HDBC-mysql

This is a "native" HDBC driver for MySQL that makes use of libmysqlclient to communicate with a MySQL server. By way of synopsis:

import Control.Monad
import Database.HDBC
import Database.HDBC.MySQL
main = do conn <- connectMySQL defaultMySQLConnectInfo {
                       mysqlHost     = "db1.example.com",
                       mysqlUser     = "scott",
                       mysqlPassword = "tiger"
                    }

          rows <- quickQuery' conn "SELECT 1 + 1" []
          forM_ rows $ \row -> putStrLn $ show row

At the moment, please consider this to be "alpha" software. As far as I can tell, it works. There are some limitations that you should be aware of.

Caveats

• This works with MySQL server and client libraries 5.0.75. I haven't tried with 4.x nor 5.1. I suspect that using a server version less than 4.1 won't work, due to lack of support for prepared statements.

• MySQL DATETIME and TIMESTAMP columns have no time zone information, because they just don't. So, I'm just converting them blindly to SqlEpochTime values, and assuming the times are UTC. This is all fine if you're actually running your server in UTC, but it will probably be confusing if you're not. It might be possible to interpret the times using the current connection's default time zone setting, instead. Is that better? Or worse?

• Regardless, the types I convert to are now deprecated, and I need to implement the new types (SqlLocalDate, etc.)

• Out of the box, MySQL probably uses MyISAM tables to store its data, and MyISAM tables don't support transactions. Yet, I'm going to blindly respond "yes" if you ask whether the driver itself supports transactions, and assume that you know enough to use InnoDB tables in the database if you want to make use of HDBC's transactional support. I suppose I might be able to discover what the default table type is, and say "no" if it's not a table type that supports transactions, but... meh.

• I'm not sure that I can tell the difference between a MySQL TEXT and a MySQL BLOB column. If you ask about the metadata of either, I'll tell you it's a SqlBinaryT.

• The statement and table metadata could stand to be improved a bit. In particular, it would be nice if "describeTable foo" and "describeResults" on "SELECT * FROM foo" returned the same thing. (They're sorta close, I guess...)

• Thread-safety could be an issue. In the driver code, there's definitely a race condition between "prepare" and "disconnect", for example. I haven't even looked at thread-safety issues for the MySQL driver. I'm not sure if I should worry about it, or if we assume that's going to be dealt with at a higher level.

• We might crash if someone opens a connection, prepares a statement, explicitly disconnects the connection, and then tries to play with the statement.

• It probably makes sense to marshall to the SqlByteString type when retrieving BLOB data.

Testing

There's a little test program that runs a query and spews out the results. To compile it,

ghc -idist/build -L/opt/local/lib/mysql5/mysql -lmysqlclient --make Test

I'm still trying to get the Makefile right so that it can build the test sources: it's not there yet. Here's how I've been doing it, for now:

cd testsrc
ghc --make -package HUnit -package HDBC -Wall -i../dist/build -i.. -L/opt/local/lib/mysql5/mysql -lmysqlclient -o runtests runtests.hs

One issue is that I want the location of the MySQL library to come from the configuration data, rather than be hard-coded.

Developing hdbc-mysql with nix

# Nix shell
nix-shell -p haskell.packages.ghc801.ghc gcc mysql57 pkgconfig zlib openssl haskellPackages.haddock

# Use Cabal to build
cabal clean && cabal build

# Build test script
ghc -idist/build -L/nix/store/vksgj509kdnk3rva0x64qwp21nzrxy9b-mariadb-10.1.16/lib -lmysqlclient --make Test

# Run test script
./Test