Simple ArcObjects Tip for Joining Tables
Rene Rubalcava | May 17, 2011
I have recently been doing a lot of work on our server side functions using ESRI ArcObjects. I'm not what one might call an ArcObjects expert, and I know my way around some C# and .NET. Currently, if you want to learn ArcObjects, you need to wade through the online documentation, struggle with the search features and rummage over forum postings. The ArcObjects book lays a pretty good foundation, but is pretty outdated (it uses VB6 for examples), but with a little leg work, most devs could probably pick it up.
I thought I would share a cool little tip I learned while working with some large data. We have a parcel data set (ArcSDE) that has approximately 2 million features with unique id's. I needed a function that would let us find a parcel quickly. That's the easy part, but we also have a table in the SDE that has the detailed information for the parcel features, including addresses, land use and so forth. I also needed to return this information to a web application. This can be accomplished a couple of ways, you can build a relationship or you can do a join in the ArcGIS Server MapService via your mxd.
The classic table join slowed the service to a crawl. Even after applying tablur indexes. This is 2 million features after all. A relationship may also be a good choice and can be accessed via ArcObjects. I don't have direct access to the SDE, and besides we update this data on an annual basis, so I don't know if that also requires the relationships to be rebuilt on the SDE every time the Feature and other Table are updated. One of my goals when developing my server functions is to lessen the maintenance need of our IT group for our GIS data, so I wanted to try to figure out a way to do this kind of query on the fly and to do it quickly.
Up until now I had been using IQueryFilter to query features in my functions. This won't let me do table joins though. Then I discovered the IQueryDef interface which will let you do table joins, and you use the where clause to define that join.
IQueryDef queryDef = workSpace.CreateQueryDef(); queryDef.Tables = "sde.Parcels, sde.Parcels_Details"; queryDef.WhereClause = "sde.Parcels.AIN = sde.Parcels_Details.AIN";
This looked just like what I needed. It worked ok, but it was still joining some 2 million records. At first I was defining my parcel query with a second IQueryFilter. Then it occurred to me that I could just find my feature with all the joined data by looking for it with IQueryDef.
queryDef.WhereClause = "sde.Parcels.AIN = sde.Parcels_Details.AIN AND sde.Parcels.AIN = '" + ainString + "'";
This way the join only occurs on that single feature instead of 2 million features. This method is really quick now. For the record, we use WebOrb in-house for our server functions, and this allows me to use Remote Objects with the ESRI Flex API. This little function is proving to be a valuable resource in our current day-to-day use.
One other item that took me a while to figure out was how to get an IFeatureCursor out of the IQueryDef. I was able to do that in the following steps.
IFeatureDataset fd = workSpace.OpenFeatureQuery("sde.Parcels", queryDef); // use the IQueryDef to get an IFeatureDataset IFeatureClassContainer fcc = fd as IFeatureClassContainer; // cast it as am IFeatureClassContainer IFeatureClass featureClass = fcc.get_ClassByName("sde.Parcels"); // pull the IFeatureClass out of the container IQueryFilter qf = new QueryFilterClass(); // just need an empty QueryFilter IFeatureCursor fc = featureClass.Search(qf, false); // now I have an IFeatureCursor that I can use to access the Feature attributes and geometries
Once I do this, I use the IFeature with my .NET to Flex classes library to send a FeatureSet to my ESRI Flex Application.
So that is my ArcObjects tip of the day. There might be some better ways of performing this function or a quicker way of getting to the IFeatureCursor from IQueryDef. I am always open to some helpful tips. Maybe it's common knowledge for seasoned ArcObjects devs, but I thought it was pretty cool.