MySQL, locking tables

[gunhelstr]

Does any of Qt's QSql.....-classes take care of locking/unlocking tables in MySQL database?
I can't find anything about that in Qt Assistent.

[wysota]

Qt sql classes use client libraries to achieve their tasks. If you want to issue a table lock command, you have to specifically ask for it using a "LOCK TABLE" query. Qt won't lock tables on its own. It doesn't need locking to perform its tasks. I don't know if this answers your question though

[gunhelstr]

What is client libraries in this context?

What if you have a system with many connections to the same database? Don't you have to lock tables to make sure your lookups are consistent every time?

[wysota]

What is client libraries in this context?

SQL database management systems (like MySQL) provide libraries, which allow other programs to query databases, etc. (the library is called libmysqlclient when it comes to MySQL). Qt uses those libraries to make "lowlevel" (database) calls to provide "high level" (QSQL driver) functionality.

What if you have a system with many connections to the same database? Don't you have to lock tables to make sure your lookups are consistent every time?

It is the database management system's responsibility to provide query consistency. Qt doesn't handle that. It just uses the client library to issue queries to the database.

BTW. To assure the consistency of a query the DBMS often lock single rows, but they avoid locking whole tables if they can. If you want to lock the whole table, you have to make a query for it yourself.

[stevey]

Do you need to explicitly lock a row for some reason, or do you just want locking as part of a "transactional" process?

If you call .transaction() on your QSqlDatabase you can insert new rows, then they won't appear until you .commit() the transaction after all your queries are performed.

Transactions usually work on select statements as well locking rows that you return with select statements, although I don't know if MySql in fact does that. Transactions are pretty new to MySql and I haven't actually tried 'select' based transaction to know if it works.

This of course may not be what you're after, but I hope it helps


Steve