SQLAlchemy doesn't do anything explicit as far as pessimistic locking, this comes down to the isolation level that is set on the current transaction and by default it makes a psycopg2 connection and does not change anything about it.
so to get an intro to what happens when you make a psycopg2 connection you can look at https://www.psycopg.org/docs/usage.html?highlight=locking#transactions-control, which then gets into the question of isolation levels at https://www.postgresql.org/docs/current/transaction-iso.html . On the SQLAlchemy side, the isolation level of your psycopg2 connection is set using the isolation level execution option, which you can see documented at https://docs.sqlalchemy.org/en/13/dialects/postgresql.html#transaction-isolation-level . to view the current isolation level as retrieved from the database, use get_isolation_level(): https://docs.sqlalchemy.org/en/13/core/connections.html#sqlalchemy.engine.Connection.get_isolation_level . On Mon, Jun 29, 2020, at 8:00 PM, gbr wrote: > Hi, > > I'm using SQLAlchemy's Core to interface a postgres database (via psycopg2) > component alongside Flask-SQLAlchemy in a Flask app. Everything was working > fine until I recently discovered what seems to be a deadlock state which is > caused by two queries locking each other (at least that's my working > hypothesis). > > Postgres offers a wide range of locking mechanisms, so I wonder which one is > used by SQLAlchemy (i.e. where is it set?). > > > This is how the database engine is being created (through Flask-SQLAlchemy. > > ``` > db = sqlalchemy.create_engine(...) > > # `db` reference is reused in process > > # this is how queries are executed > rv = db.session.execute(select(...)) > ``` > > Is there any way I can check the transaction handling/locking used? > > -- > SQLAlchemy - > The Python SQL Toolkit and Object Relational Mapper > > http://www.sqlalchemy.org/ > > To post example code, please provide an MCVE: Minimal, Complete, and > Verifiable Example. See http://stackoverflow.com/help/mcve for a full > description. > --- > You received this message because you are subscribed to the Google Groups > "sqlalchemy" group. > To unsubscribe from this group and stop receiving emails from it, send an > email to sqlalchemy+unsubscr...@googlegroups.com. > To view this discussion on the web visit > https://groups.google.com/d/msgid/sqlalchemy/4825d83c-688a-4f7a-a30d-6a69e5d3341do%40googlegroups.com > > <https://groups.google.com/d/msgid/sqlalchemy/4825d83c-688a-4f7a-a30d-6a69e5d3341do%40googlegroups.com?utm_medium=email&utm_source=footer>. -- SQLAlchemy - The Python SQL Toolkit and Object Relational Mapper http://www.sqlalchemy.org/ To post example code, please provide an MCVE: Minimal, Complete, and Verifiable Example. See http://stackoverflow.com/help/mcve for a full description. --- You received this message because you are subscribed to the Google Groups "sqlalchemy" group. To unsubscribe from this group and stop receiving emails from it, send an email to sqlalchemy+unsubscr...@googlegroups.com. To view this discussion on the web visit https://groups.google.com/d/msgid/sqlalchemy/65adc852-0358-471b-aa9a-0f24acdee3ee%40www.fastmail.com.
