Do prepared statements persist between requests?

Question

If I want to execute the same query on two different requests, and I use prepared statements with Doctrine2... Will the prepared statement be sent only the first time and be stored by the database for some time? Or will it be removed after each script finishes?

Answer 1

On PostgreSQL a prepared statement is valid only till the end of the session and is not saved in the memory and shared between many sessions, see doc: http://www.postgresql.org/docs/9.2/static/sql-prepare.html

Prepared statements only last for the duration of the current database session. When the session ends, the prepared statement is forgotten, so it must be recreated before being used again. This also means that a single prepared statement cannot be used by multiple simultaneous database clients; however, each client can create their own prepared statement to use.

However, they also say, that PostgreSQL may (but not need to) save a plan for this query in memory for future reusing:

If a prepared statement is executed enough times, the server may eventually decide to save and re-use a generic plan rather than re-planning each time. This will occur immediately if the prepared statement has no parameters; otherwise it occurs only if the generic plan appears to be not much more expensive than a plan that depends on specific parameter values. Typically, a generic plan will be selected only if the query's performance is estimated to be fairly insensitive to the specific parameter values supplied.

To examine the query plan PostgreSQL is using for a prepared statement, use EXPLAIN. If a generic plan is in use, it will contain parameter symbols $n, while a custom plan will have the current actual parameter values substituted into it.