE and READ ONLY transactions seems like a good one for read-heavy systems.

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