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I wrote an application (using Qt in C++) which inserts data into a SQLite database. Another application reads data from the same database, but I noticed that it is not possible to read uncommitted data. I would like instead to be able to read data even if it is not committed yet.
Reading the SQLite documentation I found the pragma read_uncommitted which, from what I understand, should be what I need. Problem is that, even if I set that to true, I cannot get uncommitted data.
I tried to run my application which performs the insertion process, and, at the same time, start the sqlite3 client. I set the pragma to true and I try to count the records inside the table. What I get is always 0 (database was empty before my insertion process started), until the entire process finishes where I immediately get the entire data.
Am I doing something wrong? Isn't this pragma supposed to make the sqlite3 client behave differently?
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Dirty Reads A dirty read occurs when a transaction reads data that has not yet been committed.
To prevent dirty reads, the database engine must hide uncommitted changes from all other concurrent transactions. Each transaction is allowed to see its own changes because otherwise the read-your-own-writes consistency guarantee is compromised.
SQLite transaction statementsBy default, SQLite operates in auto-commit mode. It means that for each command, SQLite starts, processes, and commits the transaction automatically. To start a transaction explicitly, you use the following steps: First, open a transaction by issuing the BEGIN TRANSACTION command.
Uncommitted read (UR) The uncommitted read isolation level allows an application to access the uncommitted changes of other transactions. Moreover, UR does not prevent another application from accessing a row that is being read, unless that application is attempting to alter or drop the table.
I answer to myself: no, it seems it is not possible. The read_uncommitted isolation mode requires to enable the shared cache, which is possible currently only for different threads living in the same process. This seems the best place to study this: http://www.sqlite.org/sharedcache.html.
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