Connecting to SQL Server from Linux using FDWs

There are two PostgreSQL FDWs (currently maintained) I know of for connecting to SQL Server from a Linux/Unix PostgreSQL box. There is the TDS Foreign Data wrapper (tds_fdw driver) which relies on the Free TDS driver. This is a fairly light-weight FDW since it just relies on TDS which is commonly already available on Linux installs or an easy install away. Unfortunately when I tried to use it on windows (compiling my usual mingw64 way), while it compiled and installed, it crashed when I attempted to connect to my SQL Server 2008 R2 box table, so I gave up on it for the time being as a cross-platform solution. One thing I will say about it is that it accepts ad-hoc queries from what I can see, as a data source, which is pretty nice. So we may revisit it in the future to see if we can get it to work on windows. I'm not sure if tds_fdw would support SQL Server spatial geometry columns though would be interesting to try.

The second option, which as you may have noticed, we spent much time talking about is the ogr_fdw foreign data driver. ogr_fdw utilizes UnixODBC on Linux, iODBC on MacOSX and Windows ODBC on windows for connecting to SQL Server. The ogr_fdw big downside is that it has a dependency on GDAL, which is a hefty FOSS swiss-army knife ETL tool that is a staple of all sorts of spatial folks doing both open source and proprietary development. The good thing about ogr_fdw, is that since it is a spatial driver, it knows how to translate SQL Server geometry to it's equivalent PostGIS form in addition to being able to handle most of the other not-so spatial columns.

Although GDAL is big, the good news is that, PostGIS relies on it as well since PostGIS 2.0 to support raster functionality. That was one of my ulterior motives for pushing raster into the PostGIS extension in 2.0: There will come a day when PostgreSQL will need to reach out to vast different kinds of spatial and not-so spatial data and GDAL would be a convenient ring to do so, so lets start planting the roots.. This makes compiling and installing ogr_fdw on Linux pretty trivial if you already have PostGIS with raster support installed and even easier now that PGDG Yum packages it in the repo.

Until recently, I've only used ogr_fdw on Windows and have been very happy with it connecting to all sorts of datasources from open-street map extract, dbase files, excel spreadsheets, MS Access databases, and SQL Server, it's a real gem. That is not to say it couldn't stand for many improvements. For a good chunk of these like OSM and MySQL and SQLite, GDAL doesn't rely on ODBC and uses the native drivers directly. Recently people have been writing me about how they can use it on Linux to connect to SQL Server. Yes, my jaw dropped, Linux people want to connect their PostgreSQL to SQL Server, why the heck would they want to do that. I thought maybe it's a good idea to try this out myself to experience first hand issues people are running into rather than simply relaying the information between people on what they tried that worked and didn't work. So here is my naive attempt to do so and distill the body of information that people have been sending me.

There are two UnixODBC drivers you can use for connecting to SQL Server. There is the TDS based one, and in theory, if you are on CentOS/Red Hat EL (5,6) or SUSE Linux EL 11, you can also use the Microsoft provided via: which I have not attempted, but may in a future article.

We'll use the more commonly available TDS driver which I think works on pretty much all Linux/Unix and MacOSX systems. This I am doing on a CentOS 7 box.

If you installed ogr_fdw or PostGIS using, yum, then you probably have UnixODBC installed, to verify do this:

sudo yum list installed | grep unixODBC

We get this for output

unixODBC.x86_64                  2.3.1-10.el7                          @base

I should note, that if GDAL isn't compiled with UnixODBC support, you have a much harder hurdle to jump. Luckily as far as I can tell, I think GDAL is generally compiled with UnixODBC support on Linux and possibly on Mac as well.

Next see what drivers you have installed already

odbcinst -d -q

On our CentOS 7 box, got returned:


Setting up UnixODBC connection using TDS ODBC Driver

Here are the following steps. Much of this information is gleaned from unixODBC - MS SQL Server. You need to do this if FreeTDS didn't show up as an option when querying odbc manager.

  1. Install FreeTDS Driver
    sudo yum install freetds

    This is a RedhatEL/CentOS/Fedora way, on Ubuntu you'd probably need to replace yum install with apt-get sudo yum install freetds

    Dependencies Resolved
     Package                                        Arch                                          Version                                                       Repository                                   Size
     freetds                                        x86_64                                        0.91-12.git0a42888.el7                                        epel                                        1.1 M
    Transaction Summary
    Install  1 Package
    Total download size: 1.1 M
    Installed size: 9.8 M
    Is this ok [y/d/N]: y
  2. Verify you got FreeTDS compiled with UnixODBC support.
    tsql -C

    Output should look something like

    Compile-time settings (established with the "configure" script)
                                Version: freetds v0.91
                 freetds.conf directory: /etc
         MS db-lib source compatibility: yes
            Sybase binary compatibility: yes
                          Thread safety: yes
                          iconv library: yes
                            TDS version: 4.2
                                  iODBC: no
                               unixodbc: yes
                  SSPI "trusted" logins: no
                               Kerberos: yes
  3. Find where the library is installed
    ldconfig -p | grep

    Should output something like (libc6,x86-64) => /lib64/
  4. Now connect as postgres account. Create a driver template file, doesn't really matter where you put it or call it, since it will be copied by the installer, contents should contain

    Description     = FreeTDS for connecting to Sybase and SQL Server
    Driver          = /lib64/

    Make sure to replace the driver with whatever path ldconfig gave. We called ours tds.driver.template

  5. Install the driver template
    su postgres    
    odbcinst -i -d -f tds.driver.template

    Should get an output:

    odbcinst: Driver installed. Usage count increased to 1.
        Target directory is /etc
  6. Verify you now have TDS driver
    odbcinst -d -q

    Output should now include FreeTDS

    [root@centie source]# odbcinst -d -q
  7. Register a connection to your SQL Server box

    To do this we need to create a datasource template file and register that with odbcinst manager similar to what we did with the driver as follows:

    Create a file call it tds-testmssql.datasource.template and has contents as follows. Note the [..] is the name you want to refer to your data source and should be different for each database you want to connect to.

    Driver  = FreeTDS
    Description     = My test database
    Trace   = No
    Server      =
    Port      = 1433
    Database        = Northwind
    TDS Version      = 7.3

    using a server name or fully qualified domain name instead of the ip of the SQL Server works just as well.

    Note: From other people's experiences I been informed that TDS Version needs to be 7.3 or above to work with ogr_fdw.

    Then run:

    odbcinst -i -s -f tds-testmssql.datasource.template
  8. Test by connecting:
    isql -v MSSQLTDSTest your_user_name your_password

    You should be able to execute queries on console. For a simple exercise try this:


    Should output something like:

    | Microsoft SQL Server 2008 R2 (SP3) - 10.50.6220.0 (X64)
            Mar 19 2015 12:32:14
            Copyright (c) Microsoft Corporation
            Standard Edition (64-bit) on Windows NT 6.3  (Build 9600: )

    Type: quit

    To get out.

Connecting to SQL Server using ogr_fdw

Connecting to SQL Server using ogr_fdw is more or less the same at this point regardless of if you are using Windows or Unix/Linux.

Someone mentioned to me that their list of tables that have compound keys didn't show up as options on Linux, but did when using ogr_fdw under windows. I haven't experimented with that to see if I can replicate the issue.